Question

Which is a free-living nitrogen-fixing bacteria?
A) Pseudomonas
B) Azotobacter
C) Nitrosomonas
D) Rhizobium

Answer 1

The process of conversion of free nitrogen into nitrogenous compounds to make it available to the plant for absorption is termed nitrogen fixation. With the help of living organisms, the process of conversion of atmospheric nitrogen into nitrogenous compounds occurs and is called biological nitrogen fixation. They include both free-living and symbiotic forms.

Complete answer:
The biological nitrogen fixation is of two types – non-symbiotic (these are the free-living bacteria and include both the aerobic as well as anaerobic bacteria) and symbiotic (they live in the tissues of higher plants in symbiotic association).

Let us look into the given options:
A) Pseudomonas – The bacteria that help in reducing nitrates or ammonia present in the soil to release nitrogen gas back into the atmosphere and this process is called Denitrification.
B) Azotobacter – It is free-living nitrogen-fixing bacteria. It is an aerobic bacteria and is able to pick up free atmospheric nitrogen and fix it in some organic compounds like amino acids.
C) Nitrosomonas – These are the nitrifying bacteria and help to convert ammonia to nitrite ions. The process of conversion of ammonia to nitrates is called nitrification.
D) Rhizobium – This is the symbiotic bacteria that live in the soil to form root nodules in plants belonging to the family Leguminosae, such as beans, gram, groundnut and soybean. The small, irregular outgrowths on the roots are called root nodules. When a root hair comes in contact with Rhizobium, it curls or is deformed.

So, the correct answer is an option (A) Azotobacter.

Note: By the process of nitrogen fixation, atmospheric nitrogen can be converted into nitrate, which is taken up by the plants. Plants convert this nitrate into amino acids, nucleotides, proteins and other nitrogenous compounds. In the absence of a nitrogen fixation process, these substances will not be formed.