Carrine Blank

Washington Univ.

3 October, 2003

Using the geochemical record to date divergences on the bacterial and archaeal trees - reconstructing microbial communities on the Archean Earth.

Using whole genome sequences that have recently become available, we are now able to produce highly resolved phylogenetic trees of the prokaryotes (the Bacteria and Archaea). We can use the order of ancient divergence events on this tree to better understand the changes in the geologic and geochemical record of the Archean and Paleoproterozoic Earth, and similarly to assign age constraints for the origination of these lineages. Using these comparisons, we can, for the first time, assign several age constraints to major lineages in the Archeal domain - a group of prokaryotes that, with the exception of the methanogens, leave few distinctive marks in the geologic record. We can also assign potential ages to the origination of major lineages in the Bacterial domain, such as the origination age of the Cyanobacteria, the sulfate reducing bacteria, and metal sulfide oxidizing bacteria. These ages could help us understand why the early Earth experienced rapid oxidation around 2.2 Ga, why there was a rapid onset of sulfur isotopic fractionation around 2.4 Ga, and why then sulfate increased in the oceans after 2.2 Ga. These ages can also help us better understand when biogeochemically important microbial metabolisms, such as nitrate reduction, sulfur oxidation, methanogenes, and photosynthesis, may have originated.