All of the following are required in the reactions of microbial respiration except a. final electron acceptor. b. oxygen. c. cytochromes. d. electron transport system. e. a source of electrons.

B. Oxygen

Explanation:

There are two main types of microbial respiration, aerobic and anaerobic, in aerobic respiration, oxygen works as the final electron acceptor, while in anaerobic, other elements, different than oxygen, complete the electron transport system.

So, in general oxygen is not always a requirement, what's needed is the final electron acceptor, in low oxygen environments, nitrates, sulfates and iron compounds, other than oxygen, are used.