Abstract

Hydrogen (H-2) is enriched in hot springs and can support microbial primary production. Using a series of geochemical proxies, a model to describe variable H-2 concentrations in Yellowstone National Park (YNP) hot springs is presented. Interaction between water and crustal iron minerals yields H-2 that partition into the vapour phase during decompressional boiling of ascending hydrothermal fluids. Variable vapour input leads to differences in H-2 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-2 in YNP were examined to determine the fate of H-2. SJ3 had the most H-2, 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-2 oxidation and carbon dioxide fixation. These observations suggest a link between geologic processes that generate and source H-2 to hot springs and the distribution of organisms that use H-2 to generate energy.