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.