The eurcaryotic pattern of two-center photosynthesis yielding ATP and NADPH, in which water is broken down and O2 is released, is followed also by blue-green algae but not by bacteria.

Purple nonsulfur bacteria avoid the need for a reducing agent by running the same electrons around again and again in cyclic photophosphorylation, but then must depend on a citric acid cycle for production of reducing power in the form of NADH (not NADPH).

They also possess a respiratory chain and, if grown aerobically in the dark, can obtain energy from glycolysis, the citric acid cycle, and O2, respiration just as we do. They are unique among bacteria in combining photosynthesis with respiration, and appear to be a metabolic halfway house on the road to blue-green algae and eucaryotes.

The purple and green sulfur bacteria are less versatile, and depend on noncyclic photophosphorylation for production of both ATP and NADH. This demands a source of reducing equivalents; lacking Photocenter II, they use H2S or H2, both of which are intrinsically stronger reducing agents than H20.

These sulfur bacteria are compulsory anaerobes that are poisoned by an oxygen atmosphere.

Once again the biochemical evidence suggests that early life arose under conditions where free oxygen was absent, but where hydrogen and hydrogen sulfide might be found.