Streptomycetes contributing to atmospheric molecular hydrogen soil uptake are widespread and encode a putative high-affinity [NiFe]-hydrogenase

Uptake of molecular hydrogen (H₂) by soil is a biological reaction responsible for ~80% of the global loss of atmospheric H₂. Indirect evidence obtained over the last decades suggests that free soil hydrogenases with an unusually high affinity for H₂ are carrying out the reaction. This assumption has recently been challenged by the isolation of Streptomyces sp. PCB7, displaying the high-affinity H₂ uptake activity previously attributed to free soil enzymes. While this finding suggests that actinobacteria could be responsible for atmospheric H₂ soil uptake, the ecological importance of H₂-oxidizing streptomycetes remains to be investigated. Here, we show that high-affinity H₂ uptake activity is widespread among the streptomycetes. Among 14 streptomycetes strains isolated from temperate forest and agricultural soils, six exhibited a high-affinity H₂ uptake activity. The gene encoding the large subunit of a putative high-affinity [NiFe]-hydrogenase (hydB-like gene sequence) was detected exclusively in the isolates exhibiting highaffinity H₂ uptake. Catalysed reporter depositionfluorescence in situ hybridization (CARD-FISH) experiments targeting hydB-like gene transcripts and H₂ uptake assays performed with strain PCB7 suggested that streptomycetes spores catalysed the H₂ uptake activity. Expression of the activity in term of biomass revealed that 10⁶-10⁷ H₂-oxidizing bacteria per gram of soil should be sufficient to explain in situ H₂ uptake by soil. We propose that specialized H₂-oxidizing actinobacteria are responsible for the most important sink term in the atmospheric H₂ budget.