Light-harvesting complexes surrounding a reaction centre (RC) harvest photons in the form of resonance energy, exciting chlorophyll pigments P870 or P960 located in the RC. Excited electrons are cycled from P870 to quinones QA and QB, then passed to cytochrome bc1, cytochrome c2, and back to P870. The reduced quinone QB attracts two cytoplasmic protons and becomes QH2, eventually being oxidized and releasing the protons to be pumped into the periplasm by the cytochrome bc1 complex. The resulting charge separation between the cytoplasm and periplasm generates a proton motive force used by ATP synthase to produce ATP energy. Electron donors for anabolism Purple bacteria are anoxygenic because they do not use water as electron donor to produce oxygen. Purple sulfur bacteria (PSB), use sulfide, sulfur, thiosulfate or hydrogen as electron donors. In addition, some species use ferrous iron as electron donor and one strain of Thiocapsa can use nitrite. Finally, even if the purple sulfur bacteria are typically photoautotrophic, some of them are photoheterotrophic and use different carbon sources and electron donors such as organic acids. Purple nonsulfur bacteria typically use hydrogen as an electron donor, but can also use sulfide at lower concentrations compared to PSB and some species can use thiosulfate or ferrous iron as electron donor. In contrast to the purple sulfur bacteria, the purple nonsulfur bacteria are mostly photoheterotrophic and can use a variety of organic compounds as both electron donor and carbon source, such as sugars, amino acids, organic acids, and aromatic compounds like toluene or benzoate. Purple bacteria lack external electron carriers to spontaneously reduce NAD(P)+ to NAD(P)H, so they must use their reduced quinones to endergonically reduce NAD(P)+. This process is driven by the proton motive force and is called reverse electron flow. Ecology Distribution Purple bacteria inhabit illuminated anoxic aquatic and terrestrial environments. Even if sometimes the two major groups of purple bacteria, purple sulfur bacteria and purple nonsulfur bacteria, coexist in the same habitat, they occupy different niches. Purple sulfur bacteria are strongly photoautotrophs and are not adapted to an efficient metabolism and growth in the dark. A different speech applies to purple nonsulfur bacteria that are strongly photoheterotrophs, even if they are capable of photoautotrophy, and are equipped for living in dark environments. Purple sulfur bacteria can be found in different ecosystems with enough sulfate and light, some examples are shallow lagoons polluted by sewage or deep waters of lakes,