Figure 1. An integral membrane-localized photosynthetic unit from an anoxygenic photosynthetic bacterium, comprising a reaction centre (RC) surrounded by 15 light-harvesting complexes I (LHI), and each LHI surrounded by eight light-harvesting complexes II (LHII). RC taken from Rhodopseudomonas viridis (Deisenhofer and Michel, (1989)). Conserved helices from the L and M polypeptides of the RC are drawn. The special pair bacteriochlorophylls, and bacteriophaeophytins are also drawn. The stoichiometry is 1:15:120, for RC:LHI:LHII, respectively. Light can be captured by any of the three photosynthetic complexes. When captured by the LHII or LHI complexes the energy can resonate within the complexes until an RC⁰ is available, even an RC⁰ within a different photosynthetic unit.

Figure 2. A water ecosystem. Gradients of light, oxygen and redox determine the localization of the different groups of photosynthetic bacteria. Oxygenic photosynthesis occurs at the surface, carried out by plants, algae and cyanobacteria. They absorb most light, leaving near IR and IR radiation to go through to the deeper layers. Anoxygenic photosynthesis, carried out by bacteria, occurs when oxygen becomes unavailable, in the deeper layers. h, light; IR, infrared; LH, light-harvesting; mV, millivolts; nm, nanometres; PS, photosynthesis; RC, reaction centre. Reprinted with modifications from Pfennig (1978) with the kind permission of Plenum Press.