Geochemical data places the origin of bacterial life as we know it at approximately 4 billon years ago. The origin of oxygen evolving microorganisms, called cyanobacteria, is placed at about 3.5 billion years ago as based on oxidation records of the earths crust and on fossel records (reviewed by Xiong and Bauer, 2002).
Microbiological research has demonstrated that there are currently five known branches of microorganisms that are capable of undertaking chlorophyll based photosynthesis. These five branches are purple bacteria (i.e. Rhodobacter ), green sulfur bacteria (i.e. Chlorobium ), green gliding bacteria (i.e. Chloroflexus ), the Gram positive organism Heliobacteria, and the oxygen evolving cyanobacteria (i.e. Synechococcus ). Cyanobacteria are the only known bacterial group that are capable of evolving oxygen as a byproduct of photosynthesis. Phylogenetic analyses indicate that cyanobacteria are closely related to plant and algal chloroplasts which is the organelle that houses the photosystem in eukaryotic cells.
To obtain a better understanding of the origin of photosynthesis, we have undertaken detailed phylogenetic analyses of various photosynthetic genes that are present in all five branches of photosynthetic microorganisms and in chloroplasts. The results of this analysis demonstrated that purple bacteria represent the oldest lineage of photosynthetic microorganisms. This result has established that non-oxygen evolving (anoxygenic) photosynthesis predates oxygen evolving photosynthesis and also places the origin of photosynthesis as a very early metabolic process among microorganisms.
Additional phylogenetic approaches has allowed us to propose a model (figure 4 at right) on the origin of photosynthesis which states that photosynthesis is a process that was derived from cytochromes that predate components of the photosystem (Xiong and Bauer, 2002)
Origin of Microbial life
More recently, we have undertaken a more ambitious project centered on the origin of microbial life. For this analysis, we have teamed up with Dr. Lisa Pratt, a geo-microbiologist at IU that is undertaking geochemical analysis of a region of Western Oregon known as Warner Valley . This area has a series of shallow alkaline (pH 8 to 10) lakes and ponds that undergo frequent seasonal periods of drying (summer and fall) coupled with periods of hydration (winter and spring). These frequent seasonal fluctuations lead to significant variations in salinity. This fluctuating environmental condition of hydration is thought to mimic environmental events that occurred during early Earth history and during dehydration of the planet Mars.
One area that we have a particular interest in is Anderson lake. This is a very shallow lake that has exceptionally high concentration of arsenic. Phylogenetic analysis of the microbial biomass is ongoing using via 16S rRNA amplification and sequencing.
References:
Xiong, J., K. Inoue & C. E. Bauer. 1998. Tracking molecular evolution of photosynthesis by characterization of a major photosynthesis gene cluster from Heliobacillus mobilis. Proc. Natl. Acad. Sci. 95, 14851-14856.
Xiong J., K. Inoue, M. Nakahara & C. E. Bauer. 2000. Molecular evidence for the early evolution of photosynthesis . Science. 289, 1724-1730 (Featured on cover) {discussed in Science Perspective in same issue pp1703; in Trends in Plant Sciences 6, 4 (2001); in The Scientist, 14, 1 (2001)}
Xiong, J. & C. E. Bauer. 2002. A cytochrome b origin of photosynthetic reaction centers. J. Mol. Biol. 322, 1025-1037.
Xiong, J., & C. E. Bauer. 2002. Complex evolution of photosynthesis. Ann Rev. Plant Physiol. 53, 503-521.
