Ecological traits of high-affinity hydrogen-oxidizing soil bacteria involved in the hydrogen cycle

Every year, soil microbial-mediated hydrogen (H₂) oxidation removes about 80 % of the global atmospheric H₂, an indirect greenhouse gas. Soil-dwelling high-affinity H₂ oxidizing bacteria use this trace gas as an energy source to persist when other substrates are limited or to meet their maintenance energy requirements during dormancy. However, there is limited knowledge of the distribution, composition, diversity, and functions of this group of bacteria, particularly their ecological traits (i.e., characteristics that influence their interactions with the environment and other organisms). This is because the high-affinity H₂-oxidizing bacteria are not phylogenetically conserved, potentially due to the horizontal transfer of their functional gene, which still needs to be demonstrated. This makes it difficult to answer ecological questions related to the distribution, functional role, and ecological contribution of H₂-oxidizing bacteria in the soil H₂ cycle, as well as their responses to environmental factors. Such information is needed to estimate the contribution of the H₂-oxidizing bacteria to the global H₂ cycle. Although many H₂-oxidizing bacteria are not culturable, they may share similar ecological traits when responding to environmental changes, such as pH, moisture content, and H₂ concentrations. Therefore, a community or guild-level trait-based approach (defined as the analysis of functional traits shared by groups of bacteria (guilds) that perform similar ecological roles) could be useful to synthesize complex genomic and phylogenetic information. This review discusses the impact of soil environmental factors on soil H₂ uptake (by oxidation), and identifies ecological response traits under controlled conditions. Our approach connects the biological activity of the H₂-oxidizing bacteria to their resident environment, for scaling up and estimating the capacity of soil microbial communities to mitigate global warming linked to increased atmospheric H₂.