Purple photosynthetic bacteria are metabolically versatile, anoxygenic phototrophs that produce bacteriochlorophylls a or b and display a wide range of metabolic lifestyles, which are reflected in their diverse range of habitats. Under oxic conditions, energy is derived from aerobic respiration, and the synthesis of photosynthetic pigments, and pigment-binding proteins, is repressed; when conditions shift to anoxic, the ultrastructure of the cytoplasmic membrane of purple bacteria changes, invaginations forming towards the inside of the cell creating intracytoplasmic membranes (ICMs). The photosynthetic machinery for the light-dependent reactions is housed in these ICMs. Light-harvesting (LH) antenna complexes absorb light energy, which is then transferred through a network of pigment–protein complexes, eventually promoting charge separation in the reaction centre, ultimately resulting in the formation of a proton motive force, used to drive ATP synthesis. ATP is then used to reduce inorganic carbon into organic compounds in the Calvin-Benson-Bassham cycle. Purple bacteria have evolved highly intricate assemblies of pigment–protein complexes to efficiently carry out this process. The assembly and organisation of these complexes differ among different species. In this chapter, we will discuss the structure, function and organisation of the photosynthetic components and the mechanisms underlying the photosynthetic process.
