Question

why are bacteria a necessary part of the nitrogen cycle? other living things cannot use atmospheric nitrogen. other living things cannot use the nitrogen in plants. bacteria are the main nitrogen reservoir. bacteria are the main nitrogen transpirators.

Explanation:

1. Analyze Option 1: Atmospheric nitrogen (\(N_2\)) is in a form that most living things (like plants, animals) can't directly use. Bacteria (like nitrogen - fixing bacteria) can convert atmospheric nitrogen into usable forms (e.g., ammonia, nitrates) through processes like nitrogen fixation. So this option is consistent with the role of bacteria in the nitrogen cycle.
2. Analyze Option 2: Other living things (like animals) can consume plants and use the nitrogen present in plants (e.g., in proteins, nucleic acids of plants). So this option is incorrect.
3. Analyze Option 3: The main nitrogen reservoir is the atmosphere (which is about 78% nitrogen gas), not bacteria. So this option is incorrect.
4. Analyze Option 4: Transpiration is a process related to water movement in plants, not nitrogen. Bacteria are not "nitrogen transpirators" as this term is not relevant to nitrogen cycling. So this option is incorrect.

Answer:

A. Other living things cannot use atmospheric nitrogen.