These photosynthetic bacteria have no respiratory machinery for converting extra NADH into ATP. However, they can control the relative amount of ATP and NADH they make by a kind of "short circuit” of photosynthesis. The process diagrammed on the previous page is called noncyclic photophosphorylation, since ADP is phosphorylated to ATP by light energy, without recycling electrons. Reducing power is continually used in making NADH, so an external source of reducing power is constantly needed. The bacteria also can send their electrons back around the circuit, passing them to cytochrome b6, and from there to some member of the original electron-transport chain. This process is termed cyclic photophosphorylation (right), and requires no H₂S but consequently produces no NADH. It appears to have an extra site of ATP synthesis to take advantage of the larger free energy drop between excited and unexcited chlorophyll. The mix between the noncyclic and the cyclic processes depends on the bacterium's relative need at the time for simple energy, or for reducing power for synthesis.

The other class of photosynthetic bacteria is the purple nonsulfur bacteria, which do not use H₂S as a source of reducing power, and manage quite well with cyclic photophosphorylation. This may be possible because they have made a marvelous invention: They have a citric acid cycle and respiratory machinery, and can function quite well as oxygen respirers if kept in the dark, although they much prefer to obtain their ATP energy from photosynthesis. When operating photosynthetically, they apparently use NADH from the citric acid cycle as one source of reducing power for synthesis.