Question

Azotobacter and Beijerinckia are examples of
A) Symbiotic nitrogen-fixing bacteria
B) Asymbiotic nitrogen-fixing bacteria
C) Photosynthetic
D) Disease-causing bacteria

Answer 1

Hint:-The most abundant element found in the earth’s atmosphere is nitrogen. Amino acids, DNA, and RNA of all organisms have some form of nitrogen. The nitrogen form available in the atmosphere cannot be directly utilized by plants or animals to meet all their biological needs. The biogeochemical cycle that involves the conversion of the atmospheric nitrogen to other forms (that can be utilized by organisms) and then back to the atmosphere is known as the nitrogen cycle. Nitrogen fixation is one of the steps of the nitrogen cycle.

Complete Answer:-Biological nitrogen fixation is performed by microbes that may be free-living or in a symbiotic relationship with a plant. This biological nitrogen fixation was discovered by Martinus Willem Beijerinck. He had noted the action of nitrogen-fixing bacteria in the root nodules of plants. These microbes convert the atmospheric nitrogen to ammonia which can be easily taken up by the plants.
Nitrogen-fixing bacteria may be free-living like- Clostridium Pasteurianum (obligate anaerobe), Klebsiella spp. (facultative anaerobe), Rhodobacter spp. (photosynthetic), Cyanobacteria (photosynthetic), azotobacter (obligate aerobes), Beijerinckia, (obligate aerobes), etc. These free-living, nitrogen-fixing bacteria are mainly seen in the soil.
Rhizobium, found in the root nodules of legumes is an example of symbiotic bacteria. These microbes utilize the resources of the plant to survive and in turn, they make sure that the plants' nitrogen requirements are met. Symbiotic nitrogen-fixing bacteria do most of the nitrogen fixation in soil. Azotobacter and Beijerinckia are not photosynthetic bacteria as they lack chlorophyll and cannot perform photosynthesis. Cyanobacteria are free-living, photosynthetic, nitrogen-fixing bacteria. Azotobacter and Beijerinckia are not pathogenic bacteria. Hence, they do not cause any disease.

The correct option is (B). Azotobacter and Beijerinckia are examples of Asymbiotic nitrogen-fixing bacteria.

Note:- There are two other ways by which nitrogen is fixed- atmospheric nitrogen fixation and industrial nitrogen fixation. An atmospheric nitrogen fixation is a natural event, lightning breaks nitrogen molecules and converts it to an oxide form of nitrogen. This can now be utilized by the plants.
Industrial nitrogen fixation involves laboratory methods by which ammonia is formed, combining nitrogen and oxygen. This ammonia is then used as a component of chemical fertilizers.