Evolution of thermotolerance in hot spring cyanobacteria of the genus Synechococcus

The extension of ecological tolerance limits may be an important mechanism by which microorganisms adapt to novel environments, but it may come at the evolutionary cost of reduced performance under ancestral conditions. We co mbined a comparative physiological approach with phylogenetic analyses to s tudy the evolution of thermotolerance in hot spring cyanobacteria of the ge nus Synechococcus. Among the 20 laboratory clones of Synechococcus isolated from collections made along an Oregon hot spring thermal gradient, four di fferent 16S rRNA gene sequences were identified, Phylogenies constructed by using the sequence data indicated that the clones were polyphyletic but th at three of the four sequence groups formed a clade. Differences in thermot olerance were observed for clones with different 16S rRNA gene sequences, a nd comparison of these physiological differences within a phylogenetic fram ework provided evidence that more thermotolerant lineages of Synechococcus evolved from less thermotolerant ancestors. The extension of the thermal li mit in these bacteria was correlated with a reduction in the breadth of the temperature range for growth, which provides evidence that enhanced thermo tolerance has come at the evolutionary cost of increased thermal specializa tion. This study illustrates the utility of using phylogenetic comparative methods to investigate how evolutionary processes have shaped historical pa tterns of ecological diversification in microorganisms,