Probing the geological source and biological fate of hydrogen in Yellowstone hot springs.

Hydrogen (H ) is enriched in hot springs and can support microbial primary production. Using a series of geochemical proxies, a model to describe variable H concentrations in Yellowstone National Park (YNP) hot springs is presented. Interaction between water and crustal iron minerals yields H that partition into the vapour phase during decompressional boiling of ascending hydrothermal fluids. Variable vapour input leads to differences in H concentration among springs. Analysis of 50 metagenomes from a variety of YNP springs reveals that genes encoding oxidative hydrogenases are enriched in communities inhabiting springs sourced with vapour-phase gas. Three springs in the Smokejumper (SJ) area of YNP that are sourced with vapour-phase gas and with the most H in YNP were examined to determine the fate of H . SJ3 had the most H , the most 16S rRNA gene templates and the greatest abundance of culturable hydrogenotrophic and autotrophic cells of the three springs. Metagenomics and transcriptomics of SJ3 reveal a diverse community comprised of abundant populations expressing genes involved in H oxidation and carbon dioxide fixation. These observations suggest a link between geologic processes that generate and source H to hot springs and the distribution of organisms that use H to generate energy.