Characterization of populations of aerobic hydrogen-oxidizing soil bacteria

Abstract

Freshly isolated soil bacteria were screened for different characteristics of the H2 metabolism without prior selection for growth on H2. The bacteria were isolated from different grain size fractions of a neutral meadow cambisol and an acidic forest cambisol, and then tested (1) for the ability to oxidize H2, (2) for chemolithoautotrophic growth on H2 as sole electron donor and energy source, (3) for DNA-DNA-hybridization with two hydrogenase gene fragments from Alcaligenes eutrophus and Rhizobium leguminosarum, and (4) for reduction of 2,3,5-triphenyl-2H-tetrazoliumchloride (TTC) in the presence of H2. Many (65–90%) of the isolates were able to reduce TTC, but only 30–65% were actually able to oxidize H2 indicating that the TTC test was not a specific characteristic for H2 oxidation ability. The TTC test was only reliable in pure cultures of known bacteria with optimized test conditions, here shown for Alcaligenes eutrophus, Bradyrhizobium japonicum and Nocardia opaca, but not in mixed cultures of unknown bacteria. Still less (< 30%) of the isolates were able to grow chemolithoautotrophically indicating that culturable aerobic bacteria with the ability for H2 oxidation are more abundant than bacteria with the ability for chemolithoautotrophic growth. The DNA-DNA-hybridization test failed to detect many of the bacteria with H2 oxidation activity, probably since the hydrogenase genes present in the isolates were too diverse to be all detected by the DNA probes applied.