Photosynthetic temperature adaptation during niche diversification of the thermophilic cyanobacterium Synechococcus A/B clade

Abstract

We take an in vivo fluorescence approach to investigate photosynthetic adaptation by ecologically divergent members of the A/B clade of the hot spring cyanobacterium Synechococcus, the most thermotolerant of which defines the upper thermal limit for photosynthesis. During Synechococcus diversification, both photosystem II and the light-harvesting phycobilisome have evolved greater thermostability as the group has invaded higher temperature habitats, particularly for the most thermotolerant lineage. This enhanced function at higher temperatures has come at the cost of reduced performance at lower temperatures, and these trade-offs contribute to niche specialization in the clade. Molecular evolutionary analyses revealed specific adaptive protein changes in the most thermotolerant lineage. Our study advances our understanding of the origins of Synechococcus diversity.

Figure 1

(a) Bayesian phylogeny of the 16S rRNA gene for strains in the Synechococcus A/B clade and outgroup taxon Synechococcus strain C9. Topology and branch lengths are from Ward et al. (2012). The scale bar is 0.05 nucleotide substitutions per site. All nodes in the tree have posterior probability support of 100%. (b) Temperature dependence of relative in vivo fluorescence monitored at 680 nm for Synechococcus strains excited at 437 nm. Strain colors are as in (a). Error bars are s.e. A relative fluorescence value of 1.0 corresponds to the following mean absolute fluorescence values (photons per s)±s.e.: 96 871±14 070.0 (C9); 23 575±1892.1 (OSB′); 42 808±3975.8 (OH20); 43 869±3975.8 (OH2); 21 692±690.3 (OH28). (c) Temperature dependence of relative in vivo fluorescence monitored at 680 nm for Synechococcus strains excited at 605 nm. Strain colors are as in (a). Error bars are s.e. A relative fluorescence value of 1.0 corresponds to the following mean absolute fluorescence values (photons per s)±s.e.: 315 815±19 215.7 (C9); 108 273±6220.4 (OSB′); 226 643±16 087.7 (OH20); 338 726±16 340.4 (OH2); 28 335±432.1 (OH28).