SAR11 is an ancient and diverse clade of heterotrophic bacteria that are abundant throughout the world's oceans, where they play a major role in the ocean carbon cycle. Correlations between the phylogenetic branching order and spatiotem-poral patterns in cell distributions from planktonic ocean environments indicate that SAR11 has evolved into perhaps a dozen or more specialized ecotypes that span evolutionary distances equivalent to a bacterial order. We isolated and sequenced genomes from diverse SAR11 cultures that represent three major lineages and encompass the full breadth of the clade. The new data expand observations about genome evolution and gene content that previously had been restricted to the SAR11 Ia subclade, providing a much broader perspective on the clade's origins, evolution, and ecology. We found small genomes throughout the clade and a very high proportion of core genome genes (48 to 56%), indicating that small genome size is probably an ancestral characteristic. In their level of core genome conservation, the members of SAR11 are outliers, the most conserved free-living bacteria known. Shared features of the clade include low GC content, high gene synteny, a large hypervariable region bounded by rRNA genes, and low numbers of paralogs. Variation among the genomes included genes for phosphorus metabolism, glycolysis, and C1 metabolism, suggesting that adaptive specialization in nutrient resource utilization is important to niche partitioning and ecotype divergence within the clade. These data provide support for the conclusion that streamlining selection for efficient cell replication in the planktonic habitat has occurred throughout the evolution and diversification of this clade.
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