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IntroductionPhotosynthesis is arguably the most important biological process on Earth. The Bryant laboratory studies members of four phyla of phototrophic bacteria: Cyanobacteria, Chlorobi (green sulfur bacteria), Chloroflexi (filamentous anoxygenic phototrophs), and the newly discovered Acidobacteria. As described in more detail elsewhere on this web site, the Bryant lab has recently played a leading role in obtaining genome sequences for organisms that belong to these four phyla. Genome sequences for 23 chlorophototrophs (the majority fully sequenced, closed and annotated) will have been made publicly available by the end of 2007. Only two mechanisms for converting solar radiation into chemical energy have been described (see Bryant and Frigaard, 2006). In the first process, which is dependent upon chlorophyll-containing protein complexes known as photochemical reaction centers, light causes photochemical oxidation of chlorophyll and ensuing electron transfer reactions, which secondarily are coupled to protonmotive force generation for ATP synthesis. The majority of autotrophic carbon fixation on Earth is the result of chlorophyll-based photoautotrophy—i.e., photosynthesis. Chlorophototrophic bacteria are believed to account for as much as one half of total autotrophic CO2 fixation. In the second mechanism of light energy conversion, which is dependent upon membrane-bound proteins (bacteriorhodopsin or proteorhodopsin) with a covalently bound retinal chromophore, light energy directly leads to proton translocation across a biological membrane, and the resulting protonmotive force is used for ATP synthesis without redox chemistry. Although retinalophototrophs can convert light into biochemical energy, no retinalophototroph has yet been shown to use carbon dioxide as its sole energy source, and thus these organisms are phototrophs but are not photosynthetic. Chlorophyll-based phototrophy is a uniquely bacterial invention—no archaeal organism has yet been shown to synthesize chlorophyll. There is significant diversity among chlorophototrophs with respect to photochemical reaction centers as well as their light-harvesting systems (see Fig. 1).
Although there are at least 25 major bacterial phyla (kingdoms), until very recently, only five were known to contain members that were phototrophic through the use of chlorophyll. The cyanobacteria and purple bacteria (members of the Proteobacteria) were discovered in the 19th century, and green sulfur bacteria (Chlorobi) were discovered by Nadson in 1906. In the ensuing 100 years, only two additional phyla with members that could synthesize chlorophylls were discovered. Pierson and Castenholz (1974) reported the discovery of Chloroflexus aurantiacus, a photosynthetic Chloroflexi, and in 1983 Gest and Favinger reported the discovery of Heliobacillus mobilis (a photoheterotrophic anaerobe that is a member of the Firmicutes). During a short sabbatical visit in the laboratory of collaborator David Ward in 2007, Don Bryant obtained evidence for a new chlorophototroph in metagenomic DNA sequences derived from the phototrophic microbial mat communities of Octopus and Mushroom Springs. This discovery ultimately led to the isolation and initial characerization of the first phototrophic member of the phylum Acidobacteria: Candidatus Chloroacidobacterium thermophilum. During the last 26 years at Penn State, the Bryant laboratory has studied diverse and wide-ranging aspects of the physiology and metabolism of phototrophic bacteria. Recently, there has been a strong emphasis on the identification and characterization of novel genes, which encode proteins that function in biochemical pathways and processes that can reveal new insights into the structure, function and biogenesis of the photosynthetic apparatuses of these organisms. As detailed elsewhere on this site, we have recently described the pathways for carotenoid biosynthesis in green sulfur bacteria and the cyanobacterium Synechococcus sp. PCC 7002, as well as the pathway for bacteriochlorophyll c synthesis in green sulfur bacteria. A recent and expanding interest has been the application of the 35 years of working experience with photosynthetic bacteria to biosolar hydrogen, biomass, and biofuels production. Bryant has collaborated for more than 20 years with Dr. John H. Golbeck, and several joint research projects are ongoing. Since 2005 Bryant has also collaborated extensively with Dr. David M. Ward (Montana State University) and with other members of the Thermal Biology Institute and NSF-Research Coordination Network at Montana State. These collaborations have allowed Bryant and lab members to begin to explore aspects of microbial diversity and microbial ecology through metagenomics, bioinformatics, traditional enrichment culture methods, and biochemical approaches. Click on all six phylum names to view project page of each phylum with chlorophototroph. (Internal and External Links)
Other projects: Biosolar hydrogen production using cyanobacteria or engineered PS I |
