Question

The bacterium capable of anaerobic nitrogen fixation is
A. Azotobacter
B. Rhizobium
C. Bacillus
D. Clostridium

Answer 1

Plants are unable to absorb atmospheric nitrogen directly. They receive it in the form of ammonium, nitrate, and nitrite ions, and the mechanism of absorbing the atmospheric nitrogen is known as nitrogen fixation. The bacteria are the main organisms that are capable of fixing atmospheric nitrogen to a form that can be utilised by the plants.

Complete step by step answer: Plants are considered the main sources of food. Generally, they synthesize various elements obtained from the atmosphere and the soil. Nitrogen is one of these elements which is obtained by the plants from the soil and utilizes it to synthesize protein. Unlike the other elements such as carbon dioxide and oxygen, atmospheric nitrogen cannot be obtained directly by the stomata of leaves. As the atmospheric nitrogen gas cannot be used directly by the plants. There are certain bacteria that help the plants to absorb natural nitrogen through the process of nitrogen fixation.
Nitrogen fixation is one of the most important processes of the nitrogen cycle. This is the process where nitrogen that presents in the atmosphere is transformed into ammonia and this process occurs by some certain bacteria such as Rhizobium, Azotobacter, etc. Some other natural phenomena are also responsible for nitrogen fixation. There are two types of nitrogen fixation process. One is physical nitrogen fixation and another is biological nitrogen fixation. The physical nitrogen fixation is further classified into two groups; natural nitrogen fixation and industrial nitrogen fixation.
Biological nitrogen fixation is a process of conversion of atmospheric nitrogen into the nitrogenous compounds with the help of living organisms i.e. some bacteria. There are two main types of microorganisms carried out the process. The first type is symbiotic bacteria or non-free-living while the other type of bacteria is free-living or non-symbiotic.
Generally, there are two types of bacteria, aerobic and anaerobic, which are responsible for the symbiotic nitrogen fixation. Clostridium are capable of nitrogen fixation and are an anaerobic bacterium. It is a saprophytic bacteria that grows in conditions that are devoid of oxygen.
The Azotobacter and Rhizobium are aerobic bacteria. Azotobacter fixes nitrogen under aerobic conditions in the presence of oxygen.
Therefore, the correct option is option D.

Note: Nitrogen fixation is a process that takes place as the plant is not capable to utilize the nitrogen directly obtained from the atmosphere. This process takes place with the help of some bacteria. They are known as a nitrogen fixer. Some of them are aerobic and some are anaerobic. Clostridium is an example of an anaerobic nitrogen fixer and the Azotobacter and Rhizobium are examples of aerobic nitrogen fixers.