The electron transport chain of green sulfur bacteria—such as is present in the model organism Chlorobaculum tepidum—uses the reaction center bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663, which passes it on down an electron transport chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD to NADH. P840 regeneration is accomplished with the oxidation of a sulfide ion from hydrogen sulfide (or of hydrogen or ferrous iron) by cytochrome c.