BIOGEOGRAPHY AND NESTED PATTERNS OF GENETIC DIVERSITY IN THE DIATOM THALASSIOSIRA ROTULA

Diatoms are some of the most biogeochemically important organisms on our planet, responsible for up to 40% of primary production in the ocean. Diatoms are also the most diverse group of algae, exhibiting high levels of cryptic speciation, and remarkable clonal diversity nested within species and populations. As single cells within a dynamic fluid environment, diatoms possess the potential to disperse throughout the globe. Yet, despite the global dispersal potential of these ecologically important organisms, little is understood about the structure and distribution of diatom diversity throughout the globe, or its ecological significance. The goal of this work was to explore nested patterns of diversity in the diatom Thalasiosira rotula by mapping its distribution over global geographic space, capturing the variations in genetic composition over time, and inferring its ecological significance. A series of molecular markers was used to target different scales of genetic subdivision from the cryptic species to the population to the individual. Ribosomal genes (rDNA) were used to define lineages nested within the species, and clarify the relationship between T. rotula and its cryptic sister species, Thalassiosira gravida. Microsatellite markers were developed for T. rotula, and offered a high-resolution glimpse into the population-level subdivisions within the species. Microsatellite analysis was robust enough to identify individuals within these populations. Ribosomal DNA analysis confirmed that T. rotula and T. gravida are, indeed, separate species. These genes also demonstrated that T. rotula is subdivided into three lineages diverged by 0.6 ± 0.3% at the internal transcribed spacer gene. Each lineage exhibits a unique geographic distribution, and harbors high levels of physiological diversity. Over 400 global samples of T. rotula collected from 8 locations in 2010 offered a ‘snapshot’ of diversity nested within the species. Microsatellites revealed high levels of population divergence among these global samples. Variation over time at a single site was as great as variation between samples collected thousands of kilometers apart. Within a single site, Narragansett Bay, changes in population structure over the course of three years demonstrated that populations are comprised of many clonal lineages, harbor high adaptive potential, and may respond quickly to changes in the marine environment and local ecology. The structure of diversity among global samples was used to infer the importance of dispersal versus environment in isolating populations and maintaining diversity. At all levels of molecular diversity, geographic distance demonstrated only a weak relationship to genetic distance. This suggests that global connectivity among populations is not limited by geographic distance. Nonetheless, T. rotula does not represent a single panmictic population. Significant genetic structure was detected on all levels of molecular diversity examined. A series of Mantel tests revealed that temperature and chlorophyll a were most highly correlated with genetic relatedness among global populations, despite our exploration of other variables including nutrient concentration, temperature, irradiance, salinity, and cell abundance. This suggests that certain populations may be better adapted to thrive in competitive high-chorophyll phytoplankton ‘bloom’ periods, whereas others may only thrive during non-bloom periods. Taken together, these data suggest that environmental and ecological selection may heavily influence the genetic structure of diatom populations, and help to explain the extraordinary diversity harbored within and among diatom species.