The adjustment of photosynthetically grown cells of Rhodospirillum rubrum to aerobic light conditions

Abstract

The growth of photosynthetically precultured cells of Rhodospirillum rubrum under aerobic condition in light is investigated. Special emphasis is given to the question of whether the photosynthetic electron transport chain is influenced under these conditions. Light-induced absorbance changes under anaerobic conditions show that although in whole cells a variation can be noted, the reactions of isolated membranes decrease only very slowly and parallel to each other. The photophosphorylation activity remains constant on a bacteriochlorophyll basis. On a cell mass basis this activity decreases parallel to the decreasing bacteriochlorophyll content. Light-dependent NAD+ reduction by ascorbate-DCPI remains constant on a bacteriochlorophyll basis, whereas succinate supported NAD+ reduction in light increases. On a cell mass basis the activity of succinate supported NAD+ reduction stays nearly constant, thus showing similar responses to the presence of oxygen in light as the NADH oxidase system. NADH oxidase activity increases on a bacteriochlorophyll basis and does not change on a cell mass basis. Parallel to the NADH-oxidase system, oxygen uptake in the dark by whole cells does not change after aerobiosis in light. Light inhibits respiration even after several generations of growth in the presence of oxygen; however, the inhibition decreases slowly. Light inhibition of respiration can be totally overcome by the addition of the uncoupler CCCP. These results indicate that light-dependent electron transport is not influenced significantly by the presence of oxygen. Although the respiratory system is formed, cells preferentially grow photosynthetically. Respiration takes over when the amount of bacteriochlorophyll reaches very low values.