Q.Some of the microbes used as biofertilizers are prokaryotes. Name the taxonomic group they come under. With the help of an example, mention how they act as biofertilizers.
The prokaryotic microbes used as biofertilizers belong to the taxonomic group Bacteria, and they enrich soil fertility by fixing atmospheric nitrogen, often in symbiotic association with plants.
When we talk about biofertilizers, we are referring to living microorganisms that, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. Among these, several are prokaryotes — organisms without a membrane-bound nucleus. The taxonomic group they come under is Bacteria.
The NCERT textbook specifically highlights cyanobacteria (often called blue-green algae) and certain bacteria like Rhizobium, Azospirillum, and Azotobacter as examples of prokaryotic biofertilizers. Cyanobacteria, though photosynthetic, are prokaryotic bacteria, not true algae. So the group is unequivocally Bacteria (domain Eubacteria, if you are thinking of the three-domain system, but for NCERT purposes, the kingdom is Monera or simply Bacteria).
Cyanobacteria are sometimes mistaken for algae because they are photosynthetic and aquatic, but they lack a true nucleus and membrane-bound organelles — they are bacteria.
Now, how do these bacteria act as biofertilizers? The most important mechanism is biological nitrogen fixation. Nitrogen is an essential nutrient for plants, but atmospheric nitrogen (N₂) is inert and cannot be used directly. Certain bacteria possess the enzyme nitrogenase, which converts atmospheric nitrogen into ammonia (NH₃), a form plants can assimilate.
Let us take the classic example: Rhizobium. This bacterium lives in a symbiotic relationship with leguminous plants (like peas, beans, and groundnuts). Rhizobium infects the root hairs of the legume and induces the formation of root nodules. Inside these nodules, the bacteria are protected from oxygen (which would otherwise inhibit nitrogenase) and are supplied with carbohydrates from the plant. In return, Rhizobium fixes atmospheric nitrogen into ammonia, which the plant uses to make proteins and nucleic acids. When the plant dies or the nodules decay, this fixed nitrogen is released into the soil, enriching it for subsequent crops.
The key point is that Rhizobium is symbiotic — it requires a host plant to fix nitrogen. In contrast, Azotobacter is a free-living nitrogen-fixing bacterium that works independently in the soil.
Another prokaryotic example is cyanobacteria like Anabaena or Nostoc. These are often used in paddy fields. They can fix atmospheric nitrogen on their own (free-living) and also add organic matter to the soil through photosynthesis. Some cyanobacteria even form symbiotic associations — for instance, Anabaena lives inside the water fern Azolla, and this combination is a traditional biofertilizer for rice.
In short, the prokaryotic microbes used as biofertilizers belong to the taxonomic group Bacteria, and they act as biofertilizers primarily by fixing atmospheric nitrogen into a usable form, as seen in the symbiotic Rhizobium that forms root nodules in legumes.
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