The Evolutionary Transition from Anoxygenic to Oxygenic Photosynthesis

This chapter reviews briefly the early geologic evidence for photosynthesis and the structural and functional differences between anoxygenic and oxygenic photosynthesis, and then goes on to discuss the evolutionary transitions that have led us to the current situation in which oxygenic photosynthetic organisms dominate the biosphere, and anoxygenic organisms have largely retreated to specific environmental niches. Understanding the historical milestone provides unique insight not only into the evolutionary process, but also into how organism-environment interactions effect global-scale changes, ranging from carbon cycling and sequestration to composition of the atmosphere and oceans. The sequence identity decreases and the structural similarity diminish with increase in evolutionary distance between proteins. The chapter also focuses on the Type II reaction centers (RCs) that are found in two classes of anoxygenic phototrophs: purple photosynthetic bacteria and green filamentous photosynthetic bacteria (RC II) as well as photosystem II of oxygenic photosynthetic organisms (PS II). A number of lines of evidence agree that early RC complexes were found in anoxygenic phototrophs and were probably protein homodimers. The transition from these primitive anoxygenic complexes with a simple protein complement and bacteriochlorophyll pigments to the oxygenic PS II with chlorophyll pigments, an Mn-containing an oxygen evolving complex (OEC), and a much more complex protein complement was a remarkably dramatic evolutionary development that is still very poorly understood.