Question

What is required for nitrogen fixation by Rhizobium?
A) $CO_2$
B) $O_2$
C) Heterocyst
D) Potassium

Answer 1

Hint: Nitrogen fixation is a natural or industrial process that causes the free nitrogen ($N_2$) that is an inert gas that is in high amount in the air that combines chemically with some other elements to form nitrogen compounds that are more-reactive, e.g., ammonia, nitrates, or nitrites.

Complete Answer:
- Nitrogen-fixing requires anaerobic conditions. Oxygen irreversibly inactivates the nitrogenase enzymes involved in nitrogen fixation and hence, nitrogen must be fixed under anaerobic conditions.
- Rhizobium functions under anaerobic conditions or can create an internal anaerobic environment in the presence of oxygen. Nodules contain an oxygen-binding heme protein called leghemoglobin.
- Yet nitrogenous compounds are found in all fertile soils, in all living things, in many foodstuffs, in coal, and in such naturally occurring chemicals like sodium nitrate (saltpeter) and ammonia. Now consider the options provided in the question,

A) $CO_2$: Carbon dioxide is an important metabolite in many bacterial species,In the genus Rhizobium, $CO_2$ is required to promote vegetative growth
B) $O_2$: Rhizobia are α- and β-proteobacteria that form a symbiotic partnership with legumes, fixing atmospheric nitrogen to ammonia and providing it to the plant. Oxygen regulation is key in this symbiosis.
C) Heterocyst: Heterocysts are highly specialized in the fixation of nitrogen under aerobic conditions.
D) Potassium: The essential role of K in photosynthesis makes K a vital contributor to effective N fixation by legumes.

Thus, the correct answer is an option (B).

Note: Rhizobium is a bacterium found in soil that helps in fixing nitrogen in leguminous plants.