Unravelling Past Population Events in Diatoms With Genomic Based Demographic Inferences

ABSTRACT The biological productivity in the Southern Ocean is mainly driven by the abundance of phytoplankton, which changes with seasons, fluctuations in sea ice and iron as well as light availability. The distribution of biological productivity is further modulated by ocean currents. Diatoms are a key element in the Southern Ocean's food web; thus, the effects of environmental changes on diatom population sizes have important implications for the overall ecosystem's productivity. In this study, we employed genomic data to assess the effects of glacial cycles on the population sizes and connectivity of the diatom Fragilariopsis kerguelensis in the Atlantic sector of the Southern Ocean. We detected three distinct variants among the 47 strains sampled along a latitudinal transect, all found in sympatry along the Polar Front. Demographic inferences identified major events during the last 100,000 years for two variants, characterized by a large decrease in effective population sizes followed by a rapid increase. These events overlapped with the change in environmental conditions during Pleistocene glacial cycles. Further research on mutation rates, generation time estimates and how changing environmental conditions influence these parameters in diatoms will improve the accuracy of the inferences drawn from these kinds of data. Although the use of genomic approaches in diatom species is in its infancy, this study highlights their potential to provide new insights into diatom past population sizes, responses and resilience to climate change and its effects on other trophic levels in the Southern Ocean.