Which of the following is associated with biological nitrogen fixation ?

Question

UnlockIAS · Accepted Answer

Correct — D, Blue-green algae. Biological nitrogen fixation — the conversion of inert atmospheric nitrogen (N2) into ammonia that plants can use — is carried out only by prokaryotes, because only they possess the enzyme nitrogenase. Blue-green algae are not true algae at all: they are cyanobacteria, prokaryotes, and they are the one group in this list that qualifies. Heterocystous forms such as Anabaena, Nostoc, Aulosira and Tolypothrix fix nitrogen in thick-walled specialised cells called heterocysts, which keep out oxygen because nitrogenase is destroyed by it. They do this free-living in the standing water of paddy fields, and also as symbionts — Anabaena azollae lives inside the leaf cavities of the water fern Azolla, which is why Azolla is spread on flooded rice fields as a biofertiliser. This is exactly why blue-green algae are promoted as biofertilisers for rice, and why the answer is (d).

Why the other options are wrong — (a) Red algae (Rhodophyceae) are eukaryotes and have no nitrogenase, so they cannot fix atmospheric nitrogen. They are economically important for a different reason — they are the source of agar and carrageenan.  (b) Brown algae (Phaeophyceae), the group that includes the large marine kelps and Sargassum, are also eukaryotic and do not fix nitrogen; their commercial value lies in algin and their use as manure and fodder.  (c) Green algae (Chlorophyceae) such as Spirogyra, Chlorella and Ulothrix are eukaryotic photosynthesisers with no nitrogen-fixing ability; UPSC has separately tested that Spirogyra is not used as a biofertiliser while the cyanobacterium Nostoc is.

Concept: Nitrogen makes up about four-fifths of the atmosphere but is unusable by plants in its gaseous form, because the triple bond in N2 is extremely stable. Breaking it requires either industrial conditions of high temperature and pressure (the Haber-Bosch process), a lightning discharge, or the enzyme nitrogenase — and nitrogenase exists only in prokaryotes. That single fact organises the whole topic: every nitrogen fixer is a bacterium or a cyanobacterium, whether free-living (Azotobacter in aerobic soils, Clostridium in anaerobic ones, cyanobacteria in water), or symbiotic (Rhizobium in the root nodules of legumes, Frankia in Casuarina and Alnus, Anabaena azollae inside Azolla). No plant, fungus or eukaryotic alga fixes nitrogen on its own.

State PCS 2019 Q100 — Environment & Ecology PYQ with Answer

State PCS 2019 Q100 — Environment & Ecology PYQ with Answer