Class 12 Biology Chapter 10 Microbes in Human Welfare

Key Concept: Heavy Metal Detoxification, Microbial Transformation

b) Reduction by *Shewanella oneidensis*
[Solution Description]
Certain bacteria like *Shewanella oneidensis* perform enzymatic reduction of Cr(VI) to Cr(III) using chromate reductases. Cr(III) is less soluble and non-toxic, making this process critical for bioremediation.

Key Concept: Bioremediation, Microbial Degradation

b) *Pseudomonas putida* – Efficiently metabolizes hydrocarbons via aerobic pathways
[Solution Description]
Pseudomonas is highly effective for hydrocarbon degradation due to its ability to produce biosurfactants and enzymes like monooxygenases that break down complex hydrocarbons into simpler compounds. It thrives in aerobic conditions, making it ideal for oil spill remediation.

Key Concept: Earthworm Species, Optimal Conditions

c) Lumbricus terrestris
[Solution Description]
*Eisenia fetida* (red wigglers) and *Perionyx excavatus* (blue worms) thrive in decomposing matter and are ideal for vermicomposting. *Lumbricus terrestris* (common earthworm) prefers soil over compost, making it inefficient for waste processing.

Key Concept: Vermicomposting, Microbial Activity

c) 50%
[Solution Description]
Initial C:N ratio = 40:1 → Let initial carbon = 40 units, nitrogen = 1 unit.
Final C:N ratio = 20:1 → Since nitrogen is constant (1 unit), final carbon = 20 units.
Carbon lost = Initial carbon – Final carbon = 40 – 20 = 20 units.
Percentage loss = $(20/40) \times 100 = 50%$.

Key Concept: Biogas Production, Anaerobic Digestion

c) 300 L
[Solution Description]
The biogas sample contains 60% methane. To find the volume of methane in 500 L of biogas, multiply the total volume by the percentage of methane: $500 \times 0.60 = 300$ L. Thus, the volume of methane generated is 300 L.

Key Concept: Biological Oxygen Demand, Nutrient Recycling

d) 95%
[Solution Description]
To find the percentage of organic matter degraded, we first calculate the reduction in BOD. The initial BOD is 400 mg/L, and the final BOD is 20 mg/L. The reduction in BOD is $400 - 20 = 380$ mg/L. The percentage reduction is calculated as $\frac{380}{400} \times 100 = 95\%$. Therefore, 95% of the organic matter was degraded.

Key Concept: Primary treatment, Secondary treatment

b) 700 kg; 70 kg
[Solution Description]
Primary treatment removes 30% of 1000 kg:
1000 kg × 30/100 = 300 kg removed as sludge
Organic matter reaching secondary treatment = 1000 - 300 = 700 kg
Microbial flocs reduce this by 90%:
700 kg × 90/100 = 630 kg removed
Final quantity remaining = 700 - 630 = 70 kg

Key Concept: BOD, Biogas composition

c) 87.5% reduction; CH₄
[Solution Description]

To calculate the percentage reduction in BOD:

Initial BOD = 400 mg/L
Final BOD = 50 mg/L

Reduction = 400 − 50 = 350 mg/L

Percentage reduction = (350 / 400) × 100 = 87.5%

Thus, the BOD is reduced by 87.5%.

Biogas produced during anaerobic digestion mainly consists of methane (CH₄), along with smaller amounts of carbon dioxide (CO₂) and hydrogen sulfide (H₂S). Methane forms the largest proportion (approximately 60–70%) and is responsible for the fuel value of biogas.

Key Concept: Factors Affecting Composting, Role of Microbes

b) Low moisture content (20%) limiting microbial activity.
[Solution Description]
The maturation stage involves slow decomposition by fungi and actinomycetes, requiring stable conditions. Low moisture content (60%) can lead to anaerobic conditions, but low moisture directly limits microbial metabolic activity, halting the maturation process. Neutral pH (6.5-8.0) and adequate aeration are required for this stage.

Key Concept: Stages of Composting, Chemical Reactions in Composting

b) The breakdown slows down due to nitrogen deficiency.
[Solution Description]
The thermophilic stage involves high temperatures (40-70°C) where thermophilic bacteria break down proteins, fats, and complex carbohydrates. An optimal C:N ratio for efficient decomposition is 25-30:1. If the C:N ratio exceeds 40:1, nitrogen becomes limited, slowing down microbial activity. This reduces the breakdown rate of complex carbohydrates because microbes lack sufficient nitrogen for protein synthesis and energy production.

Key Concept: Biogas Production, Methanogens, Anaerobic Digestion

b) 126 m$^3$
[Solution Description]
Daily methane production per kg of waste = 0.3 m$^3$. Total waste in 7 days = $100 \times 7 = 700$ kg. Converted waste = $700 \times 0.60 = 420$ kg. Total methane produced = $420 \times 0.3 = 126$ m$^3$.

Key Concept: Sewage Treatment, BOD, Activated Sludge Process

b) 2 mg/L
[Solution Description]
Initial BOD after primary treatment = 250 mg/L. Secondary treatment reduces it to 20 mg/L. Activated sludge process reduces BOD by 90%: $20 \times 0.10 = 2$ mg/L. Final BOD is 2 mg/L.

Key Concept: Biochemical Oxygen Demand

d) Large concentration of biodegradable organic waste
[Solution Description]
The Biochemical Oxygen Demand (BOD) measures the amount of oxygen required by aerobic microorganisms to decompose organic matter present in water. A high BOD indicates that the water has a large concentration of biodegradable organic waste, which requires more oxygen for microbial degradation. This reduces the dissolved oxygen available for aquatic life, leading to pollution.
Hence, a high BOD signifies highly polluted water with excessive organic waste.

Key Concept: Hydrocarbon Degradation

d) Pseudomonas
[Solution Description]
Microorganisms such as certain species of bacteria and fungi are capable of degrading petroleum hydrocarbons in oil spills. Among these, Pseudomonas and some species of Bacillus are widely used due to their hydrocarbon-degrading abilities. They break down complex hydrocarbons into simpler, less harmful compounds.
Therefore, the correct answer is a bacterium from the genus Pseudomonas.

Key Concept: Microbial Role in Vermicomposting

c) Trichoderma
[Solution Description]
Cellulose decomposition in vermicomposting is mainly carried out by fungi like *Trichoderma* and bacteria such as *Bacillus*. Among these, *Trichoderma* is a dominant cellulolytic fungus.

Key Concept: Biochemical Oxygen Demand - BOD

c) It measures the amount of oxygen needed for bacterial decomposition of organic matter
[Solution Description]
Biochemical Oxygen Demand (BOD) measures the amount of oxygen required by microorganisms to decompose organic matter in water. High BOD implies high organic pollution, meaning more oxygen will be consumed, potentially harming aquatic life due to oxygen depletion. Hence, reducing BOD during sewage treatment ensures safer discharge of treated water into natural ecosystems.

Key Concept: Role of Microbes in Sewage Treatment

d) To promote aerobic microbial growth and reduce BOD
[Solution Description]
The primary purpose of aeration in the secondary treatment of sewage is to provide oxygen to aerobic microbes, which break down organic matter into simpler compounds. This reduces the biochemical oxygen demand (BOD) of the sewage by consuming organic pollutants. The reduction in BOD indicates that the water is less polluted and can be safely released into natural water bodies.

Key Concept: Sludge Digestion

c) Methane (CH₄)
[Solution Description]
During anaerobic sludge digestion, microorganisms decompose organic matter in the absence of oxygen. In this process, complex organic compounds are broken down step by step by different groups of microbes. The final stage, called methanogenesis, produces methane (CH₄) as the major end product.

Key Concept: Biochemical Oxygen Demand

d) Greater pollution potential due to organic waste
[Solution Description]
BOD measures the amount of oxygen consumed by microbes to decompose organic matter in water. A high BOD indicates a large amount of organic waste present, meaning higher microbial activity is required, suggesting greater pollution potential.

Key Concept: Factors Affecting Composting Efficiency

b) 25:1 to 30:1
[Solution Description]
A balanced C/N ratio is critical for microbial activity in composting. If the ratio is too high (excess carbon), decomposition slows down due to nitrogen limitation. If it's too low (excess nitrogen), ammonia volatilization occurs. The optimal range for efficient composting is between 25:1 and 30:1 (C/N ratio). This ensures sufficient nitrogen for microbial growth while maintaining enough carbon for energy.

Key Concept: Microbial Activity in Composting

b) Thermophilic bacteria and fungi
[Solution Description]
The thermophilic phase occurs at temperatures between 40-70°C. In this phase, thermophilic bacteria and fungi dominate the composting process, as they are more efficient at breaking down complex organic materials such as cellulose and lignin compared to mesophilic microbes. Examples of these microbes include Bacillus, Pseudomonas (bacteria), Aspergillus, Trichoderma (fungi), and actinomycetes like Streptomyces.

Key Concept: Biogas Production

c) Methanobacterium
[Solution Description]
Methanobacterium is a genus of methanogenic archaea that thrives in strictly anaerobic (oxygen-free) conditions. These microorganisms play a key role in biogas production by converting simple organic compounds—such as hydrogen and carbon dioxide—into methane (CH₄) and small amounts of carbon dioxide (CO₂) during the final stage of anaerobic digestion (methanogenesis).

Key Concept: Microbes in Sewage Treatment

d) To facilitate aerobic microbial degradation of organic matter
[Solution Description]
The secondary treatment involves aerobic microbes breaking down organic matter to reduce the Biochemical Oxygen Demand (BOD). Aeration provides oxygen for aerobic bacteria to thrive and degrade organic waste efficiently.

Key Concept: Microorganisms Involved

c) *Alcanivorax borkumensis*
[Solution Description]
Microbes like *Pseudomonas* and *Alcanivorax borkumensis* are known for their ability to degrade petroleum hydrocarbons in oil spills. *Bacillus subtilis* is more associated with general biodegradation, while *Escherichia coli* is not typically used for hydrocarbon degradation. *Saccharomyces cerevisiae* is a yeast used in fermentation.

Key Concept: Types of Bioremediation

a) Bioaugmentation
[Solution Description]
In situ bioremediation involves treating contaminated material directly at the site without removing it. Bioaugmentation and biostimulation are examples of in situ bioremediation, where microbes are added or nutrients are provided to enhance degradation. Landfarming and composting require the removal of contaminants and are ex situ methods.

Key Concept: Types of Organic Waste Used

c) Vegetable peels
[Solution Description]
Common organic wastes used in vermicomposting include vegetable peels, fruit waste, crop residues, and animal manure. These materials provide the necessary carbon and nitrogen required for microbial activity during composting.

Key Concept: Role of Microbes and Earthworms

c) Ingest organic material and excrete nutrient-rich castings
[Solution Description]
The primary role of earthworms in vermicomposting is to ingest organic material and excrete nutrient-rich castings, which enhances soil fertility. They also help in breaking down organic matter by grinding and mixing it, increasing the surface area for microbial action.

Key Concept: Biofertilizers

c) Rhizobium
[Solution Description]
Rhizobium is a nitrogen-fixing bacterium that forms a symbiotic relationship with the roots of leguminous plants. It induces the formation of root nodules where nitrogen fixation occurs. The bacteria convert atmospheric nitrogen into ammonium ions, which are used by the plant for amino acid synthesis. In return, the plant provides carbohydrates to the bacteria.

Key Concept: Role of microbes in sewage treatment

c) To reduce the biochemical oxygen demand (BOD) of the effluent
[Solution Description]
The primary role of aerobic microbes in the secondary treatment of sewage is to reduce the Biochemical Oxygen Demand (BOD) of the effluent. These microbes form flocs and consume organic matter present in the sewage, thereby breaking it down into simpler compounds like carbon dioxide and water. This process significantly reduces the BOD, making the water less polluting before it is discharged into natural water bodies.

Key Concept: BOD in Sewage Treatment

b) The amount of oxygen needed for microbial decomposition of organic matter
[Solution Description]
BOD measures the amount of oxygen consumed by microorganisms to decompose organic matter present in water. A higher BOD indicates greater pollution potential because more oxygen is required for decomposition.

Key Concept: Decomposition of Organic Waste

c) Methane ($CH_4$)
[Solution Description]
During anaerobic decomposition, microbes like methanogens break down organic matter and produce methane ($CH_4$) as a primary by-product along with carbon dioxide ($CO_2$). This process is used in anaerobic lagoons and sludge digesters for biogas production.

Key Concept: Composting Process

c) To support aerobic microbial activity
[Solution Description]
Oxygen is essential for aerobic decomposition carried out by microorganisms. Without oxygen, anaerobic conditions prevail, leading to foul odors and slower decomposition.

Key Concept: Types of Microbes in Composting

c) Bacteria
[Solution Description]
Bacteria, fungi, and actinomycetes are the primary decomposers in composting. Among these, bacteria are the most active in the early stages of decomposition.

Key Concept: Biogas Production

d) Methanobacterium
[Solution Description]
Methanobacterium (methanogens) are the microorganisms primarily responsible for biogas production in anaerobic conditions. They break down organic matter to produce methane and carbon dioxide.

Key Concept: Microbes in Sewage Treatment

c) To consume organic matter and reduce BOD
[Solution Description]
The primary function of aerobic microbes in secondary sewage treatment is to consume organic matter in the effluent, thereby reducing its Biochemical Oxygen Demand (BOD). These microbes grow as flocs and help break down pollutants.

Key Concept: Biocontrol agents, IPM, Pest-specific action

d) Treating crops with Bacillus thuringiensis spores to target caterpillars selectively
[Solution Description]
Bacillus thuringiensis (Bt) produces insecticidal proteins that specifically target caterpillars (Lepidoptera larvae) without affecting non-target organisms such as bees or ladybird beetles. Trichoderma is antifungal, not insecticidal. Baculoviruses are also effective but typically have a narrow host range; however, they are less commonly used compared to Bt in integrated pest management (IPM). Chemical pesticides are non-selective and harmful.

Key Concept: Rhizobium, Nitrogen fixation, Symbiotic association

d) Introducing Rhizobium to form root nodules in leguminous plants for nitrogen fixation
[Solution Description]
Rhizobium forms a symbiotic relationship with leguminous plants and fixes atmospheric nitrogen ($N_2$) into ammonia ($NH_3$), enhancing soil fertility without chemical fertilizers. It is highly specific to legumes, unlike free-living nitrogen fixers like Azospirillum or Azotobacter, which are less efficient for legumes. Cyanobacteria mainly benefit paddy fields, not legumes.

Key Concept: Organic Matter Decomposition, Mycorrhizal Associations

b) Enhances nutrient release and root phosphorus absorption
[Solution Description]
Decomposer bacteria break down organic matter into humus, releasing nutrients like potassium ($K^+$) and calcium ($Ca^{2+}$). Glomus forms mycorrhizal associations with plant roots, enhancing phosphorus uptake and improving drought resistance. This dual action enriches soil fertility and improves its structure.

Key Concept: Nitrogen Fixation, Phosphate Solubilization

c) Combined improvement of nitrogen and phosphorus availability
[Solution Description]
Rhizobium fixes atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$), enriching the soil with nitrogen. Pseudomonas solubilizes insoluble phosphates into plant-accessible forms ($PO_4^{3-}$). Together, they enhance both nitrogen and phosphorus availability in the soil, promoting plant growth.

Key Concept: Phosphate Solubilization, Biocontrol Agents

c) 194 g
[Solution Description]
Step 1: Write the solubilization reaction:
$Ca_3(PO_4)_2 + 4H^+ \rightarrow 3Ca^{2+} + 2H_2PO_4^-$.
Step 2: Calculate moles of $Ca_3(PO_4)_2$. Molar mass of $Ca_3(PO_4)_2$ is 310 g/mol. Moles of $Ca_3(PO_4)_2 = \frac{620 \text{ g}}{310 \text{ g/mol}} = 2$ mol.
Step 3: Determine moles solubilized (50% efficiency). Moles solubilized = $2 \times 0.5 = 1$ mol.
Step 4: Use stoichiometry to find moles of $H_2PO_4^-$ produced. From the reaction, 1 mole of $Ca_3(PO_4)_2$ produces 2 moles of $H_2PO_4^-$. Thus, $H_2PO_4^-$ produced = $2 \times 1 = 2$ mol.
Step 5: Calculate mass of $H_2PO_4^-$. Molar mass of $H_2PO_4^-$ is $1 \times 2 + 31 + 16 \times 4 = 97$ g/mol. Mass of $H_2PO_4^- = 2 \times 97 = 194$ g.

Key Concept: Nitrogen Fixation, Legume-Rhizobium Symbiosis

c) 12.14 kg
[Solution Description]
Step 1: Calculate the molar mass of $N_2$. The atomic mass of nitrogen (N) is 14 g/mol. So, the molar mass of $N_2$ is $2 \times 14 = 28$ g/mol.
Step 2: Convert the given mass of $N_2$ (10 kg = 10000 g) to moles. Moles of $N_2 = \frac{10000 \text{ g}}{28 \text{ g/mol}} = 357.14$ mol.
Step 3: Use the stoichiometry of the reaction. According to the equation, 1 mole of $N_2$ produces 2 moles of $NH_3$.
Step 4: Calculate moles of $NH_3$ produced. Moles of $NH_3 = 2 \times 357.14 = 714.28$ mol.
Step 5: Convert moles of $NH_3$ to mass. The molar mass of $NH_3$ is $14 + 3 \times 1 = 17$ g/mol. Mass of $NH_3 = 714.28 \times 17 = 12142.76$ g ≈ 12.14 kg.

Key Concept: Phosphate Solubilization, Mycorrhiza

c) Applying Phosphate-Solubilizing Bacteria
[Solution Description]

The problem concerns improving phosphorus availability in alkaline soils, where phosphate is commonly fixed in the form of insoluble calcium phosphate.

1. Phosphate-solubilizing bacteria such as Bacillus and Pseudomonas release organic acids (e.g., citric acid and gluconic acid). These acids lower the pH in the surrounding soil and convert insoluble calcium phosphate into soluble forms that plants can absorb. The reaction can be represented as:

   Ca₃(PO₄)₂ + 2H⁺ → 2CaHPO₄ + Ca²⁺

2. Mycorrhizal fungi such as Glomus also help by extending their hyphae beyond the root zone. This increases the surface area for absorption and allows access to phosphorus that is otherwise unavailable to plant roots.

3. Although both mechanisms improve phosphorus uptake, direct solubilization of phosphate by bacteria is especially important in high-pH (alkaline) soils where calcium phosphate fixation is high.

Key Concept: Nitrogen Fixation, Rhizobium Symbiosis

c) Conversion of atmospheric nitrogen to ammonia
[Solution Description]

The process carried out by Rhizobium in the root nodules of leguminous plants is called biological nitrogen fixation.

It occurs in the following steps:

1. Rhizobium bacteria infect the roots of leguminous plants and form specialized structures called root nodules.

2. Inside these nodules, the bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) using the enzyme nitrogenase under anaerobic conditions.

3. The reaction can be represented as:

   N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi

4. The ammonia formed is converted into ammonium ions (NH₄⁺), which are used by the plant to synthesize amino acids, proteins, and other nitrogenous compounds. In return, the plant supplies carbohydrates and a suitable environment to the bacteria.

Key Concept: Frankia, Nitrogen Fixation

c) 785
[Solution Description]
The nitrogen contribution from *Frankia* is 157 kg N/ha/year for *Alnus*. For 5 hectares:
Step 1: Multiply the nitrogen contribution per hectare by the number of hectares.
$157 \text{ kg/ha/year} \times 5 \text{ ha} = 785 \text{ kg/year}$
Thus, the total nitrogen contribution is 785 kg/year.

Key Concept: Vesicular-Arbuscular Mycorrhizal Fungi, Phosphate Solubilizing Bacteria

c) 25
[Solution Description]
Initially, the soil has 100 kg/ha of insoluble phosphorus. Microbial activity increases soluble phosphorus by 25%. To find the additional phosphorus available:
Step 1: Calculate 25% of 100 kg.
$25% \text{ of } 100 = \frac{25}{100} \times 100 = 25 \text{ kg/ha}$
Thus, 25 kg/ha of additional phosphorus is now available to the plants.

Key Concept: VAM fungi, Biofertilizers

d) $Glomus$
[Solution Description]
Vesicular-arbuscular mycorrhizal (VAM) fungi form symbiotic relationships with plant roots, enhancing nutrient uptake, especially phosphorus. Among the VAM fungi, $Glomus$ is the most abundant in soil. These fungi are crucial in agriculture and forestry for improving plant growth and stress resistance.

Key Concept: Ectomycorrhiza, Nutrient uptake

c) Fungal hyphae penetrate the root cells internally.
[Solution Description]
Ectomycorrhiza involves fungal hyphae forming a dense sheath external to the root, with some hyphae traversing intercellular spaces. They aid in absorbing nutrients like nitrogen, phosphorus, potassium, and calcium. Additionally, they protect roots from pathogens and produce growth-promoting substances. However, ectomycorrhiza does not penetrate the root cells internally, which is a characteristic of endomycorrhiza. The incorrect statement among the options would be the one describing internal penetration of root cells by fungal hyphae.

Key Concept: Ectomycorrhiza vs Endomycorrhiza, Sustainable Agriculture

c) Ectomycorrhiza
[Solution Description]
The question describes a fungal sheath around roots without penetration into root cells, which is characteristic of ectomycorrhiza. Endomycorrhiza involves penetration of root cells. Since the scenario matches the features of ectomycorrhiza and its role in sustainable agriculture (as per the syllabus), the correct answer is ectomycorrhiza.

Key Concept: VAM Fungi, Nutrient Absorption, Pathogen Resistance

d) *Glomus*
[Solution Description]
The question describes a scenario where VAM fungi provide pathogen resistance and phosphorus uptake but not nitrogen absorption. From the syllabus, *Glomus* is the most abundant VAM fungus known for enhancing phosphorus uptake and offering resistance to root-infecting pathogens. Other options like *Rhizobium* are associated with nitrogen fixation, which is not mentioned here. Thus, *Glomus* is the correct answer.

Key Concept: Azolla-Anabaena Symbiosis, Cyanobacteria Applications

d) The symbiosis allows targeted nitrogen release near rice roots
[Solution Description]
The Azolla-Anabaena symbiosis is highly effective because Anabaena azollae, residing in the leaf cavities of Azolla, fixes atmospheric nitrogen efficiently and releases nitrogenous compounds directly into the fern. This symbiotic relationship ensures a continuous supply of fixed nitrogen to the rice plants when Azolla decomposes in the waterlogged paddy fields. Free-living cyanobacteria, while also nitrogen-fixing, may not provide as consistent or localized a nutrient supply due to environmental factors like competition and dispersal.

Key Concept: Cyanobacteria, Nitrogen Fixation

c) 8 electrons and 16 ATP molecules
[Solution Description]
The nitrogenase enzyme catalyzes the reduction of atmospheric nitrogen ($N_2$) to ammonia ($NH_3$). The balanced reaction is:
$N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16Pi$
From the equation, it is clear that reducing one molecule of $N_2$ requires 8 electrons and 16 ATP molecules.

Key Concept: Diazotrophs, Associative nitrogen fixation

c) Herbaspirillum seropedicae
[Solution Description]
From the syllabus: Diazotrophs include Herbaspirillum seropedicae, H. rubrisubalbicans, Acetobacter diazotrophicus, and Bacillus azotofixans, which grow endophytically in sugarcane and rice. Among these, Herbaspirillum seropedicae is specifically mentioned as a key endophytic nitrogen-fixing bacterium associated with sugarcane and rice.

Key Concept: Azospirillum association, Nitrogen fixation efficiency

d) 24 kg nitrogen
[Solution Description]
Given:
1. Efficient strains of Azotobacter can fix 30 kg nitrogen from 1,000 kg organic matter.
2. The field has 800 kg organic matter/ha.
To find the nitrogen fixed by Azotobacter:
First, calculate the nitrogen fixed per kg of organic matter:
$\frac{30\ \text{kg}}{1000\ \text{kg}} = 0.03\ \text{kg N/kg organic matter}$
Now, multiply by the total organic matter (800 kg):
$0.03 \times 800 = 24\ \text{kg N fixed}$
Therefore, Azotobacter would fix 24 kg nitrogen from 800 kg organic matter.

Key Concept: Growth-promoting substances, Azotobacter benefits

c) By synthesizing phytohormones that stimulate plant growth
[Solution Description]
Azotobacter produces phytohormones such as auxins, gibberellins, and cytokinins, which stimulate root and shoot growth, improve nutrient uptake, and enhance overall plant development. These substances act directly on plant cells to promote division, elongation, and differentiation, leading to healthier and more vigorous plants.

Key Concept: Nitrogen fixation efficiency, Azotobacter application

c) 425 kg N
[Solution Description]
First, calculate the total organic matter across 5 hectares:
$2,000 \text{ kg/ha} \times 5 \text{ ha} = 10,000 \text{ kg organic matter}$.
Next, determine the nitrogen fixed by Azotobacter from this organic matter:
$\frac{30 \text{ kg N}}{1,000 \text{ kg organic matter}} \times 10,000 \text{ kg organic matter} = 300 \text{ kg N}$.
Now, calculate the nitrogen savings due to reduced fertilizer use (maximum savings):
$25 \text{ kg N/ha} \times 5 \text{ ha} = 125 \text{ kg N}$.
The total nitrogen saved is the sum of nitrogen fixed and nitrogen savings from fertilizer reduction:
$300 \text{ kg N} + 125 \text{ kg N} = 425 \text{ kg N}$.

Key Concept: Nitrogenase enzyme, Energy requirement in Nitrogen Fixation

c) 8 ATP
[Solution Description]
The balanced equation for nitrogen fixation shows that 16 ATP molecules are consumed to produce 2 molecules of ammonia. Thus, per molecule of ammonia, the ATP consumption is calculated as follows:
$\text{ATP per } NH_3 = \frac{16 \text{ ATP}}{2 \text{ }NH_3} = 8 \text{ ATP per } NH_3$
Hence, 8 ATP molecules are required per ammonia molecule synthesized.

Key Concept: Free-living nitrogen-fixing bacteria, Mechanism of Nitrogen Fixation

d) Clostridium
[Solution Description]
The question requires identifying the nitrogen-fixing bacteria suited for anaerobic (oxygen-deficient) environments like waterlogged soils. Among the options, Clostridium is an anaerobic bacterium that thrives in such conditions and fixes nitrogen effectively, making it ideal for paddy fields or waterlogged soils.

Key Concept: Nitrogen fixation pathway, Electron transfer

c) Oxidation of carbohydrates.
[Solution Description]
The nitrogen fixation reaction requires electrons for reducing atmospheric nitrogen ($N_2$) to ammonia ($NH_3$). These electrons are obtained through the oxidation of carbohydrates provided by the host legume plant to the Rhizobium bacteria. The equation for nitrogen fixation is:
$N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2$
The source of these electrons is the oxidation of carbohydrates, as they are broken down to release energy and electrons.
Therefore, the correct answer is: Oxidation of carbohydrates.

Key Concept: Nitrogen fixation, Leghaemoglobin role

b) It binds oxygen to protect nitrogenase from inactivation.
[Solution Description]
Leghaemoglobin is a pigment found in legume root nodules that gives them a pink or reddish color. Its primary role is to bind oxygen ($O_2$) and maintain a low oxygen concentration within the nodule. This is crucial because nitrogenase, the enzyme responsible for nitrogen fixation, is highly sensitive to oxygen and would be inhibited in its presence. By reducing the oxygen levels, leghaemoglobin creates an anaerobic environment necessary for nitrogenase activity.
Therefore, the correct answer is: It binds oxygen to protect nitrogenase from inactivation.

Key Concept: Mycorrhiza, Endomycorrhiza

b) Endomycorrhiza with Glomus fasciculatum
[Solution Description]
The question involves endomycorrhizal fungi, particularly vesicular-arbuscular mycorrhizae (VAM), which are known to enhance phosphorus uptake and provide drought resistance in plants like betel vine. Ectomycorrhizae form external sheaths and are common in forest trees, not horticultural crops. Endomycorrhizae, especially VAM fungi (e.g., Glomus), penetrate root cells and form arbuscules, facilitating nutrient exchange. Thus, the observed benefits are due to endomycorrhizal association.

Key Concept: Symbiotic nitrogen fixation, Diazotrophs

d) Azotobacter chroococcum
[Solution Description]
The question pertains to diazotrophs, specifically their role in nitrogen fixation in rice fields. Among the given options, Azotobacter is a well-known free-living aerobic heterotrophic diazotroph that fixes nitrogen efficiently. It can fix approximately 30 kg of nitrogen per 1000 kg of organic matter. Other options like Pseudomonas are not primarily nitrogen-fixers, while Rhizobium forms symbiotic associations with legumes, not rice. Therefore, Azotobacter is the most effective option for rice fields.

Key Concept: Azolla-Anabaena Symbiosis, Cyanobacteria

d) Azolla fixes atmospheric nitrogen through its symbiotic cyanobacteria, enriching the soil
[Solution Description]
Azolla harbors the nitrogen-fixing cyanobacterium Anabaena azollae in its leaf cavities. This symbiosis provides a continuous supply of biologically fixed nitrogen directly to the rice plants, improving soil fertility without chemical inputs. Over time, Azolla decomposes, releasing organic matter and growth-promoting substances, further enhancing yield sustainably.

Key Concept: Legume-Rhizobium Symbiosis, Nitrogen Fixation

c) The nodules are inactive or poorly functional due to insufficient leghaemoglobin
[Solution Description]
The pink or reddish color of healthy legume nodules is due to the presence of leghaemoglobin, which is crucial for maintaining low oxygen levels required for nitrogenase activity. A greenish color indicates inadequate leghaemoglobin production, often resulting from poor oxygen regulation or ineffective symbiosis. This reduces nitrogen fixation efficiency.

Key Concept: Mycorrhiza, Phosphate Solubilizing Microbes

c) Inoculating soil with *Bacillus* and *Glomus* species simultaneously.
[Solution Description]
Key points:
1. Insoluble phosphate requires solubilization by microbes like *Bacillus* or *Pseudomonas*.
2. Drought resistance is improved by mycorrhizal fungi (e.g., *Glomus*).
Optimal solution: Combine phosphate-solubilizing bacteria (*Bacillus*) with vesicular-arbuscular mycorrhiza (VAM fungi like *Glomus*).
Step-by-step reasoning:
- *Bacillus* converts insoluble phosphates to soluble forms, making phosphorus available.
- VAM fungi enhance nutrient uptake (including phosphorus) and improve drought tolerance.
Thus, combining both addresses the immediate phosphate issue and long-term stress resilience.

Key Concept: Nitrogen-Fixing Bacteria, Azolla-Anabaena Symbiosis

c) *Rhizobium* provides measurable fixed nitrogen (50–150 kg N/ha), while *Azolla-Anabaena*'s contribution is indirect but significant.
[Solution Description]
First, let's understand the nitrogen contributions:
1. *Rhizobium* fixes 50–150 kg N/ha in legume root nodules.
2. *Azolla-Anabaena* is not explicitly quantified here but increases rice yield by 50%, likely due to substantial nitrogen input comparable to or exceeding *Rhizobium*.
Comparing both: While *Rhizobium* provides direct nitrogen fixation (50–150 kg N/ha), *Azolla-Anabaena* enhances yield via nitrogen release and other growth-promoting effects. However, specific nitrogen quantities aren't directly comparable without additional data. The question asks for an accurate comparison.
Calculation steps: No numerical calculation needed, as the answer relies on conceptual understanding.
Conclusion: The correct option must acknowledge that *Rhizobium* provides measurable fixed nitrogen (50–150 kg N/ha), while *Azolla-Anabaena* contributes indirectly but significantly to yield.

Key Concept: Azolla-Anabaena symbiosis, Cyanobacteria, Nitrogen fixation

d) Azolla-Anabaena symbiosis provides bioavailable nitrogen to rice plants.
[Solution Description]
The Azolla-Anabaena symbiosis involves the aquatic fern Azolla harboring the nitrogen-fixing cyanobacterium Anabaena azollae in its leaf cavities. Anabaena fixes atmospheric nitrogen and releases nitrogenous compounds, which are utilized by Azolla and subsequently by rice plants when Azolla decomposes in flooded rice fields. This natural fertilization process enhances soil fertility, reduces the need for chemical fertilizers, and can increase rice yields significantly (over 50%). The symbiotic system thrives in waterlogged conditions typical of rice paddies, making it highly suitable for lowland rice cultivation.

Key Concept: Legume-Rhizobium symbiosis, Nitrogen fixation

c) Leghaemoglobin maintains an anaerobic environment for nitrogenase activity.
[Solution Description]
Leghaemoglobin is a pigment found in healthy legume root nodules that gives them their pink or reddish color. It plays a crucial role in nitrogen fixation by maintaining an anaerobic environment necessary for the nitrogenase enzyme to function. Nitrogenase is highly sensitive to oxygen, and leghaemoglobin binds with oxygen, preventing it from inhibiting nitrogenase activity. This allows Rhizobium bacteria to convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$), which is then used by the plant for amino acid synthesis.

Key Concept: Azolla-Anabaena Symbiosis, Biofertilizers

c) *Azolla* harbors *Anabaena azollae*, which fixes atmospheric nitrogen, enhancing soil fertility.
[Solution Description]
*Azolla* is a floating fern that forms a symbiotic relationship with the nitrogen-fixing cyanobacterium *Anabaena azollae*. The cyanobacterium fixes atmospheric nitrogen ($N_2$) into a usable form for plants, enriching the soil with nitrogen. This reduces dependency on synthetic nitrogen fertilizers and increases rice yields by over 50%.

Key Concept: Legume-Rhizobium Symbiosis, Nitrogen Fixation

c) The nodules contain *Rhizobium* bacteria that fix atmospheric nitrogen, reducing the need for chemical fertilizers.
[Solution Description]
The nodules on soybean roots are formed due to a symbiotic relationship with *Rhizobium* bacteria. These bacteria fix atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$), which the plant uses for amino acid synthesis. In return, the plant provides carbohydrates to the bacteria. This reduces the need for chemical fertilizers and enhances soil fertility, making it highly beneficial for organic farming.

Key Concept: Advantages of Biofertilizers

c) They improve soil structure and reduce pollution
[Solution Description]
Microbial biofertilizers improve soil structure and fertility by enhancing nutrient availability to plants. They reduce environmental pollution caused by chemical fertilizers and are cost-effective, making them an eco-friendly alternative for sustainable agriculture. Unlike chemical fertilizers, they sustain long-term soil health by supporting microbial life and humus formation.

Key Concept: Microbial Biofertilizers

d) Rhizobium
[Solution Description]
Rhizobium is a type of biofertilizer that forms a symbiotic association with the roots of leguminous plants. It helps in fixing atmospheric nitrogen ($N_2$) into organic forms, enriching the soil with nitrogen content. This process enhances soil fertility naturally and reduces dependency on chemical fertilizers.

Key Concept: Phosphate Solubilization

d) Convert insoluble phosphates into soluble forms
[Solution Description]
Phosphate-solubilizing bacteria convert insoluble phosphates ($PO_4^{3-}$) into soluble forms ($H_2PO_4^-$, $HPO_4^{2-}$), making phosphorus available for plant uptake. This improves soil fertility by increasing nutrient accessibility.

Key Concept: Nitrogen Fixation

c) Rhizobium
[Solution Description]
The correct microbe is *Rhizobium*, which forms nodules on the roots of leguminous plants and converts atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) through biological nitrogen fixation. This process enriches soil nitrogen content naturally.

Key Concept: Azolla-Anabaena Symbiosis

c) Fixes atmospheric nitrogen and improves soil fertility
[Solution Description]
Anabaena azollae is a blue-green alga that forms a symbiotic relationship with Azolla, a freshwater fern. It fixes atmospheric nitrogen and releases nitrogenous compounds, enriching the soil with bioavailable nitrogen. When used as an organic manure in rice cultivation, this system increases crop yield by over 50% due to improved soil fertility.

Key Concept: Legume-Rhizobium Symbiosis

b) Leghaemoglobin
[Solution Description]
The pigment responsible for the pink or reddish color of healthy legume nodules is leghaemoglobin. It is produced in root nodules formed due to the symbiotic relationship between legume plants and nitrogen-fixing bacteria like Rhizobium. Leghaemoglobin maintains a low oxygen concentration, which is essential for the functioning of the nitrogenase enzyme involved in nitrogen fixation.

Key Concept: Phosphate Solubilization

d) Phosphate-solubilizing bacteria like Bacillus.
[Solution Description]
Phosphate-solubilizing bacteria, such as Bacillus and Thiobacillus, and fungi like Sclerotium and Fusarium, break down insoluble inorganic phosphates into soluble organic phosphates. This process enhances phosphorus availability for plant uptake, promoting growth.

Key Concept: Nitrogen Fixation in Biofertilizers

d) Rhizobium converts atmospheric nitrogen into ammonium ions.
[Solution Description]
Rhizobium is a nitrogen-fixing bacterium that forms symbiotic associations with leguminous plants. It converts atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$) through the reaction:
$N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2$
This fixed nitrogen is then utilized by the plant for amino acid synthesis, enhancing soil fertility.

Key Concept: Vesicular-Arbuscular Mycorrhizal Fungi

c) Nodules
[Solution Description]
Vesicular-Arbuscular Mycorrhizal (VAM) fungi penetrate root cells and form vesicles (storage structures) and arbuscules (exchange sites). They do not form nodules, which are associated with nitrogen-fixing bacteria like *Frankia* or rhizobia.

Key Concept: Phosphate Solubilizing Microbes

b) Pseudomonas
[Solution Description]
Phosphate solubilizing bacteria (PSB) and fungi convert insoluble phosphates into soluble forms for plants. Among the given options, *Pseudomonas* is a well-known phosphate-solubilizing bacterium that produces organic acids to dissolve phosphates.

Key Concept: Vesicular-Arbuscular Mycorrhizae - VAM

b) They increase absorption of nutrients like phosphorus and provide resistance to drought.
[Solution Description]
VAM fungi enhance nutrient absorption, particularly phosphorus and copper, improve drought resistance, and protect against certain root pathogens. These benefits make them highly valuable in agriculture and forestry.

Key Concept: Ectomycorrhiza and Endomycorrhiza

a) The fungus forms a sheath externally in ectomycorrhiza and penetrates cells in endomycorrhiza.
[Solution Description]
Ectomycorrhiza forms a dense sheath of fungal hyphae around the root surface but does not penetrate the cells, while endomycorrhiza involves fungal hyphae living inside the root cells. Additionally, ectomycorrhiza is common in forest trees like pine and oak, whereas endomycorrhiza is found in orchids and horticultural crops such as coffee and cardamom.

Key Concept: Vesicular-Arbuscular Mycorrhiza (VAM

c) Glomus
[Solution Description]
Among vesicular-arbuscular mycorrhizal (VAM) fungi, $Glomus$ is the most abundant in the soil. These fungi enhance nutrient absorption (especially phosphorus and copper) and provide drought resistance to plants.

Key Concept: Ectomycorrhiza

b) Fungal hyphae form a dense sheath outside the root
[Solution Description]
Ectomycorrhiza involves fungal hyphae forming a dense sheath external to the root. The fungus absorbs nutrients like nitrogen, phosphorus, potassium, and calcium for the host plant while protecting it from pathogens. It also produces growth-promoting substances such as cytokinins.

Key Concept: Symbiotic Relationship of Cyanobacteria

c) It fixes nitrogen and enriches soil fertility for rice cultivation.
[Solution Description]
The symbiotic relationship between *Azolla* (a floating fern) and *Anabaena azollae* (a cyanobacterium) is highly beneficial in rice cultivation. *Anabaena* fixes atmospheric nitrogen and provides nitrogenous compounds to the host plant (*Azolla*), which improves soil fertility. This reduces the need for chemical fertilizers and enhances crop yield, especially in paddy fields.

Key Concept: Nitrogen Fixation by Cyanobacteria

c) Heterocysts
[Solution Description]

Key Concept: Chemical Reaction in Nitrogen Fixation

c) N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
[Solution Description]

The nitrogen fixation reaction catalyzed by Azospirillum is correctly represented by the following balanced equation:

N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi

This equation shows that atmospheric nitrogen (N₂) is reduced to ammonia (NH₃) using protons (H⁺), electrons (e⁻), and energy supplied in the form of ATP. Hydrogen gas (H₂) is released as a by-product of the reaction.

Key Concept: Nitrogen Fixation by Azospirillum

b) Converts atmospheric nitrogen into ammonia for plant uptake
[Solution Description]
Azospirillum is a nitrogen-fixing bacterium that forms loose associations with cereal crops like rice and maize. It enhances soil fertility by converting atmospheric nitrogen into ammonia, which plants can use. Additionally, it produces growth-promoting substances that aid plant development. Its primary role is as a biofertilizer in organic farming to reduce dependency on chemical fertilizers.

Key Concept: Application Benefits

b) 10-25 kg/ha
[Solution Description]
The syllabus states that Azotobacter application can save 10-25 kg/ha of nitrogen fertilizer. This range indicates the variability depending on soil conditions and crop types. Thus, the correct answer falls within this range.

Key Concept: Nitrogen Fixation Efficiency

c) 150 kg
[Solution Description]
According to the given data, 30 kg of nitrogen is fixed from 1,000 kg of organic matter. To find the nitrogen fixed for 5,000 kg of organic matter, we use proportionality:
Nitrogen fixed per kg of organic matter = $\frac{30 \text{ kg}}{1000 \text{ kg}} = 0.03 \text{ kg/kg}$
For 5,000 kg of organic matter:
$0.03 \text{ kg/kg} \times 5000 \text{ kg} = 150 \text{ kg}$
Therefore, 150 kg of nitrogen will be fixed from 5,000 kg of organic matter.

Key Concept: Free-living Nitrogen-fixing Bacteria

d) Azotobacter
[Solution Description]
The question asks about a free-living nitrogen-fixing bacterium that thrives in neutral to alkaline soils and produces phytohormones (auxins and gibberellins). From the key bacteria listed, Azotobacter fits this description as it is aerobic, fixes nitrogen, and secretes growth-promoting substances.

Key Concept: Nitrogen fixation by Rhizobium

c) It reduces atmospheric nitrogen to ammonium ions
[Solution Description]
Rhizobium bacteria live symbiotically in the root nodules of legumes, where they convert atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$). The process involves the enzyme nitrogenase, which requires electrons obtained from the oxidation of carbohydrates provided by the host plant. The fixed nitrogen (ammonium ions) is then assimilated into amino acids, which are transported to other parts of the plant for protein synthesis.

Key Concept: Role of leghaemoglobin in root nodules

c) Maintains low oxygen levels for nitrogenase activity
[Solution Description]
Leghaemoglobin is a pigment found in healthy legume root nodules, giving them a pink or reddish color. Its primary function is to create an anaerobic environment necessary for the activity of the nitrogenase enzyme, which is responsible for nitrogen fixation. Nitrogenase is highly sensitive to oxygen, and leghaemoglobin binds with free oxygen, maintaining low oxygen concentration while facilitating oxygen supply for bacterial respiration.

Key Concept: Azolla-Anabaena Symbiosis

b) It enhances soil nitrogen content through biological nitrogen fixation.
[Solution Description]
The Azolla-Anabaena symbiosis is beneficial in rice fields because Anabaena fixes atmospheric nitrogen, releasing nitrogenous compounds that enhance soil fertility. This leads to increased rice yields by over 50% without relying on chemical fertilizers.

Key Concept: Legume-Rhizobium Symbiosis

b) Leghaemoglobin
[Solution Description]
The reddish color of root nodules in legume-Rhizobium symbiosis is due to the presence of leghaemoglobin. This pigment helps maintain an oxygen-free environment, which is necessary for nitrogenase activity—the enzyme responsible for nitrogen fixation.

Key Concept: Azolla-Anabaena Symbiosis

c) Leaf cavities
[Solution Description]
Anabaena azollae, the blue-green alga involved in nitrogen fixation, resides in the leaf cavities of the Azolla fern. This symbiotic relationship allows the alga to fix atmospheric nitrogen, which is then utilized by the Azolla plant and benefits surrounding crops like rice.

Key Concept: Legume-Rhizobium Symbiosis

c) Presence of leghaemoglobin
[Solution Description]
The pink or reddish color in healthy legume root nodules is due to the presence of leghaemoglobin. This protein is crucial because it creates a low oxygen environment necessary for the nitrogenase enzyme to function effectively, allowing Rhizobium bacteria to fix atmospheric nitrogen.

Key Concept: Azolla-Anabaena symbiosis

c) Fixes atmospheric nitrogen and improves soil fertility
[Solution Description]
The blue-green alga Anabaena azollae, residing in the leaf cavities of Azolla, fixes atmospheric nitrogen and releases nitrogenous compounds into the surrounding environment. This symbiotic relationship enhances soil fertility, leading to an increase in rice crop yields by over 50% when used as a biofertilizer.

Key Concept: Legume-Rhizobium symbiosis

c) Pink or reddish hue due to leghaemoglobin
[Solution Description]
Healthy legume nodules are typically pink or reddish in color due to the presence of leghaemoglobin, which helps maintain low oxygen concentration for nitrogenase activity. This pigment is essential for protecting nitrogenase from oxygen inhibition.

Key Concept: Phosphate Solubilizing Microorganisms

c) Pseudomonas
[Solution Description]
Phosphate solubilizing microorganisms (PSMs), such as Pseudomonas, convert insoluble inorganic phosphates into soluble forms through the production of organic acids and chelating agents. These microorganisms are important biofertilizers because they enhance phosphorus availability, which is crucial for plant growth. Among the given options, Pseudomonas is a well-known phosphate solubilizing bacterium, whereas other options either do not perform this function or are less common in agricultural applications.

Key Concept: Legume-Rhizobium Symbiosis

b) Maintains low oxygen levels to facilitate nitrogen fixation
[Solution Description]
Leghaemoglobin is a pigment found in root nodules formed during the symbiotic association between legume plants and Rhizobium bacteria. Its primary role is to maintain a low oxygen concentration in the nodule, as nitrogenase, the enzyme responsible for nitrogen fixation, is highly sensitive to oxygen. By binding to oxygen, leghaemoglobin creates an anaerobic environment necessary for nitrogen fixation while also supplying oxygen for bacterial respiration.

Key Concept: Biocontrol Agents

c) Pseudomonas fluorescens
[Solution Description]
Biocontrol agents are microbes used to manage pests and diseases without harmful chemicals. Pseudomonas species are known to control fungal diseases such as damping-off in cotton. They act by producing antifungal compounds that inhibit the growth of pathogenic fungi, thereby protecting the crops.

Key Concept: Legume-Rhizobium Symbiosis

a) The bacteria provide carbohydrates to the legume in exchange for fixed nitrogen.
[Solution Description]
The Legume-Rhizobium symbiosis involves nitrogen-fixing bacteria such as Rhizobium forming nodules on legume roots. These bacteria convert atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$), which are used by the plant for amino acid synthesis. In return, the legume provides carbohydrates to the bacteria. This mutualistic relationship enhances soil fertility by increasing nitrogen availability.

Key Concept: Cyanobacteria

d) Cyanobacteria
[Solution Description]
Cyanobacteria, also known as blue-green algae, are often used in paddy fields because they fix atmospheric nitrogen and enhance soil fertility, making them a natural biofertilizer for rice cultivation.

Key Concept: Rhizobium

c) Rhizobium
[Solution Description]
Rhizobium forms a symbiotic relationship with leguminous plants, fixing atmospheric nitrogen into a usable form for the plant. This makes it an essential biofertilizer for crops like peas and beans.

Key Concept: Phosphate Solubilization

a) Bacillus
[Solution Description]
Phosphate-solubilizing bacteria like *Bacillus* and *Pseudomonas*, as well as fungi like *Aspergillus* and *Penicillium*, convert insoluble phosphates into soluble forms. From the options, *Bacillus* is the correct choice.

Key Concept: Nitrogen Fixation

a) Rhizobium
[Solution Description]
The bacteria that fix atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) include *Rhizobium*, *Azotobacter*, and *Azospirillum*. Among the given options, *Rhizobium* is one of the key nitrogen-fixing bacteria.

Key Concept: Azolla-Anabaena Symbiosis

b) It fixes atmospheric nitrogen
[Solution Description]
Anabaena azollae fixes atmospheric nitrogen and releases nitrogenous compounds in the leaf cavities of Azolla, making it an important biofertilizer in rice cultivation.

Key Concept: Legume-Rhizobium Symbiosis

d) Rhizobium
[Solution Description]
The correct answer is Rhizobium, which forms root nodules in legumes for nitrogen fixation. Other options do not primarily form such a symbiotic relationship with legumes.

Key Concept: Examples of Biofertilizers

c) Rhizobium
[Solution Description]
Rhizobium bacteria form a symbiotic association with leguminous plants by residing in their root nodules. This relationship enables the fixation of atmospheric nitrogen into a form usable by plants, enriching the soil naturally.

Key Concept: Advantages of Biofertilizers

c) They are cost-effective and sustainable for long-term agriculture
[Solution Description]
Biofertilizers enrich the soil with essential nutrients, improve soil structure, reduce dependency on chemical fertilizers, and enhance plant growth sustainably. Among the given options, their cost-effectiveness and sustainability for long-term agricultural practices make them advantageous over chemical fertilizers.

Key Concept: Vesicular-Arbuscular Mycorrhizae

c) Vesicles and arbuscules
[Solution Description]
Vesicular-arbuscular mycorrhizal (VAM) fungi form vesicles and arbuscules within the root cortex of plants. These structures enhance nutrient uptake, particularly phosphorus and copper.

Key Concept: Phosphate Solubilizing Microbes

c) Bacillus
[Solution Description]
Phosphate solubilizing bacteria and fungi, such as Bacillus, Pseudomonas, and Aspergillus awamori, convert insoluble inorganic phosphates into soluble forms usable by plants. Among the options, Bacillus is a well-known phosphate solubilizing bacterium.

Key Concept: VAM Fungi

c) Glomus
[Solution Description]
The question asks for the most abundant VAM fungus in soil. From the syllabus, it is explicitly mentioned that among VAM fungi, Glomus is the most abundant.

Key Concept: Ectomycorrhiza

b) Ectomycorrhiza
[Solution Description]
The question is about identifying the type of mycorrhiza where fungal hyphae form a sheath outside the root and also enter intercellular spaces. From the syllabus, this describes ectomycorrhiza, as endomycorrhiza involves fungal hyphae living within root cells.

Key Concept: Endomycorrhiza

b) Arbuscular mycorrhiza
[Solution Description]
Vesicular-arbuscular mycorrhizal (VAM) fungi, such as Glomus, penetrate roots and form vesicle and arbuscule structures inside the root cortex. These fungi are a subset of endomycorrhiza.

Key Concept: Ectomycorrhiza

a) Ectomycorrhiza
[Solution Description]
Ectomycorrhiza is characterized by fungal hyphae forming a dense sheath on the exterior of the plant root, while some hyphae also grow into intercellular spaces. This is in contrast to endomycorrhiza, where fungal hyphae live within the cells of the root.

Key Concept: Symbiotic Associations of Cyanobacteria

b) Anabaena azollae
[Solution Description]
Anabaena azollae is a cyanobacterium that forms a symbiotic relationship with the aquatic fern Azolla. This association is widely used as a biofertilizer in rice cultivation, significantly increasing crop yield.

Key Concept: Nitrogen Fixation by Cyanobacteria

b) Fixing atmospheric nitrogen into ammonia
[Solution Description]
Cyanobacteria play a crucial role in organic farming due to their ability to fix atmospheric nitrogen, which enriches the soil with essential nutrients for plant growth. This function reduces reliance on chemical fertilizers and promotes sustainable agriculture.

Key Concept: Diazotrophs

c) Herbaspirillum seropedicae
[Solution Description]
The syllabus mentions diazotrophs like Herbaspirillum seropedicae, H. rubrisubalbicans, Acetobacter diazotrophicus, and Bacillus azotofixans grow endophytically in sugarcane and rice plants. These bacteria fix atmospheric nitrogen and make it available to the host plants.

Key Concept: Loose Association of Azospirillum

b) Rice and sorghum
[Solution Description]
According to the syllabus, Azospirillum lipoferum shows loose association with roots and above-ground parts of cereal crops like rice, sorghum, and maize. These crops benefit from the nitrogen-fixing ability of Azospirillum, which enhances soil fertility.

Key Concept: Efficiency of Nitrogen Fixation

c) 30 kg
[Solution Description]
Efficient strains of Azotobacter can fix approximately 30 kg of nitrogen from 1,000 kg of organic matter, contributing significantly to soil nitrogen availability without synthetic fertilizers.

Key Concept: Nitrogen Fixation by Azotobacter

a) Fixing atmospheric nitrogen and enriching soil
[Solution Description]
Azotobacter is a free-living nitrogen-fixing bacterium that helps in converting atmospheric nitrogen into a usable form for plants, reducing the need for chemical fertilizers. It plays a key role in organic farming by enhancing soil fertility naturally.

Key Concept: Cyanobacteria in paddy fields

b) Anabaena
[Solution Description]
The correct answer is Anabaena, which is an autotrophic cyanobacterium that fixes atmospheric nitrogen and is commonly used in paddy fields as a biofertilizer. It also aids in reclamation of saline and alkaline soils.

Key Concept: Free-living nitrogen-fixing microbes

b) Azotobacter
[Solution Description]
The correct answer is Azotobacter, as it is a well-known free-living nitrogen-fixing bacterium that can fix up to 30 kg of nitrogen from 1,000 kg of organic matter. It helps in reducing the dependency on chemical fertilizers.

Key Concept: Leghaemoglobin

c) Presence of leghaemoglobin
[Solution Description]
Healthy legume nodules appear pink or reddish due to the presence of leghaemoglobin, a pigment that binds oxygen and creates an anaerobic environment required for nitrogenase activity, enabling nitrogen fixation.

Key Concept: Rhizobium symbiosis

b) Converts atmospheric nitrogen into ammonium ions
[Solution Description]
Rhizobium bacteria form symbiotic associations with legume roots, leading to nitrogen fixation. The bacteria convert atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$), which plants utilize for amino acid synthesis. This process benefits both the plant (by providing nitrogen) and the bacteria (by receiving carbohydrates).

Key Concept: Azolla-Anabaena Symbiosis

b) Anabaena azollae
[Solution Description]
Azolla, a floating freshwater fern, forms a symbiotic relationship with the blue-green alga Anabaena azollae. This alga lives in the leaf cavities of Azolla and fixes atmospheric nitrogen, providing nitrogenous compounds that benefit the fern. This symbiosis is particularly useful in rice cultivation as a natural biofertilizer.

Key Concept: Legume-Rhizobium Symbiosis

b) Fixing atmospheric nitrogen
[Solution Description]
The main benefit of Rhizobium bacteria to legume plants is nitrogen fixation. These bacteria live in root nodules of legumes and convert atmospheric nitrogen into ammonia, which the plant can use for growth. In return, the plant provides carbohydrates to the bacteria. This mutualistic relationship helps improve soil fertility.

Key Concept: Cyanobacteria

c) *Anabaena*
[Solution Description]
Cyanobacteria such as *Anabaena*, *Nostoc*, and *Aulosira* are widely used as biofertilizers in paddy fields. They fix atmospheric nitrogen, enhance soil fertility, and improve crop yield by producing organic matter and growth-promoting substances.

Key Concept: Legume-Rhizobium Symbiosis

a) Protects nitrogenase from oxygen
[Solution Description]
Leghaemoglobin is an oxygen-binding pigment found in root nodules. Its primary function is to protect the nitrogenase enzyme from oxygen inactivation, as nitrogenase is sensitive to oxygen. It binds with free oxygen and creates a low-oxygen environment, allowing nitrogen fixation ($N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i$) to occur efficiently.

Key Concept: Azolla-Anabaena symbiosis

c) Fixes atmospheric nitrogen
[Solution Description]
Anabaena azollae is a blue-green alga that lives in the leaf cavities of Azolla, a floating fern. It fixes atmospheric nitrogen and releases nitrogenous compounds, which benefit rice cultivation by enriching the soil with nutrients.

Key Concept: Legume-Rhizobium symbiosis

c) Rhizobium
[Solution Description]
Rhizobium bacteria form a symbiotic relationship with the roots of legumes, leading to the formation of root nodules. These bacteria help in fixing atmospheric nitrogen into ammonium ions ($NH_4^+$), which plants use for amino acid synthesis.

Key Concept: Azolla-Anabaena Symbiosis

c) It fixes atmospheric nitrogen for the fern
[Solution Description]
Anabaena azollae is a blue-green alga that lives in the leaf cavities of the fern Azolla. It fixes atmospheric nitrogen and releases nitrogenous compounds, enriching the soil as a biofertilizer, especially beneficial for rice cultivation.

Key Concept: Legume-Rhizobium Symbiosis

c) To fix atmospheric nitrogen for the plant
[Solution Description]
Rhizobium bacteria form a symbiotic relationship with leguminous plants by colonizing their root nodules. They fix atmospheric nitrogen into ammonium ions ($NH_4^+$), which are used by the plant for amino acid synthesis and growth, while the plant provides carbohydrates to the bacteria.

Key Concept: Biocontrol Agents

b) Bacillus thuringiensis
[Solution Description]
Bacillus thuringiensis (Bt) is a well-known biocontrol agent that produces toxins lethal to specific insect pests. It is widely used in organic farming to reduce reliance on chemical pesticides. Bt targets pests like caterpillars and mosquito larvae without harming non-target organisms.

Key Concept: Biofertilizers

a) Rhizobium
[Solution Description]
Rhizobium is a nitrogen-fixing bacterium that forms root nodules in legume plants. It engages in a mutualistic relationship where the bacteria supply ammonium ions ($NH_4^+$) to the plant, and in return, the plant provides carbohydrates. This symbiosis is crucial for nitrogen fixation in organic farming.

Key Concept: Frankia, Reafforestation, Soil Fertility

b) Planting Alnus rubra in Pennsylvania to restore nitrogen-depleted mine spoils.
[Solution Description]
Frankia is extensively used in reafforestation projects to restore nitrogen-deficient soils, such as mine spoils or sandy tracts. Examples include planting Alnus rubra in Pennsylvania (U.S.A.) for mine spoil restoration and Casuarina sp. in Senegal and coastal India to stabilize sandy soils.
Step-by-step explanation:
1. Frankia symbiotically fixes nitrogen in non-leguminous trees like Alnus and Casuarina.
2. These trees are planted in degraded areas (e.g., mine spoils, eroded lands) to restore nitrogen levels.
3. Case studies include Pennsylvania (Alnus rubra) and Senegal (Casuarina sp.).
4. The fixed nitrogen benefits subsequent crops by improving soil fertility.

Key Concept: Frankia, Nitrogen Fixation, Symbiotic Relationships

b) Frankia-infected plants release 60-157 kg N/ha/year, aiding soil nitrogen replenishment.
[Solution Description]
Frankia is a nitrogen-fixing actinomycete that forms symbiotic nodules with non-leguminous plants like Alnus and Casuarina. These nodules help in atmospheric nitrogen fixation, enriching soil fertility. Frankia-infected plants can release 60-157 kg N/ha/year. The nodule formation process involves root hair penetration and proliferation of cortical cells, similar to Rhizobium-legume symbiosis but without leghaemoglobin.
Step-by-step explanation:
1. Frankia is an actinomycete, not a bacterium or fungus.
2. It forms nodules on non-leguminous plants like Alnus (alder) and Casuarina.
3. The nitrogen fixation rate ranges between 60-157 kg N/ha/year.
4. Unlike Rhizobium, Frankia does not require leghaemoglobin for oxygen regulation.
5. The nodules are used in reafforestation to restore nitrogen-depleted soils.

Key Concept: Role of carbon source, efficiency of phosphate solubilization

b) Glucose is metabolized faster, enhancing organic acid production
[Solution Description]
The efficiency of phosphate solubilization by bacteria depends on the type of carbon source available, as it influences organic acid production. Glucose is directly metabolized via glycolysis, producing organic acids like gluconic acid more efficiently than sucrose, which requires additional enzymatic breakdown. Thus, glucose promotes higher phosphate solubilization.

Key Concept: Phosphate solubilizing bacteria, mechanisms of phosphorus solubilization

c) Nitrogen fixation in root nodules
[Solution Description]
Phosphate solubilizing bacteria (PSB) convert insoluble phosphates into soluble forms primarily through the secretion of organic acids (e.g., gluconic acid, citric acid), chelation of cations, and enzymatic activity. However, nitrogen fixation is not a mechanism associated with PSB; it is a process carried out by nitrogen-fixing bacteria like Rhizobium or Azotobacter.

Key Concept: Mycorrhiza Types, Nutrient Absorption, Symbiotic Relationships

c) Enhanced absorption of nitrogen and phosphorus by the fungal hyphae
[Solution Description]
Ectomycorrhizal fungi form a dense sheath around the roots of host plants (e.g., pine, oak) and enhance nutrient absorption by converting complex organic molecules into simpler forms. They absorb essential nutrients like nitrogen, phosphorus, potassium, and calcium, while also protecting roots from pathogens and producing growth-promoting substances like cytokinins. This makes them indispensable for afforestation in nutrient-poor soils like mined wastelands.

Key Concept: Ectomycorrhiza, Endomycorrhiza, VAM Fungi

c) Vesicular-arbuscular mycorrhiza (VAM)
[Solution Description]
Vesicular-arbuscular mycorrhizal (VAM) fungi, such as those from the genera Glomus, Acaulospora, and Gigaspora, penetrate roots to form specialized structures called vesicles and arbuscules within the cortex. These fungi enhance nutrient absorption (especially phosphorus and copper) and provide drought resistance. Ectomycorrhiza forms an external sheath, while endomycorrhiza involves internal colonization but may not always form vesicular-arbuscular structures.

Key Concept: Azolla-Anabaena symbiosis, Applications of Biofertilizers

b) Significant increase in rice yield due to nitrogen fixation
[Solution Description]
The Azolla-Anabaena symbiotic system involves Anabaena azollae fixing atmospheric nitrogen inside Azolla's leaf cavities, releasing nitrogenous compounds. This enhances soil fertility, leading to higher rice yields (over 50% increase). Additionally, the system reduces dependency on chemical fertilizers, improves soil organic matter, and sustains long-term agricultural productivity without environmental harm.

Key Concept: Cyanobacteria, Nitrogen fixation, Advantages of Biofertilizers

c) Improved protein synthesis from higher nitrogen availability
[Solution Description]
Cyanobacteria fix atmospheric nitrogen ($N_2$) into ammonium ions ($NH_4^+$), which are essential for amino acid synthesis in plants. A 20% increase in $NH_4^+$ concentration means more nitrogen is available for rice plants. This enhanced nitrogen availability leads to increased protein synthesis, promoting plant growth and higher crop yield. Additionally, cyanobacteria also produce growth-promoting substances and help reclaim saline soils, further benefiting rice cultivation.

Key Concept: Azotobacter, Free-living Diazotrophs

c) 15 kg
[Solution Description]
To find the nitrogen fixed by Azotobacter in 500 kg of organic matter:
Step 1: Given that 30 kg nitrogen is fixed from 1,000 kg of organic matter.
Step 2: For 500 kg of organic matter, the nitrogen fixed is proportional:
$\text{Nitrogen fixed} = \left(\frac{30}{1000}\right) \times 500 = 15 \, \text{kg}$
Therefore, the correct answer is 15 kg.

Key Concept: Diazotrophs, Nitrogen Fixation Efficiency

c) 20%
[Solution Description]
To solve this question, we need to calculate the percentage of nitrogen fertilizer saved by using Azospirillum lipoferum.
Step 1: The nitrogen fixed by Azospirillum lipoferum = 20 kg/ha.
Step 2: Recommended chemical nitrogen fertilizer for maize = 100 kg/ha.
Step 3: Percentage of nitrogen saved = (Nitrogen fixed by biofertilizer / Total recommended nitrogen) × 100
$\text{Percentage saved} = \left(\frac{20}{100}\right) \times 100 = 20%$
Therefore, the correct answer is 20%.

Key Concept: Types of diazotrophs, Associative nitrogen fixation, Application in crops

b) Azospirillum with farmyard manure in millet fields
[Solution Description]
The syllabus mentions that Azospirillum shows loose association with cereal crops like rice and sorghum, and when combined with farmyard manure, it can save 15-25 kg equivalent of nitrogen per hectare. It also highlights that a combination of chemical fertilizer with Azospirillum yields significantly higher results.
Thus, the correct scenario involves Azospirillum applied with farmyard manure or chemical fertilizer for cereals like rice/sorghum, leading to nitrogen savings.

Key Concept: Loose association of nitrogen-fixing bacteria, Diazotrophs, Efficiency of nitrogen fixation

d) 60 kg
[Solution Description]
The syllabus states that the most efficient strains of Azotobacter can fix 30 kg of nitrogen from 1000 kg of organic matter. For 2000 kg of organic matter, the calculation is as follows:
Step 1: Nitrogen fixed per 1000 kg = 30 kg
Step 2: For 2000 kg, nitrogen fixed = 30 kg $\times$ 2 = 60 kg
Therefore, the expected nitrogen fixation is 60 kg.

Key Concept: Symbiotic relationship, Nitrogen assimilation

c) The fern absorbs ammonia for growth; it suffers nitrogen deficiency without the alga's input.
[Solution Description]
The fern benefits by absorbing the nitrogenous compounds (such as ammonia, $NH_3$) released by Anabaena in its leaf cavities, which enhances its growth. If the alga loses its nitrogen-fixing ability, the fern would no longer receive these compounds, leading to stunted growth due to nitrogen deficiency, as Azolla depends on the alga for this nutrient in their symbiotic relationship.

Key Concept: Nitrogen fixation efficiency, Azolla-Anabaena symbiosis

c) 110 kg
[Solution Description]
First, calculate the total nitrogen fixed by the Azolla-Anabaena system: 50 kg per hectare. Urea contains 46% nitrogen, so to find the equivalent urea needed to provide 50 kg of nitrogen, we use the formula:
$\text{Mass of urea} = \frac{\text{Required nitrogen}}{\%\text{N in urea}} \times 100 = \frac{50\,\text{kg}}{46} \times 100 \approx 108.7\,\text{kg}$
Therefore, approximately 108.7 kg of urea is replaced per hectare.

Key Concept: Genetic enhancement of nitrogen fixation, nif genes, Rhizobial inoculation

c) Introducing additional copies of nif and nod genes into Rhizobium
[Solution Description]
The nitrogen-fixing ability of legumes depends on genes like nif (nitrogenase), nod (nodulation), fix (electron transport), and ruf (regulatory). To enhance nitrogen fixation:
1. Transferring nif genes directly increases nitrogenase activity, which catalyzes $N_2$ reduction.
2. Combining nif with fix or nod genes improves electron supply and nodule formation, respectively.
3. This approach enhances nitrogen fixation efficiency, potentially increasing nitrogen fixation levels to approximately 50–150 kg ha⁻¹, compared to simple inoculation alone.
The correct answer involves targeting multiple symbiotic genes.

Key Concept: Legume-Rhizobium symbiosis, Nitrogen fixation, Leghaemoglobin

c) Reduced nitrogen fixation efficiency due to nitrogenase inactivation
[Solution Description]
Leghaemoglobin plays a critical role in protecting nitrogenase enzyme from oxygen inactivation by binding free oxygen and maintaining a low oxygen concentration inside the nodule. If leghaemoglobin is inhibited:
1. Oxygen levels in the nodule would rise, leading to nitrogenase inactivation.
2. The nitrogen fixation process ($N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16Pi$) would be disrupted.
3. Consequently, ammonium ion production would decrease, affecting amino acid synthesis and plant growth.
Thus, the correct answer relates to reduced nitrogen fixation efficiency.

Key Concept: Azolla-Anabaena symbiosis, Biofertilizers in rice cultivation

c) Azolla provides nitrogen through its symbiotic cyanobacteria
[Solution Description]
Azolla forms a symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena azollae, which resides in its leaf cavities. The alga fixes atmospheric nitrogen and releases nitrogenous compounds, enriching the soil.
Step-by-step explanation:
1. Azolla provides a habitat for Anabaena in its leaf cavities.
2. Anabaena performs nitrogen fixation, converting $N_2$ into ammonium ($NH_4^+$).
3. The fixed nitrogen is released into the water or soil, becoming available to rice plants.
4. Increased nitrogen availability enhances rice growth and yield.
Thus, the primary reason for increased yield is the nitrogen contribution from the Azolla-Anabaena symbiosis.

Key Concept: Legume-Rhizobium symbiosis, Nitrogen fixation

c) The soil lacks sufficient Rhizobium populations
[Solution Description]
The pink or reddish color of healthy legume nodules is due to the presence of leghaemoglobin, which is essential for nitrogen fixation by Rhizobium bacteria. If nodules are absent or not pink, it indicates a lack of successful Rhizobium-legume symbiosis. This could occur if the soil lacks sufficient Rhizobium populations, or if environmental conditions (e.g., pH, temperature) are unfavorable for bacterial activity.
Step-by-step reasoning:
1. Leghaemoglobin is produced when Rhizobium bacteria successfully infect root hairs and form nodules.
2. Without Rhizobium, no nodules form, and nitrogen fixation cannot occur.
3. Even if nodules form but are white or green, it indicates inactive Rhizobium, possibly due to poor soil conditions.
The correct answer is that the soil lacks sufficient Rhizobium populations or has unsuitable conditions for nodulation.