Gene transfer history of carbon fixation proteins constrains marine cyanobacteria divergence times

Abstract

Carboxysomes provide an avenue for narrowing the timing of evolutionary events in groups of cyanobacteria that are ecologically dominant in modem marine environments - groups that may have an integral role in oxygenating the Earth's atmosphere. Here I show that using concatenated phylogenies of carbon fixation proteins better informs the horizontal gene transfer event that brought carboxysomes from purple sulfur bacteria into marine cyanobacteria and that this gene history aids in constraining the evolutionary timing of carbon fixation. Genes encoding the proteins for the a-carboxysomal shell as well as RuBisCO and carbonic anhydrase are co-located on the genomes of various cyanobacteria in the Prochlorococcus and Synechococcus groups. Previous studies have shown that these genes were likely horizontally transferred together from Chromatiales (purple sulfur bacteria), a group of phototrophic Gammaproteobacteria. While many of these genes are highly conserved and thus yield poorly resolved phylogenies, their concatenation clarifies a shared evolutionary history. This work integrates gene transfer with molecular clock calibration methods to determine divergence times. Accordingly, I evaluate the relationship between atmospheric evolution and the ecology of important groups of phototrophs.