Question

One of the free-living anaerobic nitrogen-fixing bacteria is
A. Azotobacter
B. Beijerinickia
C. Rhodospirillum
D. Rhizobium

Answer 1

Nitrogen-fixing bacteria are capable of reworking atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants). More than 90% of nitrogen fixation is affected by these types of organisms; therefore, they play an important role in the nitrogen cycle. Nitrogen acts as a limiting element for plant growth and production.

Complete answer: Nitrogen fixation is the process in which molecular nitrogen within the environment is converted into ammonia or related nitrogenous compounds in soil. Atmospheric nitrogen is molecular dinitrogen which is metabolically useless to all or any but some microorganisms. Biological organic process converts N2 into ammonia, which is metabolized by most organisms.
Nitrogen Fixers: Some prokaryotes like bacteria and cyanobacteria can fix atmospheric nitrogen. They're called nitrogen fixers or diazotrophs. Diazotrophs could also be asymbiotic (free living) or symbiotic:
Nitrogen Fixing Bacteria: Azotobacter, Beijerinckia and Clostridium (anaerobic) are saprophytic bacteria that perform organic processes. Desulfovibrio is a chemotrophic nitrogen fixing bacterium. Rhodopseudomonas, Rhodospirillum and Chromatium are nitrogen fixing photoautotrophic bacteria. These bacteria add up to 10-25 kg of nitrogen/ha/annum.
Nitrogen Fixing Cyanobacteria: Many cyanobacteria perform biological processes, e.g., Anabaena, Nustoc, Aulosira, Cylmdrospermum, Trichodesmium.
Symbiotic Nitrogen Fixing Cyanobacteria: Anabaena and Nostoc species are symbionts in lichens, Anthoceros, Azolla and cycad roots. Azolla pinnata has Anabaena azollae in its fronds. It's often inoculated to rice fields for organic process.
Symbiotic Nitrogen Fixing Bacteria: Rhizobium is aerobic, gram negative nitrogen fixing bacterial symbionts of Papilionaceous roots. Sesbania rostrata has rhizobium in root nodules and Aero Rhizobium in stem nodules.
Symbiotic Nitrogen Fixation: Rhizobium sp. and Frankia fix atmospheric di-nitrogen.
Diazotroph: Diazotrophs are widespread within domain Bacteria including cyanobacteria, also as green thiobacillus, Azotobacteraceae, rhizobia and Frankia. Several obligately anaerobic bacteria fix nitrogen like Clostridium spp. Some archaea also fix nitrogen.
Therefore, the correct option is c, Rhodospirillum.

Note: Cyanobacteria inhabit nearly all illuminated environments on Earth and play key roles within the carbon and biological process of the biosphere. Several cyanobacteria strains are capable of diazotrophic growth. Cyanobacteria in coral reefs can fix twice the maximum amount of nitrogen as on land—around 660 kg/ha/year.