A soil actinobacterium scavenges atmospheric H 2 using two membrane-associated, oxygen-dependent NiFe hydrogenases

In the Earth's lower atmosphere, H2 is maintained at trace concentrations (0. 53 ppmv/0. 40 nM) and rapidly turned over (lifetime ≤ 2. 1 y (-1) ). It is thought that soil microbes, likely actinomycetes, serve as the main global sink for tropospheric H2. However, no study has ever unambiguously proven that a hydrogenase can oxidize this trace gas. In this work, we demonstrate, by using genetic dissection and sensitive GC measurements, that the soil actinomycete Mycobacterium smegmatis mc (2) 155 constitutively oxidizes subtropospheric concentrations of H2. We show that two membrane-associated, oxygen-dependent NiFe hydrogenases mediate this process. Hydrogenase-1 (Hyd1) (MSMEG₂262-2263) is well-adapted to rapidly oxidize H2 at a range of concentrations Vmax (app) = 12 nmol⋅g⋅dw (-1) ⋅min (-1) ; Km (app) = 180 nM; threshold = 130 pM in the Δhyd23 (Hyd1 only) strain, whereas Hyd2 (MSMEG₂719-2720) catalyzes a slower-acting, higher-affinity process Vmax (app) = 2. 5 nmol⋅g⋅dw (-1) ⋅min (-1) ; Km (app) = 50 nM; threshold = 50 pM in the Δhyd13 (Hyd2 only) strain. These observations strongly support previous studies that have linked group 5 NiFe hydrogenases (e. g. , Hyd2) to the oxidation of tropospheric H2 in soil ecosystems. We further reveal that group 2a NiFe hydrogenases (e. g. , Hyd1) can contribute to this process. Hydrogenase expression and activity increases in carbon-limited cells, suggesting that scavenging of trace H2 helps to sustain dormancy. Distinct physiological roles for Hyd1 and Hyd2 during the adaptation to this condition are proposed. Soil organisms harboring high-affinity hydrogenases may be especially competitive, given that they harness a highly dependable fuel source in otherwise unstable environments.