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:

Atmospheric nitrogen ($N_2$) is abundant but inert. Nitrogen-fixing bacteria convert it into ammonia ($NH_3$), a form that plants and other organisms can use to build proteins and nucleic acids, making bacteria essential to the nitrogen cycle. The other options are incorrect: organisms can use nitrogen in plants, the atmosphere is the main nitrogen reservoir, and transpiration is a plant process unrelated to nitrogen cycling by bacteria.

Answer:

A. Other living things cannot use atmospheric nitrogen.