Question

Which of the following is a major group of microbes involved in nitrogen fixation?

• A. Bacteria
• B. Fungi
• C. Protozoa
• D. Archaea

Hint:

• Nitrogen fixation is the conversion of atmospheric N$_2$ to ammonia (NH$_3$).
• This process is carried out by prokaryotic microorganisms called diazotrophs.
• Major groups of diazotrophs are found within Bacteria (e.g., \textit{Rhizobium}, \textit{Azotobacter}, \textit{Clostridium}, Cyanobacteria).
• Some Archaea (e.g., certain methanogens) can also fix nitrogen.
• Fungi and protozoa (eukaryotes) do not fix nitrogen.

Answer 1

The Correct Option is A

Nitrogen fixation is the biological process by which atmospheric nitrogen gas (N$_2$), which is relatively inert and unusable by most organisms, is converted into ammonia (NH$_3$) or other biologically useful nitrogenous compounds. This process is exclusively carried out by certain prokaryotic microorganisms called diazotrophs. These diazotrophs include:

• Bacteria: This is a very diverse group containing many well-known nitrogen-fixing species. They can be:
  • Symbiotic bacteria: Such as Rhizobium species that form nodules on the roots of leguminous plants.
  • Free-living aerobic bacteria: Such as Azotobacter and Beijerinckia.
  • Free-living anaerobic bacteria: Such as Clostridium pasteurianum.
  • Cyanobacteria (Blue-Green Algae): Many species, both free-living (e.g., Anabaena, Nostoc) and symbiotic, can fix nitrogen. These are also bacteria.
• Archaea: Some species of archaea, particularly certain methanogens (e.g., Methanosarcina), are also capable of nitrogen fixation.

While some Archaea (option d) can fix nitrogen, Bacteria (option a) represent a larger, more diverse, and ecologically more prominent group of nitrogen-fixing microorganisms. Many of the most studied and agriculturally important nitrogen fixers are bacteria. Fungi (option b) and Protozoa (option c) are eukaryotes and are not known to perform nitrogen fixation themselves, although they interact with nitrogen-fixing prokaryotes in the ecosystem. Given the options, "Bacteria" is the most encompassing and generally recognized major group primarily involved in this critical biogeochemical process. \[ \boxed{\text{Bacteria}} \]