UC San Diego

Marine cyanobacteria from the genus Synechococcus are found throughout the world's oceans and as primary producers, they have a significant role in the global carbon cycle. They are also genetically diverse with at least 10 described clades and physiologically diverse capable of motility, using different sources of nitrogen or phosphorus, or modifying the composition of their photosynthetic pigments, for example. As more is learned of their diversity, however, it has become less clear how their genetic diversity reflects their physiological diversity and the ecological niches that they occupy. By developing novel, culture-independent methods, biogeographic patterns were uncovered to provide a framework to link the genetic, physiological, and ecological diversity of marine Synechococcus. Quantitative PCR was used to show that the coastal Southern California Bight is dominated by two Synechococcus clades. Relative to each other, the abundance of these clades varied seasonally indicating that they occupied distinct ecological niches. Synechococcus biogeography was explored further using a novel high-throughput hybridization method (Luminex) to examine the distribution of sub-clades within the two co-existing clades. Even at this scale of diversity, ecological differentiation was apparent. Most interestingly, in mesotrophic environments of the Southern California Bight a sub-clade was abundant in deep water (50 m) while most other sub-clades preferred surface waters. Environmental metagenomic sequences were used to investigate the role of selection in shaping the diversity of Synechococcus populations. The large majority of genes have evolved under purifying selection. Genes that were well represented in environmental populations and may be essential for the population in the coastal Southern California Bight tended to be more evolutionarily conserved. Genes that may have been positively selected were mostly hypothetical genes and were more rare in the environment. For this dissertation, the diversity, biogeography, and evolution of Synechococcus were examined at a level of detail not previously available. The results of this research formed a foundation for further investigations into the genetic and environmental factors regulating the ecological functions and niches of Synechococcus populations. More generally, this research addressed the scales at which bacterial evolution and diversification occur, and how bacterial populations are adapted to their environment