Phototrophy and phototrophs

Phototrophs are organisms that utilize light as an energy source, and two types are known in Nature. Retinalophototrophs employ retinal-binding, rhodopsin-like proteins (e.g., bacteriorhodopsin or proteorhodopsin) and light to produce stored potential energy in the form of ion gradients across the cytoplasmic membrane (either H⁺, Na⁺ or Cl^-). Retinalophototrophs occur in all three domains of life and are widespread among bacterial and archaeal taxa. Chlorophototrophs use chlorophyll (Chl)- or bacteriochlorophyll (BChl)-containing reaction center proteins to perform light-induced redox chemistry, i.e., photochemistry, which leads to oxidation of a (B)Chl molecule and the reduction of an electron acceptor, either a quinone or an Fe/S cluster. Subsequent dark electron transfer reactions produce stable reductants (quinols, reduced ferredoxin, or NADPH) as well as proton motive force that can be used to produce ATP. Seven bacterial phyla (and no archaea) contain members that can perform Chl-based phototrophy. This article briefly introduces the two types of phototrophy and describes the occurrence of these processes among archaea and bacteria, with an emphasis on Chl-based phototrophy and chlorophototrophs.