Why must atmospheric nitrogen (\(N_2\)) be 'fixed' before most organisms can use it, and which organisms are primarily responsible for this process?

• A. Because \(N_2\) is highly reactive and must be stabilized by fungi before use.
• B. Because \(N_2\) is inert and cannot be directly used by most living organisms; nitrogen-fixing bacteria convert it into usable forms.
• C. Because \(N_2\) is toxic to most organisms; only plants can convert it into ammonia.
• D. Because \(N_2\) is only present in water; algae are responsible for nitrogen fixation.

Answer: (B)

Verified step by step guidance

1. Understand the nature of atmospheric nitrogen (\(N_2\)): Atmospheric nitrogen is a diatomic molecule with a triple bond between the two nitrogen atoms. This bond is extremely strong, making \(N_2\) chemically inert and not readily usable by most organisms.
2. Recognize why nitrogen fixation is necessary: Most organisms cannot break the triple bond in \(N_2\) to access nitrogen for building essential biomolecules like amino acids and nucleotides. Nitrogen must be converted into more reactive and usable forms, such as ammonia (\(NH_3\)) or nitrate (\(NO_3^-\)).
3. Identify the organisms responsible for nitrogen fixation: Nitrogen-fixing bacteria, such as those in the genera *Rhizobium* and *Azotobacter*, are primarily responsible for converting \(N_2\) into usable forms. Some cyanobacteria also perform nitrogen fixation.
4. Understand the process of nitrogen fixation: These bacteria use the enzyme nitrogenase to catalyze the conversion of \(N_2\) into ammonia (\(NH_3\)). The reaction requires energy, typically supplied by ATP, and occurs under anaerobic conditions to protect the nitrogenase enzyme from oxygen damage.
5. Recognize the ecological importance: Nitrogen fixation is a critical process in the nitrogen cycle, enabling the incorporation of atmospheric nitrogen into ecosystems. Plants often form symbiotic relationships with nitrogen-fixing bacteria to access fixed nitrogen directly.