Physiological response of an Antarctic cryptophyte to increasing temperature, CO2, and irradiance

Abstract The Southern Ocean, a globally important CO 2 sink, is one of the most susceptible regions in the world to climate change. Phytoplankton of the coastal shelf waters around the Western Antarctic Peninsula have been experiencing rapid warming over the past decades and current ongoing climatic changes will expose them to ocean acidification and high light intensities due to increasing stratification. We conducted a multiple‐stressor experiment to evaluate the response of the still poorly studied key Antarctic cryptophyte species Geminigera cryophila to warming in combination with ocean acidification and high irradiance. Based on the thermal growth response of G. cryophila , we grew the cryptophyte at suboptimal (2°C) and optimal (4°C) temperatures in combination with two light intensities (medium light: 100 μ mol photons m −2 s −1 and high light HL: 500 μ mol photons m −2 s −1 ) under ambient (400 μ atm pCO 2 ) and high pCO 2 (1000 μ atm pCO 2 ) conditions. Our results reveal that G. cryophila was not susceptible to high pCO 2 , but was strongly affected by HL at 2°C, as both growth and carbon fixation were significantly reduced. In comparison, warming up to 4°C stimulated the growth of the cryptophyte and even alleviated the previously observed negative effects of HL at 2°C. When grown, however, at temperatures above 4°C, the cryptophyte already reached its maximal thermal limit at 8°C, pointing out its vulnerability toward even higher temperatures. Hence, our results clearly indicate that warming and high light and not pCO 2 control the growth of G. cryophila .