Aerobic H2 respiration enhances metabolic flexibility of methanotrophic bacteria

26 Methanotrophic bacteria are important soil biofilters for the climate-active gas 27 methane. The prevailing opinion is that these bacteria exclusively metabolise single28 carbon, and in limited instances, short-chain hydrocarbons for growth. This specialist 29 lifestyle juxtaposes metabolic flexibility, a key strategy for environmental adaptation 30 of microorganisms. Here we show that a methanotrophic bacterium from the phylum 31 Verrucomicrobia oxidises hydrogen gas (H2) during growth and persistence. 32 Methylacidiphilum sp. RTK17.1 expresses a membrane-bound hydrogenase to 33 aerobically respire molecular H2 at environmentally significant concentrations. While 34 H2 oxidation did not support growth as the sole electron source, it significantly 35 enhanced mixotrophic growth yields under both oxygen-replete and oxygen-limiting 36 conditions and was sustained in non-growing cultures starved for methane. We 37 propose that H2 is consumed by this bacterium for mixotrophic growth and 38 persistence in a manner similar to other non-methanotrophic soil microorganisms. 39 We have identified genes encoding oxygen-tolerant uptake hydrogenases in all 40 publicly-available methanotroph genomes, suggesting that H2 oxidation serves a 41 general strategy for methanotrophs to remain energised in chemically-limited 42 environments. 43