Seasonal genotype dynamics of a Baltic dinoflagellate – pelagic populations are homogeneous and as diverse as benthic seed banks

Genetic diversity is the basis for evolutionary adaptation and selection under changing environmental conditions. Phytoplankton populations are genotypically diverse, can become genetically differentiated within small spatiotemporal scales and many species form resting stages for seasonal survival. Resting stage accumulations (seed banks) are expected to serve as archives for genetic information, but so far, their role in phytoplankton diversity and evolution has remained unclear. In this study we used the toxic dinoflagellate Alexandrium ostenfeldii (Dinophyceae) as a model organism to investigate if (1) the benthic seed bank is more diverse than the pelagic bloom population and (2) if the pelagic population is seasonally differentiated. Cysts and bloom samples were collected in the Baltic Sea during 2015, followed by cell isolation and genotyping using restriction site associated DNA sequencing (RAD). High clonal diversity combined with intermediate to low gene diversity was found. Surprisingly, the benthic and pelagic fraction of the population were equally diverse, and the bloom was temporally homogenous, despite seasonal fluctuation of potential selection pressures. The persistence of linkage equilibrium throughout the year indicates frequent genetic recombination. Results of this study suggest that continuous benthic-pelagic coupling, combined with sexual reproduction prevent the establishment of a single clonal lineage in a dynamic environment. Together they harmonize the pelagic with the benthic population and thus prevent seasonal population differentiation. At the same time frequent sexual reproduction and benthic-pelagic coupling maintain high clonal diversity in both habitats.