Meta-omics survey of [NiFe]-hydrogenase genes fails to capture drastic variations in H2-oxidation activity measured in three soils exposed to H2

Inferences on soil biogeochemical processes based on metagenomic profiles is a challenging task due to enormous diversity of soil microbes and the fragile linkage between gene abundance and functioning. Here we used the biological sink of H2 as a case study to test the hypothesis that [NiFe]-hydrogenase gene distribution and expression profiles explain variations in H2 oxidation rate measured in soil collected in poplar monoculture, larch plantation and farmland. Shotgun metagenomic and metatranscriptomic analyses of soil samples exposed to elevated or low H2 concentration led to the identification of 45 genes encoding the large subunit of [NiFe]-hydrogenases belonging to 8 distinct phyla. Our results indicate that despite significant sequencing effort, retrieved hydrogenase sequences are not in themselves adequate surrogates of H2 oxidation activity in these soils. In fact, land-use exerted a greater influence than H2 exposure on both hydrogenase gene distribution and expression though expression of certain genes responded to H2. We argue that approaches relying on PCR/RT-PCR amplicon sequencing or quantification combined with physicochemical parameters are currently the best option to infer the activity of H2-oxidizing bacteria and probably other specialist functional guilds with similar population size in soil.