Interactive effects of ocean warming and higher irradiances on the Southern Ocean diatom Pseudo-nitzschia subcurvata

Diatoms are the dominant primary producers south of the Antarctic Polar Front (~60°S) and play a central role in Southern Ocean food webs and the biological carbon pump. Although models project that climate change will enhance primary productivity in the Southern Ocean, the interactive effects of environmental drivers, particularly temperature and light, on diatom productivity and physiology remain poorly understood. In this study, we investigated the individual and combined effects of ocean warming and increased irradiance on the growth and physiology of the widespread Southern Ocean diatom Pseudo-nitzschia subcurvata . We used an experimental matrix consisting of two temperature treatments (2 °C and 8 °C) and two light levels (20 and 200 µmol photons m −2 s −1 ), representing current and projected conditions in the Antarctic for 2100 in Coupled Model Intercomparison Project (CMIP) models. The combination of high light and high temperature yielded the highest growth rates and Particulate Organic Carbon (POC) and Nitrogen (PON) production rates. While high light individually enhanced biomass accumulation, growth was significantly higher only when high light and high temperature acted together. Combined high light and high temperature conditions also produced higher-than-expected chlorophyll-to-carbon ratios, and photosynthetic efficiency remained high across treatments. These findings suggest that under future Southern Ocean conditions, diatoms such as P. subcurvata may increase their productivity by adjusting their photosynthetic physiology. By quantifying the interactive effects of temperature and light on diatom physiology, this study provides mechanistic insights to productivity changes that can help refine predictions of climate-driven shifts in Southern Ocean primary production.