Nitrous oxide respiration in acidophilic methanotrophs.

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 (N₂O) and grow anaerobically on diverse non-methane substrates, including methanol, C-C substrates, and hydrogen. We study two strains that possess N₂O reductase genes: Methylocella tundrae T4 and Methylacidiphilum caldifontis IT6. We show that N₂O respiration supports growth of Methylacidiphilum caldifontis at an extremely acidic pH of 2.0, exceeding the known physiological pH limits for microbial N₂O consumption. Methylocella tundrae simultaneously consumes N₂O and CH₄ in suboxic conditions, indicating robustness of its N₂O reductase activity in the presence of O₂. Furthermore, in O₂-limiting conditions, the amount of CH₄ oxidized per O₂ reduced increases when N₂O is added, indicating that Methylocella tundrae can direct more O₂ towards methane monooxygenase. Thus, our results demonstrate that some methanotrophs can respire N₂O independently or simultaneously with O₂, 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 CO₂, CH_4, and N₂O.