Early photosynthetic systems, such as those in green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules than water as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as electron donors. Green nonsulfur bacteria used various amino and other organic acids as electron donors. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly reducing at that time.
With a possible exception of Heimdallarchaeota, photosynthesis is not found in archaea. Haloarchaea are phototrophic and can absorb energy from the sun, but do not harvest carbon from the atmosphere and are therefore not photosynthetic. Instead of chlorophyll they use rhodopsins, which convert light-energy to ion gradients but cannot mediate electron transfer reactions.
In bacteria eight photosynthetic lineages are currently known:
*Cyanobacteria, the only prokaryotes performing oxygenic photosynthesis and the only prokaryotes that contain two types of photosystems (type I (RCI), also known as Fe-S type, and type II (RCII), also known as quinone type). The seven remaining prokaryotes have anoxygenic photosynthesis and use versions of either type I or type II.
*Chlorobi (green sulfur bacteria) Type I
*Heliobacteria Type I
*Chloracidobacterium Type I
*Proteobacteria (purple sulfur bacteria and purple non-sulfur bacteria) Type II
*Chloroflexota (green non-sulfur bacteria) Type II
*Gemmatimonadota Type II
*Eremiobacterota Type II