Effect of light and darkness on nitrate assimilation by Rhodopseudomonas capsulata E1F1

The photosynthetic nonsulfur purple bacterium Rhodopseudomonas capsulata strain E1F1 assimilated nitrate or nitrite only in illuminated cultures under anaerobic conditions. The bacterial cells grew aerobically in the dark only when ammonia or other forms of reduced nitrogen were present in the medium. However, nitrate reductase was detected either in light-anaerobic or in dark-aerobic conditions upon addition of nitrate to the media. Changes from light-anaerobic to dark-aerobic conditions and vice versa markedly influenced growth, nitrate uptake and the nitrate reductase levels. Growth on nitrate in the light and nitrate reductase activity were dependent on the presence of molybdenum in the medium whereas the addition of tungstate inhibited both growth and enzyme activity.