Question

Which one is correct about the free-living Rhizobium sp. and its enzyme?
A) Bacteria are aerobic and nitrogenase is active.
B) Bacteria are anaerobic and nitrogenase is active (operational).
C) Bacteria aerobic and nitrogenase is inactive.
D) Bacteria are anaerobic and nitrogenase is inactive.

Answer 1

Hint: Rhizobium sp. are nitrogen-fixing bacteria and also are mostly in a symbiotic relationship with leguminous plants. Rhizobia is unable to fix nitrogen in absence of a host and thus, nitrogenase remains inactive.

Complete answer: The Rhizobium is a bacteria which needs a symbiotic relationship to fix nitrogen. It is an aerobe, rod-shaped cell and gram-negative bacteria. It fixes nitrogen using nitrogenase enzymes. The nitrogenase easily gets oxidised in the atmosphere or during its aerobic conditions of surviving. They are prokaryotes and convert the nitrogen into ammonia. The nitrogen from the atmosphere is fixed into ammonia and found in the soil. The Rhizobium is free-living but does not fix nitrogen unless they have a symbiotic leguminous plant found. They are free-living until they sense a specific flavonoid which is secreted by the roots of the leguminous plants when there is any cut or nick on the roots of the plants. This provides favourable penetration for the Rhizobium to enter the roots. The leghemoglobin in the plants help in preventing the oxidation of nitrogen and thus nitrogen is active in symbiotic association. In the absence of symbiosis and free-living conditions, Rhizobium does not fix nitrogen into ammonia.
Hence, the correct is option C.

Note: Nitrogen fixation involves converting the inert form of nitrogen into various usable nitrogen compounds by plants. It occurs in the following sequence, i.e. nitrogen fixation, nitrification, assimilation, ammonification and denitrification. Nitrogen fixation can occur both biologically and artificially.