Genomics, structural and functional relationships, metabolism, physiology and ecology of chlorophototrophic bacteria
Photosynthesis, the chlorophyll-dependent conversion of light energy into chemical energy with the ensuing reduction of carbon dioxide to biomass, is arguably the most important biological process on Earth. Among prokaryotes, the ability to use chlorophylls to capture and convert light into biochemical energy was until very recently believed to occur in members of only five eubacterial kingdoms: Cyanobacteria, Proteobacteria, Chlorobi, Chloroflexi, and Firmicutes. We recently discovered a previously unknown chlorophototroph, Candidatus Chloracidobacterium thermophilum, which is a member of the poorly characterized kingdom Acidobacteria (http://www.sciencemag.org/cgi/content/abstract/317/5837/523). Current research in my laboratory focuses on a wide variety of topics in photosynthesis in bacteria, including structure-function relationships of proteins, biogenesis of the photosynthetic apparatus, gene regulation, and photosynthetic physiology. We principally study two model organisms, the unicellular, marine cyanobacterium Synechococcus sp. PCC 7002 and the moderately thermophilic green sulfur Chlorobaculum tepidum (Chlorobi), which can easily be manipulated genetically. Because Cyanobacteria perform oxygen-evolving photosynthesis but characterized Chlorobi are obligately photoautotrophic anaerobes, these two organisms provide an interesting contrast in physiology and metabolism. We additionally study natural phototrophic communities in Yellowstone National Park.
Figure 1. Left, Octopus Spring, an alkaline siliceous hot spring in the Lower Geyser Basin of Yellowstone National Park, WY. Right, microbial mat showing the upper, 1-2 mm green chlorophototrophic community from which Candidatus Chloracidobacterium thermophilum was isolated. The lower red layers are carotenoid-containing members of the anoxic community.
Long-term objectives of my laboratory are to understand the structure, function, assembly, and regulation of expression of bacterial photosynthetic apparatuses, principally those of members of the phyla Cyanobacteria and Chlorobi. To achieve these goals, we have obtained complete genomic sequences for all of the organisms we study. These data enable bioinformatics approaches that facilitate gene discovery and characterization. Genome sequences have been determined for 3 Cyanobacteria (6 in progress), 16 Chlorobi, 7 Chloroflexi, 11 Proteobacteria, and 1 member of the Acidobacteria (see http://bmb-it-services.bmb.psu.edu/bryant/lab/index.htm). We additionally use nextGen sequencing methods for transcription profiling of Synechococcus sp. PCC 7002 and C. tepidum, as well as for profiling gene expression patterns of the entire mat community from which Cab. thermophilum was isolated (see Figure 1). In a recent Multi-omics Analysis Experiment (MOAE, the “Mother Of All Experiments”), >15,000 genes were tracked in samples collected at hourly intervals over one diurnal cycle.
The photosynthetic apparatus of cyanobacteria closely resembles that found in the chloroplasts of higher plants. We have developed sophisticated genetic tools to analyze gene function in Synechococcus sp. PCC 7002, and these have been used for metabolic engineering to improve biosolar hydrogen, biomass, and biofuels production in this robust cyanobacterium. The Chlorobi are specifically adapted for survival in low-light environments and are important in reducing carbon and nitrogen while oxidizing sulfide in anoxic environments. We have developed a reliable natural transformation method for C. tepidum and have used this capability to define the pathways for bacteriochlorophyll and carotenoid biosynthesis in this organism as well as to characterize the structure of chlorophylls in the light-harvesting organelle, the chlorosome (Figure 2).
Figure 2. Single and double-layer model of bacteriochlorophyll d structure in chlorosomes of a bchQRU mutant of Chlorobaculum tepidum; see Ganapathy et al. (2009) Proc. Natl. Acad. Sci. USA 106: 8515-8530 for details).
Students and postdoctoral associates apply a broad combination of methods including microbial ecology, microbial physiology, genomics and bioinformatics, molecular genetics, protein biochemistry, and spectroscopic methods. We collaborate extensively with Dr. John H. Golbeck, Dr. David M. Ward of Montana State University (see http://landresources.montana.edu/dward/), and Dr. G. Charles Dismukes, John W. Peters, Matthew C. Posewitz and other members of MURI project on biosolar hydrogen production (see http://www.princeton.edu/~biosolar/). NSF, DOE, NASA, Air Force Office of Sponsored Research, Pacific Northwest National Laboratory (DOE), and Penn State University currently provide financial support for our research. More detailed information about the research projects in my laboratory can be found on our laboratory website http://bmb-it-services.bmb.psu.edu/bryant/lab/index.htm) and through our recent publications.