Isolation and characterization of hydrogen-oxidizing bacteria induced following exposure of soil to hydrogen gas and their impact on plant growth

In many legumes, the nitrogen fixing root nodules produce H-2 gas that diffuses into soil. It has been demonstrated that such exposure of soil to H-2 can promote plant growth. To assess whether this may be due to H-2-oxidizing microorganisms, bacteria were isolated from soil treated with H-2 under laboratory conditions and from soils collected adjacent to H-2 producing soybean nodules. Nineteen isolates of H-2-oxidizing bacteria were obtained and all exhibited a half-saturation coefficient (Ks) for H-2 of about 1 ml l(-1). The isolates were identified as Variovorax paradoxus, Flavobacterium johnsoniae and Burkholderia spp. using conventional microbiological tests and 16S rRNA gene sequence analysis. Seventeen of the isolates enhanced (57-254%) root elongation of spring wheat seedlings. Using an Arabidopsis thaliana bioassay, plant biomass was increased by 11-27% when inoculated by one of four isolates of V. paradoxus or one isolate of Burkholderia that were selected for evaluation. The isolates of V. paradoxus found in both H-2-treated soil and in soil adjacent to soybean nodules had the greatest impact on plant growth. The results are consistent with the hypothesis that H-2-oxidizing bacteria in soils have plant growth promoting properties.