Patterns of Genetic Diversity and Co-Existence in Open Ocean Diatoms: the Effects of Water Mass Structure, Selection and Sex

The open ocean North Atlantic spring bloom influences regional ecology and global biogeochemistry. 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 this open ocean 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. Here, we show that the genetic diversity of open ocean diatoms is high and that their population structure differs dramatically from coastal diatoms. High levels of genetic diversity were observed across all water samples and did not change during the bloom. Four genetically distinct populations were identified but were not associated with different water masses, depths or time points during the bloom. Instead, all four populations co-existed within samples, spanning different water masses, stages of the bloom and depths of over >300 m. The pattern of genetically distinct, co-existing populations in the open ocean contrasts dramatically with coastal habitats, where distinct populations have not been observed to co-exist at the same time and place. It is likely that populations originate via transport from disparate locations combined with 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.