UPSC Prelims 2010 · General Studies

Question 58: Science and Technology

Which feature of some species of blue-green algae helps promote them as bio-fertilizers ?

aThey convert atmospheric methane into ammonia which the crop plants can absorb readily

bThey induce the crop plants to produce the enzymes which help convert atmospheric nitrogen to nitrates

cThey have the mechanism to convert atmospheric nitrogen into a form that the crop plants can absorb readily

dThey induce the roots of the crop plants to absorb the soil nitrates in larger quantities

AnswerThe answer is (c).

Some blue-green algae (cyanobacteria), such as Anabaena, Nostoc and Aulosira, fix atmospheric nitrogen into ammonia in special cells called heterocysts. Grown in paddy fields, they release this fixed nitrogen in a form the rice plants can absorb, which is why they are used as biofertilisers. They do not convert methane into ammonia, they do not make the crop produce enzymes, and they do not make roots absorb more nitrate. Remember: cyanobacteria as biofertiliser = nitrogen fixation.

Why each option is right or wrong

(a) They fix nitrogen gas; they do not convert methane.
(b) They fix nitrogen themselves; they do not induce the crop to produce nitrogen-fixing enzymes.
(c) Right: they convert atmospheric nitrogen into a form the crop can absorb.
(d) They do not act on roots to raise nitrate uptake; they supply fixed nitrogen.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2010, General Studies, Series A. The paper on upsc.gov.in.