Dark anaerobic dinitrogen fixation by a photosynthetic microorganism.

Photosynthetic purple bacteria can grow with dinitrogen gas as the sole nitrogen source under anaerobic conditions with light as the energy source. The bacterium Rhodopseudomonas capsulata can fix nitrogen in darkness with alternative energy conversion systems, namely, anaerobic sugar fermentation and aerobic respiration at low oxygen tension. Although growth on dinitrogen is optimal under photosynthetic conditions, the results show that reduction of dinitrogen is not obligatorily coupled to activity of the photosynthetic apparatus.