Photosynthetic Bacteria - Some microscopic algae also produce H₂ when exposed to, particularly, low levels of sunlight. The photosynthetic apparatus splits water molecules into H₂ and O₂ most likely as follows. The photo system I produces reduced ferredoxin; the ferredoxin is then reoxidized and protons (H⁺) act as electron acceptors to produce H₂.

The efficiency of H₂ production is reasonable at low light intensities (about 15% of the energy is stored as chemical energy in form of H₂). But at higher and more realistic light intensities, the efficiency is much lower. It is imperative that this problem is suitably solved for making this route of H₂ production of commercial interest.