Q.Explain the role of cyanobacteria to enrich the fertility of soil.
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Imagine you have a garden. You want your plants to grow strong and healthy, and you also want to keep pests away. There are two main ways to do this: one is with harsh chemicals (chemical fertilizers and pesticides), and the other is with living things or natural substances. Biofertilizers and biopesticides are the second way — they are nature’s own tools for farming.
Let’s start with biofertilizers. Think of them as "living manure." A chemical fertilizer is like giving a plant a direct shot of nutrients — it works fast but can burn the soil over time. A biofertilizer, on the other hand, is a preparation containing live microorganisms (like bacteria, fungi, or algae) that help the plant get nutrients from the soil or air. For example, certain bacteria can take nitrogen from the air and convert it into a form the plant can use. The plant doesn’t get fed directly; instead, the biofertilizer helps the soil become richer and more fertile naturally.
The NCERT textbook (Class 12 Biology, Chapter 10) defines biofertilizers as organisms that enrich the nutrient quality of the soil. The main examples are Rhizobium (a bacterium that lives in root nodules of legumes and fixes nitrogen), Azospirillum and Azotobacter (free-living nitrogen-fixing bacteria), and blue-green algae (like Anabaena) which also fix nitrogen.
Now, biopesticides. These are living organisms or natural substances that control pests — insects, fungi, weeds, etc. — without using synthetic chemicals. Instead of spraying a poison that kills everything (good and bad bugs alike), a biopesticide might use a specific bacterium that only harms a particular caterpillar, or a fungus that attacks a weed. The most famous example is Bacillus thuringiensis (often called Bt), a bacterium that produces a protein toxic to certain insect larvae but harmless to humans, animals, and most other insects.
The key difference between chemical and biological agents: Biofertilizers and biopesticides are renewable, eco-friendly, and do not leave toxic residues in the soil or water. They are a cornerstone of sustainable agriculture — farming that can continue for generations without destroying the land.
Why does this matter for a commerce or humanities student? Because agriculture is not just about biology — it’s about economics, policy, and human health. Chemical fertilizers and pesticides are expensive to produce, can pollute groundwater, and their overuse leads to "superpests" that become resistant. Biofertilizers and biopesticides are often cheaper in the long run, safer for farm workers, and help maintain soil health. Governments around the world (including India) promote them through subsidies and organic farming schemes. Understanding them helps you see why "organic" food costs more, why some farmers switch to natural methods, and how environmental regulations work.
Here’s a quick summary of the main types you should know (from NCERT):
- Biofertilizers:
- Rhizobium (symbiotic with legumes) …
Certain photosynthetic bacteria found in paddy fields and other soils can capture atmospheric nitrogen directly, which is otherwise unavailable to plants. …
Free-living and symbiotic cyanobacteria fix atmospheric nitrogen and add organic matter/oxygen, functioning as natural biofertilizers that improve soil fertility.
Cyanobacteria (blue-green algae) such as Nostoc, Anabaena, Oscillatoria, and Spirulina are photosynthetic, nitrogen-fixing prokaryotes found abundantly in aquatic and terrestrial (especially paddy-field) environments.
Their role in enriching soil fertility includes:
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Biological nitrogen fixation: Many cyanobacteria possess specialised cells called heterocysts, which contain the enzyme nitrogenase and can fix atmospheric nitrogen (N₂) into forms such as ammonia that plants can use. This adds nitrogenous compounds to the soil, which is especially valuable in paddy fields where cyanobacteria are used as biofertilizers.
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Addition of organic matter: As cyanobacteria grow, multiply, and eventually die, their biomass decomposes and adds organic matter to the soil, improving its structure and nutrient content.
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Showing the 12 most recent of 41 on this concept.
- CBSE 2026Set A1 markMCQQ.Which of the following cyanobacteria fixes atmospheric nitrogen?(a) Nostoc(b) Anabaena(c) Oscillatoria(d) All of these
›Reveal solutionSolution
All the listed cyanobacteria are nitrogen fixers; the correct option is (d).
Cyanobacteria (blue-green algae) are autotrophic aquatic and terrestrial microbes that can fix atmospheric nitrogen and are used as biofertilizers, especially in paddy fields. Nostoc, Anabaena and Oscillatoria are all exa …
- CBSE 2026Set A1 markMCQQ.With the help of what are butterfly caterpillars controlled?(a) Pseudomonas(b) Trichoderma(c) Bacillus thuringiensis(d) Baculovirus
›Reveal solutionSolution
Bacillus thuringiensis is used to control butterfly caterpillars; the correct option is (c).
Bacillus thuringiensis (Bt) is a bacterium used as a biocontrol agent against insect pests. Dried Bt spores are mixed with water and sprayed onto crops. When eaten by the larvae (caterpillars) of butterflies and moths, the toxin (crystal protein) is released in their gut and kills them, while being harmless to other …
- CBSE 2026Set ANNUAL1 markMCQQ.Microbes (micro-organism) found in the form of biofertilizer is(a) Agrobacterium(b) Bacillus thuringiensis(c) Baculoviruses(d) Rhizobium
›Reveal solutionSolution
Biofertilisers are living organisms that enrich soil nutrient quality; Rhizobium (a symbiotic N2-fixer in legume root nodules) is a classic bacterial biofertiliser.
Rhizobium lives symbiotically in the root nodules of leguminous plants and fixes atmospheric nitrogen into a usable form, enriching soil fertility - hence it is used as a biofertiliser. Agrobacterium is used as …
- CBSE 2026Set ANNUAL1 markQ.Which fungus is used as biological control in the treatment of plant diseases?
›Reveal solutionSolution
Trichoderma species are free-living fungi widely used as biological control agents to protect plant roots from fungal pathogens.
Trichoderma is a genus of free-living fungi commonly found in root ecosystems, and several species act as effective biocontrol agents against a number of plant pathogens, reducing the need for chemical fungicides. This is one of th …
- CBSE 2026Set ANNUAL1 markMCQQ.Cyanobacteria are useful biofertilizers in the field of(a) Wheat(b) Paddy(c) Maize(d) Sugarcane
›Reveal solutionSolution
Cyanobacteria (blue-green algae) are nitrogen-fixing biofertilizers especially valuable in flooded paddy fields.
Cyanobacteria such as Nostoc, Anabaena, and Oscillatoria are photosynthetic, nitrogen-fixing prokaryotes that enrich soil with organic matter and fix atmospheric nitrogen into forms plants can use, increasing soil fertility.
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- CBSE 2026Set ANNUAL1 markMCQQ.Which bacterial species acts as an bio-insecticide ?(a) LAB(b) Bacillus thuringiensis(c) Acetobacter aceti(d) Streptococcus
›Reveal solutionSolution
Bacillus thuringiensis (Bt) produces crystal (Cry) proteins toxic to insect larvae and is widely used as a biological insecticide.
Bacillus thuringiensis forms proteinaceous crystals during a particular phase of its growth that contain a toxic insecticidal protein. When ingested by an insect larva, the toxin is activated in the alkaline gut environment, damaging the gut lining and killing the insect — this is used commercially as Bt-based bio-insecticide (dried spore-crystal formulations) and is also the …
- CBSE 2026Set ANNUAL1 markMCQQ.Assertion (A): Rhizobium leguminosorum is a symbiotic bacterium found in the root nodules of leguminous plants. Reason (R): This bacterium has the ability to fix atmospheric sulphur into sulphur dioxide.(a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.(b) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.(c) Assertion is true but Reason is false.(d) Assertion is false but Reason is true.
›Reveal solutionSolution
Rhizobium leguminosorum is indeed a symbiotic bacterium in leguminous root nodules (Assertion true), but it fixes atmospheric NITROGEN into ammonia — not sulphur into sulphur dioxide (Reason false).
Assertion: Rhizobium species live symbiotically in the root nodules of leguminous plants (like pea, gram, beans) — true; this is a classic example of a biofertilizer relationship.
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- CBSE 2026Set ANNUAL1 markMCQQ.Which one of the following is commonly found in root ecosystem and control several plant pathogens?(a) Rhizobium(b) Bacillus thuringiensis(c) Trichoderma(d) Azospirillum
›Reveal solutionSolution
Trichoderma is a soil fungus of the root ecosystem used as a biocontrol agent against many plant pathogens.
Biocontrol agents are living organisms used to control plant diseases and pests instead of chemicals. Trichoderma species are free-living fungi very common in the root ecosystems (rhizosphere) and are effective biocontrol agents against several soil-borne plant pathogens.
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- CBSE 2025Set X11 markQ.Members of genus ____________ form mycorrhiza and absorb phosphorous from soil and supply it to the plant.
›Reveal solutionSolution
Members of the genus Glomus form mycorrhiza and supply phosphorus to plants.
Many members of the fungal genus Glomus form a symbiotic association (mycorrhiza) with the roots of higher plants. The fungal partner absorbs phosphorus from the soil and passes it to the plant; the association al …
- CBSE 2025Set ANNUAL1 markMCQQ.The Bacteria found in the root nodules of Leguminous plants is-(a) Rhizobium(b) Azotobacter(c) Staphylococus(d) Lactobacillus
›Reveal solutionSolution
Rhizobium is the nitrogen-fixing bacterium that lives symbiotically in the root nodules of leguminous plants.
Rhizobium is a gram-negative, symbiotic, nitrogen-fixing bacterium that infects the roots of leguminous plants (e.g. pea, gram, soybean) and induces the formation of root nodules. Inside the nodules it converts atmospheric nitrogen (N2) into ammonia using the nitrogenase enzyme, making fixed nitrogen available to the plant, while the plant in turn supplies the bacterium with carbohydrates. Azotobacter is a free …
- CBSE 2025Set F1 markMCQQ.Which of the following fixes nitrogen?(a) Anabaena(b) Ulva(c) Ulothrix(d) Spirogyra
›Reveal solutionSolution
Anabaena fixes atmospheric nitrogen.
Anabaena is a filamentous cyanobacterium (blue-green alga) that fixes atmospheric nitrogen (N2) into usable forms in specialised cells called heterocysts. It acts as a biofertiliser (e.g. Anabaena in the fern Azolla enriches rice fi …
- CBSE 2025Set F1 markMCQQ.Which of the following is an excellent biofertilizer?(a) Azolla(b) Marsilea(c) Pteridium(d) Salvinia
›Reveal solutionSolution
Azolla, which carries the N₂-fixing cyanobacterium Anabaena, is an excellent biofertilizer.
Biofertilizers are living organisms that enrich soil nutrients. Azolla is a free-floating aquatic fern that lives in symbiosis with the nitrogen-fixing cyanobacterium Anabaena azollae in its leaf cavities. Grown in rice paddies, this Azolla–Anabaena association fixes atmospheric nitrogen and adds it to …
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