Photosynthetic carbon dioxide assimilation by Rhodospirillum rubrum

Summary Although Rhodospirillum rubrum, grown photoheterotrophically on malate, assimilates carbon dioxide less rapidly than it does when grown autotrophically, the difference is less marked than previously suggested.The rate of photoassimilation of carbon dioxide varies during batch culture on malate, reaching a maximum at about mid-exponential phase. It also varies with density and growth rate in a turbidostat continuous-flow culture on malate and increases with decreasing growth rate in a chemostat continuous-flow culture growing with limiting malate concentrations.The changing rates of carbon dioxide photoassimilation during photoheterotrophic growth under the various conditions are paralleled by changing activities of ribulose diphosphate carboxylase.Under conditions of maximum carbon dioxide fixation the rate by photoheterotrophic cultures approaches that shown by the bacterium growing autotrophically and is assimilated eight to ten times more slowly than is malate in chemostat cultures.The rate of carbon dioxide fixation also increases to that shown by autotrophic cells when photoheterotrophic cultures are deprived of malate, but without subjecting them to the conditions required for autotrophic growth.