Nitrous oxide respiration in acidophilic methanotrophs.

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 (NO) and grow anaerobically on diverse non-methane substrates, including methanol, C-C substrates, and hydrogen. We study two strains that possess NO reductase genes: Methylocella tundrae T4 and Methylacidiphilum caldifontis IT6. We show that NO respiration supports growth of Methylacidiphilum caldifontis at an extremely acidic pH of 2.0, exceeding the known physiological pH limits for microbial NO consumption. Methylocella tundrae simultaneously consumes NO and CH in suboxic conditions, indicating robustness of its NO reductase activity in the presence of O. Furthermore, in O-limiting conditions, the amount of CH oxidized per O reduced increases when NO is added, indicating that Methylocella tundrae can direct more O towards methane monooxygenase. Thus, our results demonstrate that some methanotrophs can respire NO independently or simultaneously with O, 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, CH and NO.