Photosynthesis in Rhodospirillum rubrum. II. Photoheterotrophic Carbon Dioxide Fixation

Abstract

The contribution of the reductive pentose phosphate cycle to the photometabolism of carbon dioxide and to carbon metabolism in R. rubrum grown photohetero-trophically with L-malate as the carbon source is nil, unlike auto-trophically grown R. rubrum. Glycolic acid appears to be the first stable product of CO2 fixation in R. rubrum cultured photohetero-trophically on L-malate. The results obtained in C14O2 fixation experiments suggest that the photometabolism of CO2 through glycolate into malate is a major pathway of CO2 fixation in such cells. However, L-malate was a much more efficient precursor of phosphate esters, and of glutamic acid, than was carbon dioxide: L-malate is therefore, in this case, a far more important source of cell carbon than is carbon dioxide. The products of the light-dependent incorporation of CO2 and of acetate were investigated in R. rubrum grown photohetero-trophically on acetate. Carboxylation reactions and the reductive pentose phosphate cycle are apparently of greater significance in the photometabolism of acetate heterotrophs than in malate hetero-trophs; the photometabolism of the acetate photoheterotrophs seems to be intermediate between the photoheterotrophy of malate heterotrophs and strict autotrophy.