H(2)-saturation of high affinity H(2)-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.