Cyanobacteria carry out photosynthesis using two photosystems as in green plants. Furthermore, other photosynthetic bacteria, like as the purple photosynthetic bacterium Rhodospirillum rubrum which have only a one photosystem reaction center. This can hold out cyclic electron transport, producing a proton gradient and thus synthesizing ATP (cyclic photophosphorylation). In other words, a noncyclic pattern of electron transport can be carried out in that the electrons from the cytochromes pass to NAD+ (rather than NADP as in green plants) to create NADH. The electron donor is, for instance, hydrogen sulfide (H2S) that produces sulfur (S). The Hydrogen gas (H2) and a various of organic compounds can also be used as electron donors by particular photosynthetic bacteria. Because H2O is not used as electron donor, no oxygen is formed.