Hydrogen Oxidation Benefits Alphaproteobacterial Methanotrophs Under Severe Methane Limitation.

Hydrogen (H) and methane (CH) 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 H 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 H2s, Methylocapsa aurea KYG, and "Methylosinus acidophilus" 29 encode diverse hydrogenases, instantly oxidise H when cultivated under CH-limited and low-oxygen conditions, under which hydrogenase transcription is upregulated compared to CH-replete conditions. H exposure accelerated the maximum H oxidation rates but caused no upregulation of hydrogenases. Furthermore, while CH oxidation activity was affected by substrate-limited growth conditions, H oxidation rates remained unaffected, and H supply to CH-limited chemostats caused increased biomass yield. Moreover, CH oxidation was severely inhibited by sulfide (HS), while H and methanol oxidation rates were only moderately affected. In summary, the ability to conserve energy from H oxidation increases resilience and enhances growth of alphaproteobacterial methanotrophs in CH-limited environments, which revises the ecological role of these MOB in ecosystems with naturally fluctuating CH and H concentrations.