text: Facultative anaerobes are able to grow in both the presence and absence of oxygen due to the expression of both aerobic and anaerobic respiratory chains using either oxygen or an alternative electron acceptor. For example, in the absence of oxygen, E. coli can use fumarate, nitrate, nitrite, dimethyl sulfoxide, or trimethylamine oxide as an electron acceptor. This flexibility allows facultative anaerobes to survive in a number of environments, and in environments with frequently changing conditions.
  Several species of protists use a facultative anaerobic metabolism to enhance their ATP production, and some can produce dihydrogen through this process.