Abstract
Phaeocystis antarctica , a dominant phytoplankton species found in the Southern Ocean, is known for colony formation and large seasonal blooms. Blooms play a key role in Southern Ocean biogeochemistry, contributing to carbon export and the production of dimethylsulfoniopropionate (DMSP) and its climatically active breakdown product, dimethyl sulfide (DMS). Here we provide evidence of phagotrophy, i.e., mixotrophic behavior in the alga P. antarctica. Mixotrophy is widespread among marine protists and was recently identified in the temperate sister species P. globosa. We examined potential triggers for bacterivory using a 4 × 4 factorial design manipulating irradiance (diurnal changes versus static) and nutrient availability and evaluated bacterivory using fluorescent microspheres as tracers. Our results show that bacterivory in P. antarctica was driven more strongly by variability in irradiance than by nutrient availability, with greater ingestion occurring in treatments with diurnal cycles. Because macronutrient limitations are generally negligible in the Southern Ocean, bacterial ingestion of P. antarctica may represent an adaptive response to seasonal changes in light availability.