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

P>Uptake of molecular hydrogen (H-2) by soil is a biological reaction responsible for similar to 80% of the global loss of atmospheric H-2. Indirect evidence obtained over the last decades suggests that free soil hydrogenases with an unusually high affinity for H-2 are carrying out the reaction. This assumption has recently been challenged by the isolation of Streptomyces sp. PCB7, displaying the high-affinity H-2 uptake activity previously attributed to free soil enzymes. While this finding suggests that actinobacteria could be responsible for atmospheric H-2 soil uptake, the ecological importance of H-2-oxidizing streptomycetes remains to be investigated. Here, we show that high-affinity H-2 uptake activity is widespread among the streptomycetes. Among 14 streptomycetes strains isolated from temperate forest and agricultural soils, six exhibited a high-affinity H-2 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 high-affinity H-2 uptake. Catalysed reporter deposition-fluorescence in situ hybridization (CARD-FISH) experiments targeting hydB-like gene transcripts and H-2 uptake assays performed with strain PCB7 suggested that streptomycetes spores catalysed the H-2 uptake activity. Expression of the activity in term of biomass revealed that 106-107 H-2-oxidizing bacteria per gram of soil should be sufficient to explain in situ H-2 uptake by soil. We propose that specialized H-2-oxidizing actinobacteria are responsible for the most important sink term in the atmospheric H-2 budget.