Variation in survival and growth following prolonged darkness in a polar diatom species

Summary Phytoplankton are the major primary producers in the Southern Ocean, participate in the global carbon cycle, nutrient cycles, and are at the base of the food-web. These polar ecosystems are unique in their extended periods of darkness in the winter. Prolonged darkness has the potential to exert selection that affects the composition of diatom communities if there is differential survival of diatoms in the dark, variation in population growth rates in subsequent light periods, or both. We tested whether prolonged darkness has the potential to exert within-species selection on a model polar diatom species by exposing 5 strains of the polar diatom Porosira glacialis to prolonged darkness at two different temperatures in the laboratory. We measured population survival in the dark, growth rate upon re-illumination, and between strain variability in these traits. We found a pronounced decline in survival and growth rate with time spent in the dark, as well as important intraspecific variation in these. Higher temperature exacerbated declines in growth and survival. Our results show that the darkness of polar night can exert selection within diatom species, with implications for phytoplankton community composition and subsequent impacts on Southern Ocean biogeochemical cycles.