Phylogenetic tree of the sequenced cyanobacterial strains and prx information. A Maximum likelihood tree for 36 sequenced cyanobacteria constructed based on 16s rRNA was reconstructed as described in the Methods . The General Time Reversible (GTR) substitution model was selected assuming an estimated proportion of invariant sites and four gamma-distributed rate categories to account for rate heterogeneity across sites. The reliability on internal branches was assessed using the bootstrapping method (400 bootstrap replicates). Percentages in brackets represent total PRXs as a percentage of the total proteins. Prochlorococcus marinus MIT 9313 are absent from this tree because they are partial genomes and have no complete rRNA genes.