Simultaneous phototrophic and chemotrophic growth in the purple sulfur bacterium Thiocapsa roseopersicina M1

The anoxygenic phototrophic purple sulfur bacterium Thiocapsa roseopersicina was grown in illuminated continuous cultures with thiosulfate as growth limiting substrate. Aeration resulted in completely colorless cells growing chemotrophically, whereafter the conditions were changed to a 23 h oxic/1 h anoxic regime. After 11 volume changes at a dilution rate of 0.031 h −1 (35% of μ max ) a time dependent equilibrium was established. During the 23 h oxic periods bacteriochlorophyll a synthesis (BChl a ) was not observed, whereas during the 1 h anoxic periods synthesis was maximal (i.e. 1.1 μg (mg protein) −1 h −1 ). As a result the BChl a concentration gradually increased from zero to an average value over 24 h of 1.9 μg (mg protein) −1 . Concomitantly, the protein concentration increased from 13.9 mg 1 −1 during continuous oxic conditions to 28.8 mg 1 −1 . For comparison, the protein concentration during fully phototrophic growth at an identical thiosulfate concentration in the inflowing medium was 53.7 mg 1 −1 . The specific respiration rate was 8 μmol O 2 (mg protein) −1 h −1 during full chemotrophic growth and gradually decreased to 3.5 μmol O 2 (mg protein) −1 h −1 after 11 volume changes at the regime employed. These data show that T. rosepersicina is able to simultaneously utilize light and aerobic respiration of thiosulfate as sources of energy. The ecological relevance of the data is discussed.