Genetically distinct populations of a diatom co-exist during the North Atlantic spring bloom

The annual North Atlantic spring bloom influences the ecology and biogeochemistry of the Atlantic Ocean. Diatoms dominate the peak of the bloom and significantly impact productivity and export of organic carbon from the bloom. Despite their key role in a yearly event with global impacts, the genetic diversity and population structure of diatoms that comprise the bloom are unknown. Here, we investigated the population genetics of the diatom Thalassiosira gravida sampled during the 2008 North Atlantic Bloom Experiment using newly developed microsatellite markers. High levels of genetic diversity (clonal diversity: 0.970; expected heterozygosity: 0.884) were observed across all water samples and did not change during the bloom. Four genetically distinct populations were identified (F-ST: 0.036-0.093) but were not associated with different water masses, depths, or time points during the bloom. Instead, all four populations co-existed within individual water samples, spanning different water masses, stages of the bloom, and depths of over 300 m. The co-existence of multiple, genetically distinct populations during the bloom event suggested large-scale admixture, with populations originating via transport from disparate locations combined with potential overwintering capacity in the water column or sediments. The pattern of co-existence suggests that the open ocean may serve as a gene pool that harbors different populations that are then available for selection to act upon, which may contribute to the ecological and biogeochemical success of diatoms and influence their long-term evolutionary survival.