Fotosintesis

Review

TRENDS in Microbiology

Vol.14 No.11

Prokaryotic photosynthesis and phototrophy illuminated
Donald A. Bryant1 and Niels-Ulrik Frigaard2
1 2

Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA Institute of Molecular Biology and Physiology, University of Copenhagen, Sølvgade 83H, 1307 Copenhagen K, Denmark

Genomesequencing projects are revealing new information about the distribution and evolution of photosynthesis and phototrophy. Although coverage of the five phyla containing photosynthetic prokaryotes (Chlorobi, Chloroflexi, Cyanobacteria, Proteobacteria and Firmicutes) is limited and uneven, genome sequences are (or soon will be) available for >100 strains from these phyla. Present knowledge ofphotosynthesis is almost exclusively based on data derived from cultivated species but metagenomic studies can reveal new organisms with novel combinations of photosynthetic and phototrophic components that have not yet been described. Metagenomics has already shown how the relatively simple phototrophy based upon rhodopsins has spread laterally throughout Archaea, Bacteria and eukaryotes. In thisreview, we present examples that reflect recent advances in phototroph biology as a result of insights from genome and metagenome sequencing. Photosynthesis and phototrophy Photosynthesis is arguably the most important biological process on Earth, and only two mechanisms for collecting light energy and converting it into chemical energy have been described (Box 1). The first mechanism, which is dependentupon photochemical reaction centers (RCs; see Glossary) that contain (bacterio)-chlorophyll [(B)Chl], is found in five bacterial phyla: Cyanobacteria, Proteobacteria, Chlorobi, Chloroflexi and Firmicutes. All currently described Chlorobi and Cyanobacteria strains are photoautotrophs but only some strains of Chloroflexi [filamentous anoxygenic phototrophs (FAPs)], Proteobacteria (purple sulfur andpurple non-sulfur bacteria) and Firmicutes (heliobacteria) are phototrophic (Figure 1). The second mechanism employs rhodopsins, retinalbinding proteins that respond to light stimuli [1]. Several homologous types of rhodopsins are known in microbes and include energy-conserving transmembrane proton pumps [bacteriorhodopsin (BR), proteorhodopsin (PR), xanthorhodopsin] (Figure 2), transmembrane chloridepumps (halorhodopsins) and light sensors (sensory rhodopsins) [1,2]. Here, we review how genome and metagenome sequencing studies are providing new insights into the physiology, metabolism and evolution of the organisms that perform these two processes for the capture of light energy.
Corresponding author: Bryant, D.A. (dab14@psu.edu). Available online 25 September 2006.
www.sciencedirect.comGlossary
Anoxygenic photosynthesis: photosynthesis performed by organisms that do not evolve oxygen; it uses electron donors other than water for carbon dioxide reduction. Bacteriorhodopsin (BR): a rhodopsin first identified in haloarchaea; translocates protons to the periplasm after light-induced isomerization of retinal. Chlorobi: bacterial phylum that includes the green-colored andbrown-colored green sulfur bacteria; these bacteria have type 1 reaction centers (containing BChl a and Chl a) and chlorosomes containing BChl c, d or e. They fix carbon by the reverse tricarboxylic cycle and oxidize sulfide, sulfur, thiosulfate, Fe2+ or H2. Chloroflexi: bacterial phylum that includes the filamentous anoxygenic phototrophs (FAPs), formerly known as the green gliding or green filamentousbacteria. Cyanobacteria: bacterial phylum that includes all oxygen-evolving photosynthetic bacteria; they have Chl a-containing type 1 and type 2 reaction centers and fix carbon by the reductive pentose-phosphate (Calvin–Benson–Bassham) cycle; most have phycobilisomes as light-harvesting antennae (but see Prochlorophytes). Filamentous anoxygenic phototrophs (FAPs): Chloroflexi that have BChl...