Abstract
In the Earth's lower atmosphere, H-2 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 H-2. 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 H-2. We show that two membrane-associated, oxygen-dependent [NiFe] hydrogenases mediate this process. Hydrogenase-1 (Hyd1) (MSMEG_2262-2263) is well-adapted to rapidly oxidize H-2 at a range of concentrations [V-max(app) = 12 nmol.g.dw(-1).min(-1); K-m(app) = 180 nM; threshold = 130 pM in the Delta hyd23 (Hyd1 only) strain], whereas Hyd2 (MSMEG_2719-2720) catalyzes a slower-acting, higher-affinity process [V-max(app) = 2.5 nmol.g.dw(-1).min(-1); K-m(app) = 50 nM; threshold = 50 pM in the Delta hyd13 (Hyd2 only) strain]. These observations strongly support previous studies that have linked group 5 [NiFe] hydrogenases (e. g., Hyd2) to the oxidation of tropospheric H-2 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 H-2 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.