INVITED: PATTERN AND TIMING OF CYANOBACTERIAL EVOLUTIONARY DIVERSIFICATION

Cyanobacteria have played a significant role in Earth history as primary producers and the ultimate sources of atmospheric oxygen. Accordingly, an understanding of cyanobacterial evolution provides a key to the elucidation of early biological and environmental history. Molecular fossils suggest that cyanobacteria appeared no later than 2,700 Ma. However, to date, how and when the group diversified has remained unclear. In tandem, paleontology and molecular phylogeny provide constraints on the timing and pattern of cyanobacterial diversification. 16SrRNA, rbcL, hetR genes were isolated and sequenced from 20 species distributed among 15 cyanobacterial genera, with particular care taken to represent the known diversity of filamentous taxa. Cyanobacteria form a morphologically diversified group. In traditional classifications, morphology has been used to divide the clade into five subsections. Particularly in filamentous cyanobacteria of subsections IV and V (formerly orders Nostocales and Stigonematales), vegetative cells can differentiate in heterocysts or akinetes in response to the environmental conditions. Phylogenetic trees constructed by NJ, MP, and ML methods support the hypothesis that cyanobacteria capable of cell differentiation form a monophyletic clade among undifferentiated filaments and coccoid forms. The geological record may provide both upper and lower bounds on the origin of heterocystous cyanobacteria. Akinetes are common in ca. 1,500 Ma cherts from tidal flat carbonates of the Billyakh Group, Siberia. Rare akinetes have also been found in silicified carbonates of the ca. 1,650 Ma Amelia Dolomite, northern Australia. What may be the earliest known akinetes are preserved in 2,000 Ma Francevillian cherts from West Africa. Geochemical evidence suggests that oxygen first reached levels that would compromise nitrogen-fixation (and hence select for heterocyst differentiation) 2,400-2,200 Ma. Integrating phylogenetic analyses and geochemical data, it is suggested that heterocyst- and akinete-bearing cyanobacteria diverged once between 2,400 and 2,000 Ma. This constraint on the timing of a relatively late branch point within the cyanobacteria provides a calibration point that may illuminate molecular evolutionary studies of deep nodes in the Tree of Life.