Question

A free living anaerobic bacterium capable of fixing nitrogen is
A. Azotobacter
B. Rhizobium
C. Clostridium
D. Streptococcus

Answer 1

Hint:Azotobacter is an aerobic bacterium, and Clostridium is an anaerobic bacterium. Examples of symbiotic bacteria include Rhizobium, Bradyrhizobium.

Complete answer:
An anaerobic organism or anaerobic is any organism that does not need oxygen for development. It can react adversely or even die in the presence of oxygen. The aerobic organism, on the other hand, is an organism that requires an oxygen-rich environment. Anaerobic can be unicellular or multicellular. Nitrogen-fixing bacteria are certain bacteria that fix the nitrogen in the atmosphere and make it available to plants.
Azotobacter vinelandii is a soil bacterium close to the Pseudomonas gene that fixes nitrogen in aerobic conditions and at the same time defends nitrogenase from oxygen destruction.
Rhizobium is aerobic, which has a lot to do with the fact that nitrogen fixation is an energy consuming process that involves a significant amount of energy that could not possibly be produced by anaerobic pathways.
Clostridium species are fermentative, Gram-positive, anaerobic, spore-forming bacteria belonging to the Firmicutes phylum. Poison-producing species can cause mild-to-fatal food poisoning, most commonly known as Clostridium perfringens and Clostridium botulinum. Clostridium pasteurianum is the first recognized free-living nitrogen-fixing bacteria.
Streptococcus is a gram-positive aerobic organism that causes many disorders, such as pharyngitis, pneumonia, wound and skin infections, sepsis, and endocarditis.

Thus, the correct answer is option C. i.e., Clostridium.

Note:There are 2 groups of nitrogen-fixing bacteria, free-living (non-symbiotic) bacteria and non-free living (symbiotic) bacteria. Examples of free-living bacteria include cyanobacteria such as Anabaena and Nostoc, Azotobacter, Clostridium.