HR: 1340h
AN: B33B-0261    [Abstracts]
TI: Phylogenomic Methods to Guide Paleontological Searches for the Early Cyanobacteria
AU: * Blank, C E
EM: blank@levee.wustl.edu
AF: Washington University, Dept. of Earth & Planetary Sci Campus Box 1169 One Brookings Dr., Saint Louis, MO 63130 United States
AB: Phylogenomic methods can help paleontologists target their searches for early microbial microfossils and potentially help them better interpret the early fossil record. In this study, the deep-branching relationships in the cyanobacteria were resolved using whole genome sequences, multiple genes for taxa lacking genomes, and intein presence/absence in the DnaE protein. Once a framework tree was produced, characters were mapped onto the tree. Characters included morphology (unicellular vs. filamentous), habitat (marine vs. freshwater), metabolism (use of sulfide as electron donor, nitrogen fixation), presence/absence of complex morphological traits (akinetes, heterocysts, hormogonia), salt tolerance, and thermal tolerance. It was found that the earliest cyanobacteria were unicellular coccoids, with cell diameters < 2 microns, that lived in freshwater environments. This suggests that paleontologists should focus their searches for the earliest cyanobacteria to freshwater deposits (lakes, streams) and to small diameter coccoids (not mats, not filaments). The earliest "cyanobacterial" microfossils (Eosynechococcus and Eoentophysalis) are large-diameter coccoids found in shallow marine platform carbonates. Because these cells have large diameters, if they were cyanobacteria one would also expect to see their sister taxa in the fossil record (i.e., large-diameter filamentous forms with sheaths, also akinetes). Because these are not found until 2.0 Ga (and akinetes until 1.5 Ga), this suggests that these earliest microfossils are not cyanobacteria. There are several instances in the cyanobacterial tree where ancestors with low salt tolerance gave rise to lineages that grow in brackish, marine, and/or hypersaline environments. This suggests that either the cyanobacteria first originated on continents and later colonized more saline environments, or that the cyanobacteria first originated in shallow "seas" that were not very saline but gradually became more saline by about 2.0 Ga. Because the continents were likely harsh environments (due to lack of an ozone layer and increased chemical and physical weathering), and because there is geochemical evidence of chemical stratification of the early oceans, the latter hypothesis (that the early shallow seas were not very saline) deserves further attention.
DE: 9619 Precambrian
DE: 4840 Microbiology
DE: 4803 Bacteria
DE: 3030 Micropaleontology
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting