Eco-physiological response of Phaeocystis antarctica and Fragilariopsis sp. to increases in irradiance and temperature.

Phaeocystis antarctica and Fragilariopsis are key phytoplankton taxa in the Southern Ocean that have different bloom magnitude and phenology, reflecting their differing physiological traits. Here, we investigate the physiological response of Fragilariopsis sp. and colony-forming P. antarctica to warmer and high irradiance conditions using chemostat experiments under low light/low temperature (LL/LT) and high light/high temperature (HL/HT). C:N and C:Chla ratios increased under HL/HT in both species, whereas the Fragilariopsis sp. Si:C ratio showed no significant variation between treatments despite Si:N being 1.4-fold higher under HL/HT. The P. antarctica colony to a single-cell ratio exhibited a 2.3-fold increase under HL/HT but with no change in the size of individual cells. On the contrary, Fragilariopsis sp. cell size decreased 1.3-fold without affecting cellular silica content. DMSPt:C and DMS:C increased in both species under HL/HT with no effect of treatment on DMSPt:DMS for either species. Primary pigment markers for taxonomic identification were unaffected by treatment, but pigments in the xanthophyll cycle increased under HL/HT with higher concentrations in Fragilariopsis sp. and a higher rate of epoxidation in P. antarctica. Results indicate a greater tolerance in P. antarctica to increased irradiance and warming than previously described, suggesting that it may be competitive with Fragilariopsis sp. under conditions usually associated with diatom dominance.