Abstract
Aerobic methanotrophic bacteria are considered strict aerobes but are often highly abundant in hypoxic and even anoxic environments. Despite possessing denitrification genes, it remains to be verified whether denitrification contributes to their growth. Here, we show that acidophilic methanotrophs can respire nitrous oxide (N(2)O) and grow anaerobically on diverse non-methane substrates, including methanol, C-C substrates, and hydrogen. We study two strains that possess N(2)O reductase genes: Methylocella tundrae T4 and Methylacidiphilum caldifontis IT6. We show that N(2)O respiration supports growth of Methylacidiphilum caldifontis at an extremely acidic pH of 2.0, exceeding the known physiological pH limits for microbial N(2)O consumption. Methylocella tundrae simultaneously consumes N(2)O and CH(4) in suboxic conditions, indicating robustness of its N(2)O reductase activity in the presence of O(2). Furthermore, in O(2)-limiting conditions, the amount of CH(4) oxidized per O(2) reduced increases when N(2)O is added, indicating that Methylocella tundrae can direct more O(2) towards methane monooxygenase. Thus, our results demonstrate that some methanotrophs can respire N(2)O independently or simultaneously with O(2), which may facilitate their growth and survival in dynamic environments. Such metabolic capability enables these bacteria to simultaneously reduce the release of the key greenhouse gases CO(2), CH(4,) and N(2)O.