• H2 oxidation kinetics;
  • Shewanella putrefaciens;
  • Bradyrhizobium japonicum;
  • Fe(III) reduction;
  • H2 threshold

Abstract Bradyrhizobium japonicum and Shewanella putrefaciens were unable to oxidize hydrogen at atmospheric concentrations (0.55 ppmv), neither in suspension nor when added to sterile soil. The Km-value of S. putrefaciens for H2 (39 ppmv in gas phase, 0.22 μM in aqueous phase), using Fe(III) as electron acceptor, showed a 4–5-fold higher affinity for H2 than that of B. japonicum (1200 ppmv; 0.84 μM) or other hydrogen-oxidizing bacteria. However, the Vmax (4.54 fmol H2 h−1 cell −1) and threshold (> 0.5 ppmv; 0.35 nM) of S. putrefaciens and the Vmax (7.19 fmol H2 h−1 cell−1) and threshold (> 0.5 ppmv; 0.35 nM) of B. japonicum were in the same order of magnitude as data for Knallgas bacteria from relevant literature. To enable hydrogen oxidation in soil the soil-samples with S. putrefaciens even had to be supplemented with Fe(III). Fresh soil, on the other hand, oxidized hydrogen very efficiently below atmospheric mixing ratios, demonstrating that there must be other oxidation activities in soil.