Early evolution of photosynthesis.

Photosynthesis is the only significant solar energy storage process on Earth and is the source of all of our food and most of our energy resources. An understanding of the origin and evolution of photosynthesis is therefore of substantial interest, as it may help to explain inefficiencies in the process and point the way to attempts to improve various aspects for agricultural and energy applications. Awealth of evidence indicates that photosynthesis is an ancient process that originated not long after the origin of life and has evolved via a complex path to produce the distribution of types of photosynthetic organisms and metabolisms that are found today (Blankenship, 2002; Björn and Govindjee, 2009). Figure 1 shows an evolutionary tree of life based on smallsubunit rRNA analysis. Of the three domains of life, Bacteria, Archaea, and Eukarya, chlorophyll-based photosynthesis has only been found in the bacterial and eukaryotic domains. The ability to do photosynthesis is widely distributed throughout the bacterial domain in six different phyla, with no apparent pattern of evolution. Photosynthetic phyla include the cyanobacteria, proteobacteria (purple bacteria), green sulfur bacteria (GSB), firmicutes (heliobacteria), filamentous anoxygenic phototrophs (FAPs, also often called the green nonsulfur bacteria), and acidobacteria (Raymond, 2008). In some cases (cyanobacteria and GSB), essentially all members of the phylum are phototrop2hic, while in the others, in particular the proteobacteria, the vast majority of species are not phototrophic. Overwhelming evidence indicates that eukaryotic photosynthesis originated from endosymbiosis of cyanobacterial-like organisms, which ultimately became chloroplasts (Margulis, 1992). So the evolutionary origin of photosynthesis is to be found in the bacterial domain. Significant evidence indicates that the current distribution of photosynthesis in bacteria is the result of substantial amounts of horizontal gene transfer, which has shuffled the genetic information that codes for various parts of the photosynthetic apparatus, so that no one simple branching diagram can accurately represent the evolution of photosynthesis (Raymond et al., 2002). However, there are some patterns that can be discerned from detailed analysis of the various parts of the photosynthetic apparatus, so some conclusions can be drawn. In addition, the recent explosive growth of available genomic data on all types of photosynthetic organisms promises to permit substantially more progress in unraveling this complex evolutionary process. While we often talk about the evolution of photosynthesis as if it were a concerted process, it is more useful to consider the evolution of various photosynthetic subsystems, which have clearly had distinct evolutionary trajectories. In this brief review we will discuss the evolution of photosynthetic pigments, reaction centers (RCs), light-harvesting (LH) antenna systems, electron transport pathways, and carbon fixation pathways. These subsystems clearly interact with each other, for example both the RCs and antenna systems utilize pigments, and the electron transport chains interact with both the RCs and the carbon fixation pathways. However, to a significant degree they can be considered as modules that can be analyzed individually.