Photophysiology in two major Southern Ocean phytoplankton taxa: Photoprotection in Phaeocystis antarctica and Fragilariopsis cylindrus

Phytoplankton communities in the Ross Sea, Antarctica, are characterized by the haptophyte Phaeocystis antarctica Karsten, which dominates deep mixed layers, and diatoms, including Fragilariopsis cylindrus Grunow, that thrive in shallower mixed layers. To investigate whether differences in photoprotective strategies explain these distributions, photosynthetic parameters, pigments, and fluorescence properties were measured in cultures grown under several irradiance regimes and during acclimation to increased irradiance. In P. antarctica, cellular concentrations of all pigments declined with increasing growth irradiance under continuous light, but xanthophyll cycle pigment concentrations increased with increasing irradiance under dynamic conditions without changes in chlorophyll. In contrast, F. cylindrus exhibited declines in chlorophyll cell(-1) with increasing irradiance under both continuous and dynamic conditions, but xanthophyll cycle cell(-1) pigments increased under continuous irradiance and declined under dynamic irradiance. P. antarctica did not exhibit non-photochemical quenching (NPQ) unless exposed to irradiance in excess of the mean growth irradiance. F. cylindrus exhibited NPQ in response to lower irradiances but displayed less photoinhibitory quenching than P. antarctica after exposure to very high irradiance. Inhibitor experiments suggest that both taxa rely upon xanthophyll cycle photoprotection to maintain photosynthetic performance but only P. antarctica relies heavily upon protein synthesis, presumably for D1 protein repair. F. cylindrus can thrive in shallow mixed layers because its high capacity for heat dissipation minimizes photoinhibition. P. antarctica utilizes xanthophyll cycle photoprotection to a lesser degree, but is able to dominate deeper mixed layers by effectively repairing the photodamage incurred when it is mixed to the surface.