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

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Abstract
Recently reported research from this laboratory demonstrated the autotrophic growth of certain H2 uptake-positive strains of R. japonicum and defined minimal conditions for such growth. Ribulose 1,5-bisphosphate carboxylase was detected in autotrophically growing cells, but at low specific activity. Growth rates were low, and growth ceased at low cell densities. Improved autotrophic growth rates of R. japonicum SR through the use of a modified mineral salts/vitamins medium and a programmed increase in O2 tension as autotrophic growth proceeds are reported here. Under these conditions, ribulose 1,5-bisphosphate carboxylase activity increased greater than 10-fold and crude-extract-uptake-hydrogenase activities were from 20-47 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 was partially repressed. The contribution of CO2 fixation to organic C accumulation in autotrophic cells was assessed as sufficient to support observed growth. Enzymological determination of the product of C fixation established a stoichiometric ratio of 1.9 mol of 3-phosphoglycerate/mol of CO2 fixed and unequivocally assigns the role of C fixation catalysis to ribulose 1,5-bisphosphate carboxylase. Ammonium served best as a N source, nitrate was less effective, and N2 would not support autotrophic growth. Ecological, evolutionary and practical considerations of autotrophy in the rhizobia are briefly discussed.