Molecular hydrogen in seawater supports growth of diverse marine bacteria.

Molecular hydrogen (H 2 ) is an abundant and readily accessible energy source in marine systems, but it remains unknown whether marine microbial communities consume this gas. Here we use a suite of approaches to show that marine bacteria consume H 2 to support growth. Genes for H 2 -uptake hydrogenases are prevalent in global ocean metagenomes, highly expressed in metatranscriptomes and found across eight bacterial phyla. Capacity for H 2 oxidation increases with depth and decreases with oxygen concentration, suggesting that H 2 is important in environments with low primary production. Biogeochemical measurements of tropical, temperate and subantarctic waters, and axenic cultures show that marine microbes consume H 2 supplied at environmentally relevant concentrations, yielding enough cell-specific power to support growth in bacteria with low energy requirements. Conversely, our results indicate that oxidation of carbon monoxide (CO) primarily supports survival. Altogether, H 2 is a notable energy source for marine bacteria and may influence oceanic ecology and biogeochemistry.