Nitrogen fixation is carried out primarily by which of the following?

• A. Fungi
• B. Animals
• C. Green plants
• D. Certain prokaryotes such as bacteria and archaea

Answer: (D)

Verified step by step guidance

1. Understand the concept of nitrogen fixation: Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds that can be used by living organisms. This is essential for the nitrogen cycle, as most organisms cannot use atmospheric nitrogen directly.
2. Identify the organisms capable of nitrogen fixation: Nitrogen fixation is primarily carried out by certain prokaryotes, including bacteria and archaea. These organisms possess the enzyme nitrogenase, which facilitates the conversion of nitrogen gas into ammonia.
3. Clarify why other options are incorrect: Fungi, animals, and green plants cannot fix atmospheric nitrogen directly because they lack the enzyme nitrogenase. Instead, they rely on fixed nitrogen compounds produced by nitrogen-fixing prokaryotes.
4. Provide examples of nitrogen-fixing prokaryotes: Common examples include free-living bacteria like Azotobacter and symbiotic bacteria such as Rhizobium, which form mutualistic relationships with the roots of leguminous plants. Cyanobacteria, a type of archaea, also play a role in nitrogen fixation in aquatic environments.
5. Summarize the correct answer: Nitrogen fixation is carried out primarily by certain prokaryotes, such as bacteria and archaea, due to their ability to produce the enzyme nitrogenase.