Photofermentation is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of photosynthetic bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion. Photofermentation differs from dark fermentation because it only proceeds in the presence of light.
For example, photo-fermentation with Rhodobacter sphaeroides SH2C (or many other purple non-sulfur bacteria) can be employed to convert small molecular fatty acids into hydrogen and other products.