Temperatures above thermal optimum reduce cell growth and silica production while increasing cell volume and protein content in the diatom Thalassiosira pseudonana

Temperature plays a fundamental role in determining phytoplankton community structure, distribution, and abundance. With climate models predicting increases in ocean surface temperatures of up to 3.2°C by 2100, there is a genuine need to acquire data on the phenotypic plasticity, and thus performance, of phytoplankton in relation to temperature. We investigated the effects of temperature (14–28°C) on the growth, morphology, productivity, silicification and macromolecular composition of the marine diatom Thalassiosira pseudonana . Optimum growth rate and maximum P:R ratio were obtained around 21°C. Cell volume and chlorophyll a increased with temperature, as did lipids and proteins. One of the strongest temperature-induced shifts was the higher silicification rates at low temperature. Our results reveal temperature-driven responses in physiological, morphological and biochemical traits in T. pseudonana ; whereby at supra-optimal temperatures cells grew slower, were larger, had higher chlorophyll and protein content but reduced silicification, while those exposed to sub-optimal temperatures were smaller, heavily silicified with lower lipid and chlorophyll content. If these conserved across species, our findings indicate that as oceans warm, we may see shifts in diatom phenotypes and community structure, with potential biogeochemical consequences of higher remineralisation and declines in carbon and silicon export to the ocean interior.