Read the given passage and answer the following question:
Microbes like Saccharomyces cerevisiae are utilised in the commercial production of ethanol. Enzyme production is another application of microbes, with lipases being used in detergent formulations to effectively remove oily stains from laundry. It is common to observe that commercially bottled fruit juices appear clearer compared to homemade ones, as the former undergo clarification processes employing pectinases and proteases. Streptokinase, produced by the bacterium Streptococcus and genetically modified, serves as a "clot buster" in the removal of blood clots from the vessels of individuals who have experienced a heart attack. Cyclosporin A, another bioactive molecule, is derived from the fungus Trichoderma polysporum and is employed as an immunosuppressive agent in organ transplant patients. Statins, on the other hand, are produced by the yeast Monascus purpureus and have been commercialised as agents to lower blood cholesterol levels. They function by competitively inhibiting the enzyme responsible for cholesterol synthesis. Biofertilisers are organisms that enhance the nutrient quality of the soil and can be derived from bacteria, fungi, and cyanobacteria. Rhizobium bacteria form nodules on the roots of leguminous plants and convert atmospheric nitrogen into organic forms, which serve as nutrients for the plants. Additionally, certain free-living bacteria like Azospirillum and Azotobacter can fix atmospheric nitrogen in the soil, thereby enriching its nitrogen content. Fungi also engage in symbiotic associations with plants, known as mycorrhiza. Many members of the Glomus genus form mycorrhiza, wherein the fungal symbiont absorbs phosphorus from the soil and transfers it to the plant. These associations offer various benefits to the plants, including enhanced resistance to root-borne pathogens, increased tolerance to salinity and drought, and an overall improvement in plant growth and development.
Which of the following is an autotrophic, nitrogen-fixing microbe that serves as an important biofertiliser in paddy fields?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Anabaena
Biofertilisers are organisms that enrich the soil. Some fix atmospheric nitrogen. The question asks for one that is autotrophic (makes its own food by photosynthesis) and is used in paddy fields. These are the cyanobacteria (blue green algae), such as Anabaena, Nostoc and Oscillatoria. They fix nitrogen and also add organic matter to the soil.
Option (1) Azotobacter — Incorrect. Azotobacter is a bacterium that fixes nitrogen while free-living in the soil. It is a biofertiliser, but it is not a cyanobacterium, and the book does not name it as the biofertiliser of paddy fields.
Option (2) Anabaena — Correct. Anabaena is a cyanobacterium: autotrophic and nitrogen-fixing. Cyanobacteria are an important biofertiliser in paddy fields.
Option (3) Rhizobium — Incorrect. Rhizobium fixes nitrogen in a symbiotic association inside the root nodules of leguminous plants. It is not autotrophic and not the paddy-field biofertiliser.
Option (4) Glomus — Incorrect. Glomus is a fungus that forms mycorrhiza with plant roots and absorbs phosphorus for the plant. It does not fix nitrogen and cannot photosynthesise.
NCERT Extract:
Microbes as Biofertilisers – “… Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.”