0Phototrophic bacteria are species that use inorganic compounds as thier source of electrons (i.e. photolithotrophs). For example, Chromatium okenii uses H2S as its electron donor, oxidizing it to elemental sulfur.
H2S --->S +2e- +2H+
Some phototrophic bacteria use organic compounds such as fatty acids and alcohols as electron donors and are therefore photoorganotrophs. For example, Rhodospirillum rubrum can use succinate as an electron donor.
Succinate--->fumarate +2e- +2H+
Certain phototrophic bacteria are not restricted to being phototrophic. As indicated before, chemotrophs rely on chemical compounds rather than light for their energy, and under some circumstances a phototrophic bacterium can grow as a chemotroph. For example, in the absence of O2 (i.e., under anaerobic conditions) R. rubrum is dependent on light as its source of energy and lives as a photoorganotroph; however, in presence of O2 it can grow in the dark as a chemoorganotroph.
