Physiology and iron modulate diverse responses of diatoms to a warming Southern Ocean

Climate change-mediated alteration of Southern Ocean primary productivity is projected to have biogeochemical ramifications regionally, and globally due to altered northward nutrient supply^1,2. Laboratory manipulation studies that investigated the influence of the main drivers (CO₂, light, nutrients, temperature and iron) on Southern Ocean diatoms revealed that temperature and iron exert major controls on growth under year 2100 conditions^3,4. However, detailed physiological studies, targeting temperature and iron, are required to improve our mechanistic understanding of future diatom responses. Here, I show that thermal performance curves of bloom-forming polar species are more diverse than previously shown⁵, with the optimum temperature for growth (T_opt) ranging from 5–16 °C (the annual temperature range is −1–8 °C). Furthermore, iron deficiency probably decreases polar diatom T_opt and T_max (the upper bound for growth), as recently revealed for macronutrients and temperate phytoplankton⁶. Together, this diversity of thermal performance curves and the physiological interplay between iron and temperature may alter the diatom community composition. T_opt will be exceeded during 2100 summer low iron/warmer conditions, tipping some species close or beyond T_max, but giving others a distinct physiological advantage. Future polar conditions will enhance primary productivity^2,3,4, but will also probably cause floristic shifts, such that the biogeochemical roles and elemental stoichiometry of dominant diatom species will alter the polar biogeochemistry and northwards nutrient supply.