THE EVOLUTIONARY DIVERSIFICATION OF CYANOBACTERIA: PALENOTOLOGICAL AND MOLECULAR-PHYLOGENETIC PERSPECTIVES

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. In tandem, paleontology and molecular phylogeny provide constraints on the timing and pattern of cyanobacterial diversification.

16SrRNA and rbcL (large subunit of RubisCO) genes were isolated from 16 species distributed among 15 cyanobacterial genera, with particular care taken to represent the known diversity of filamentous taxa. Phylogenetic trees constructed by several methods support the hypothesis that cyanobacteria capable of cell differentiation (heterocysts and akinetes) form a monophyletic clade among undifferentiated filaments and coccoid forms. The phylogenies also support the view that the Stigonematales, traditionally recognized as heterocystous cyanobacteria with complex branching patterns, are nested within the broader grouping of heterocyst- and akinete-bearing taxa.

The geological record may provide both upper and lower bounds on the origin of heterocystous cyanobacteria. Akinetes are common in ca. 1500 Ma cherts from tidal flat carbonates of the Billyakh Group, Siberia. Rare akinetes have also been found in silicified carbonates of the ca. 1650 Ma Amelia Dolomite, northern Australia, where they occur with other microfossils representing a broad cross-section of cyanobacterial diversity. What may be the earliest known akinetes are preserved in 2100 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) 2400-2200 Ma. By inference, earlier branches of the cyanobacterial tree diverged during the Archean or earliest Proterozoic, although the geological record of this interval is difficult to interpret. Molecular fossils suggest that the cyanobacteria evolved at least 2700 Ma, and isotopic data are consistent with, although not indicative of, a much earlier origin. An improved microfossil record or the identification of clade-specific biomarkers will be necessary to tie the Archean record closely to molecular phylogenies.