Bacterial photosynthesis is often anaerobic. Instead of using H2O like plants as the electron donor for photosynthesis, purple and green sulfur bacteria use H2S. Some nonsulfur photosynthetic bacteria utilize organic compounds as electron donors.

Contemplating the basic chemistry of anaerobic photosynthesis is a good exercise for gaining fluency in bioenergetics because it is just unfamiliar enough to make you see things in new ways.