Hydrogen Oxidation Benefits Alphaproteobacterial Methanotrophs Under Severe Methane Limitation.

Hydrogen (H2) and methane (CH4) are produced in the anoxic layers of wetlands and sediments. In the overlaying oxygenated surface layers, these gases become available for oxidation by aerobic hydrogenotrophic and methanotrophic microorganisms. While H2 oxidation by verrucomicrobial methane-oxidising bacteria (MOB) is extensively studied, less is known about this metabolism in MOB from the class Alphaproteobacteria, which frequently inhabit wetlands. We show that Methylocystis bryophila H2sT, Methylocapsa aurea KYGT, and "Methylosinus acidophilus" 29 encode diverse hydrogenases, instantly oxidise H2 when cultivated under CH4-limited and low-oxygen conditions, under which hydrogenase transcription is upregulated compared to CH4-replete conditions. H2 exposure accelerated the maximum H2 oxidation rates but caused no upregulation of hydrogenases. Furthermore, while CH4 oxidation activity was affected by substrate-limited growth conditions, H2 oxidation rates remained unaffected, and H2 supply to CH4-limited chemostats caused increased biomass yield. Moreover, CH4 oxidation was severely inhibited by sulfide (H2S), while H2 and methanol oxidation rates were only moderately affected. In summary, the ability to conserve energy from H2 oxidation increases resilience and enhances growth of alphaproteobacterial methanotrophs in CH4-limited environments, which revises the ecological role of these MOB in ecosystems with naturally fluctuating CH4 and H2 concentrations.