Impact of genome content on the distribution and physiology of marine Cyanobacteria

The ocean plays a key role in the global carbon and nutrients cycles and account for 50% of the world's carbon dioxide uptake from the atmosphere converting it to oxygen. Two central players in the ocean carbon and nutrient cycles are the smallest but predominant marine cyanobacteria Prochlorococcus and Synechococcus. Together, they are responsible for 25.2% of the ocean's net primary productivity. Their abundance relies heavily on environmental nutrient concentrations, in particular nitrogen and phosphorus, temperature and light availability. Changes in environmental conditions are a threat to global ocean cycles and could potentially alter their activity, distribution and diversity in the oceans.;Genomic studies provide detailed information on the many genes that drive biogeochemical patterns and serve as an important step toward understanding the role of marine cyanobacteria in ocean environment. Moreover, we have evidence that biogeography linked with environmental variables can influence the diversity and distribution of marine cyanobacteria. Therefore, the goal of my dissertation has been to understand the impact of genome content on the distribution and physiology of Prochlorococcus and Synechococcus. I used a combination of laboratory experiments and field metagenomics to link variations in genome content with changes in physiology and distribution. I first examined how variations in growth rate affect the physiology of Synechococcus strains in a chemostat experiment under phosphate limitation. We found that there are physiological differences between strains related to growth at difference phosphate levels and we estimated new Ks,p value and C:N:P ratios for Synechococcus that can be use to better parameterize ocean biogeochemical models. We suggest that differences in genome content could potentially be responsible for physiological differences. Previous studies in natural populations have found differences in physiology between different strains of Prochlorococcus and Synechococcus but whole genome content variations have never been investigated in Synechococcus field populations. In order to assess genomic diversity associated with Synechococcus we developed a method for targeted metagenomic sequencing of marine cyanobacteria. We combined cell sorting and metagenomics to improve on current methods and resolve phylogenetically field populations of interest as well as specific genes. We then applied our method to assess the genomic diversity of Synechococcus populations in the Western North Atlantic Ocean. We found that we can distinguish spatial and temporal variations of Synechococcus clades and genome content that are correlated to ambient nutrient concentrations, temperature, and depths.;These results suggest that genome content along with environmental variables shape the diversity and distribution of marine cyanobacteria. Thereby, through my dissertation I linked genetic adaptation, physiology and ocean biogeochemical cycles to help understand how changes in ocean environment could affect the distribution and diversity of the most dominant and ecologically important marine cyanobacteria.