H2-saturation of high affinity H2-oxidizing bacteria alters the ecological niche of soil microorganisms unevenly among taxonomic groups

Soil microbial communities are continuously exposed to H 2 diffusing into the soil from the atmosphere. N 2 -fixing nodules represent a peculiar microniche in soil where H 2 can reach concentrations up to 20,000 fold higher than in the global atmosphere (0.530 ppmv). In this study, we investigated the impact of H 2 exposure on soil bacterial community structure using dynamic microcosm chambers simulating soil H 2 exposure from the atmosphere and N 2 -fixing nodules. Biphasic kinetic parameters governing H 2 oxidation activity in soil changed drastically upon elevated H 2 exposure, corresponding to a slight but significant decay of high affinity H 2 -oxidizing bacteria population, accompanied by an enrichment or activation of microorganisms displaying low-affinity for H 2 . In contrast to previous studies that unveiled limited response by a few species, the relative abundance of 958 bacterial ribotypes distributed among various taxonomic groups, rather than a few distinct taxa, was influenced by H 2 exposure. Furthermore, correlation networks showed important alterations of ribotype covariation in response to H 2 exposure, suggesting that H 2 affects microbe-microbe interactions in soil. Taken together, our results demonstrate that H 2 -rich environments exert a direct influence on soil H 2 -oxidizing bacteria in addition to indirect effects on other members of the bacterial communities.