Chemoautotrophic growth of hydrogen-uptake-positive strains of Rhizobium japonicum

Abstract

Recently reported research from this laboratory has demonstrated the autotrophic growth of certain hydrogen-uptake-positive strains of Rhizobium japonicum and defined minimal conditions for such growth. Ribulose 1,5-bisphosphate carboxylase has been detected in autotrophically growing cells, but at low specific activity. Moreover, growth rates were low, and growth ceased at low cell densities. We report here improved autotrophic growth rates of R. japonicum SR through the use of a modified mineral salts/vitamins medium and a programmed increase in oxygen tension as autotrophic growth proceeds. Under these conditions, ribulose, 1,5-biphosphate carboxylase activity increased greater than 10-fold and crude-extract-uptake-hydrogenase activities were from 20 to 47 times those heretofore reported for free-living R. japonicum. It is likely that previous assays for these enzymes were done on preparations of cells in which their synthesis had been partially repressed. The contribution of CO2 fixation to organic carbon accumulation in autotrophic cells was assessed as sufficient to support observed growth. Enzymological determination of the product of carbon fixation has established a stoichiometric ratio of 1.9 mol of 3-phosphoglycerate per mol of CO2 fixed and unequivocally assigns the role of carbon fixation catalysis to ribulose 1,5-bisphosphate carboxylase. Ammonium served best as a nitrogen source, nitrate was less effective, and gaseous nitrogen would not support autotrophic growth. Ecological, evolutionary, and practical considerations of autotrophy in the rhizobia are briefly discussed in the light of our findings.