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

Abstract

Climate change-mediated alteration of Southern Ocean primary productivity is projected to have biogeochemical ramifications regionally, and globally due to altered northward nutrient supply1,2. 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 conditions3,4. 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 (Topt) ranging from 5–16 °C (the annual temperature range is −1–8 °C). Furthermore, iron deficiency probably decreases polar diatom Topt and Tmax (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. Topt will be exceeded during 2100 summer low iron/warmer conditions, tipping some species close or beyond Tmax, but giving others a distinct physiological advantage. Future polar conditions will enhance primary productivity2–4, 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. Climate change will alter primary productivity in the Southern Ocean, and warming and iron limitation will influence the composition of diatoms in the region. Optimum growth temperatures are wider than expected, but limited iron will affect which species flourish.