Photopigments in Rhodopseudomonas capsulata cells grown anaerobically in darkness.

The phototrophic bacterium Rhodopseudomonas capsulata can obtain energy for dark anaerobic growth from sugar fermentations dependent on accessory oxidants such as trimethylamine-N-oxide or dimethyl sulfoxide. Cells grown for one to two subcultures in this fashion, with fructose as the energy source, showed approximately a twofold increase in bacteriochlorophyll content (per milligram of cell protein) and developed extensive intracytoplasmic membranes in comparison with cells grown photosynthetically at saturating light intensity. Cells harvested from successive anaerobic dark subcultures, however, showed progressively lower pigment contents. After ca. 20 transfers, bacteriochlorophyll and carotenoids were barely detectable, and the amount of intracytoplasmic membrane diminished considerably. Spontaneous mutants incapable of producing normal levels of photosynthetic pigments arose during prolonged anaerobic dark growth. Certain mutants of this kind appear to have a selective advantage over wild-type cells under fermentative growth conditions. Of four pigment mutants characterized (two being completely unable to produce bacteriochlorophyll), only one retained the capacity to grow photosynthetically.