PHYLOGENOMIC DATING AND EARLY EVOLUTIONARY PATTERNS IN THE CYANOBACTERIA

We used a phylogenomic approach to study the evolution of the Cyanobacteria in the context of the early geologic record. A well-resolved cyanobacterial backbone tree was constructed using multiple genes from whole genome sequences. Additional taxa were added using a seperate alignment that contained morphological characters, 16S and 23S rRNA, rpoC, rpoD, tufA, and gyrB genes. Additional morphological characters (e.g., presence or absence of sheath, unicellular vs. filamentous growth, presence or absence of heterocysts) were coded as characters (where absence=0; presence=1), and mapped onto the backbone cyanobacterial tree. The distribution of traits was compared with those found in the fossil record.

Our analyses show that the earliest cyanobacterial lineages lived in freshwater environments, and that multiple cyanobacterial lineages later diverged into marine (also brackish and hypersaline) environments. This suggests that paleontologists might find the earliest cyanobacterial microfossils in freshwater Paleoproterozoic deposits. However, these earliest cyanobacteria also had small diameter coccoid cells (< 2 um), and thus distinguishing them from other prokaryotic cells may prove difficult.

Several morphological traits were found to have later arisen in multiple cyanobacterial lineages independently: sheath, filamentous growth, nitrogen fixation, thermophily, motility, use of sulfide as an electron donor, and large cell diameters. Not surprisingly, several complex traits had a single ancestor: hormogonia, heterocysts, and akinetes. Within the Nostocales, hormogonia and heterocysts arose before akinetes.

An earlier study suggested that the cyanobacteria first appeared just before the rise in oxygen at 2.35 Ga. Cyanobacterial microfossils first appear ~ 2.15 Ga, as solitary and colonial coccoids in shallow water platform deposits. Geochemical studies indicate a highly stratified ocean with shallow waters dominated by riverine input and deeper waters by hydrothermal input. If the early cyanobacteria lived in freshwater, either they arose in lakes and streams and later colonized shallow water marine environments ~2.2 Ga, or perhaps the shallow water platform "marine" environments were not particularly saline in composition.