Pelagic and ice‐associated microalgae under elevated light and pCO2: Contrasting physiological strategies in two Arctic diatoms

Sea ice retreat, changing stratification, and ocean acidification are fundamentally changing the light availability and physico‐chemical conditions for primary producers in the Arctic Ocean. However, detailed studies on ecophysiological strategies and performance of key species in the pelagic and ice‐associated habitat remain scarce. Therefore, we investigated the acclimated responses of the diatoms Thalassiosira hyalinaand Melosira arcticatoward elevated irradiance and CO2partial pressures (pCO2). Next to growth, elemental composition, and biomass production, we assessed detailed photophysiological responses through fluorometry and gas‐flux measurements, including respiration and carbon acquisition. In the pelagic T. hyalina, growth rates remained high in all treatments and biomass production increased strongly with light. Even under low irradiances cells maintained a high‐light acclimated state, allowing them to opportunistically utilize high irradiances by means of a highly plastic photosynthetic machinery and carbon uptake. The ice‐associated M. arcticaproved to be less plastic and more specialized on low‐light. Its acclimation to high irradiances was characterized by minimizing photon harvest and photosynthetic efficiency, which led to lowered growth. Comparably low growth rates and strong silification advocate a strategy of persistence rather than of fast proliferation, which is also in line with the observed formation of resting stages under low‐light conditions. In both species, responses to elevated pCO2were comparably minor. Although both diatom species persisted under the applied conditions, their competitive abilities and strategies differ strongly. With the anticipated extension of Arctic pelagic habitats, flexible high‐light specialists like T. hyalinaseem to face a brighter future.