Seascape genomics reveals high genetic diversity and apparent panmixia in endemic Southern Ocean phytoplankton

The adaptive potential and resilience of cold‐adapted phytoplankton to climate change have the potential to impact polar marine food webs and global biogeochemical cycles. Two central but poorly constrained facets of adaptive potential in polar phytoplankton are standing genetic diversity and its spatial structure. Here, we partially sequenced 30 field isolate genomes of a commonly‐occurring endemic Southern Ocean (SO) phytoplankton, the diatom Actinocyclus actinochilus , and analyzed patterns in standing genetic diversity and geographic structure. Across a > 4000 km high‐latitude SO transect, A. actinochilus exhibited high genome‐wide nucleotide diversity ( π = 0.05) suggesting large effective population size. Divergence among genomes was low and did not resolve into genetically distinct populations, suggesting a panmictic population structure that we hypothesize is maintained by SO circumpolar currents driving high levels of zonal gene flow. Our findings have implications for SO phytoplankton resilience and highlight a plausible buffering mechanism for SO phytoplankton in response to rapid environmental change.