Intraspecific Variability in Thermal Tolerance Buffers Southern Ocean Diatoms from Biogeographic Range Contraction in a Warming Ocean

Diatoms are the dominant primary producers in the Southern Ocean (SO) and are largely responsible for the region’s significant deep-sea carbon export. While predicted increases in sea surface temperature (SST) may affect the success and biogeography of these diatoms, current understanding is limited by how few characterized SO thermal performance curves (TPCs) exist. Furthermore, the extent of intraspecific trait variability in TPCs is unknown for SO diatoms, as is the role that intraspecific variability may play in moderating temperature-driven shifts in geographic range. Here, we measured 1250 growth rates for a suite of SO diatoms to examine both inter- and intraspecific variation in TPCs. Predicted thermal optima and maxima of strains isolated from the SO ranged over 10 deg C and coefficients of variation reached 50% among strains within species. To investigate how this variability could moderate the effects of temperature-driven change in the SO, we used our empirically derived TPCs and earth system model output to estimate shifts in viable area between now and 2100. Initial results suggest that intraspecific trait variation could significantly reduce estimates of range contraction compared to estimates run with lower and no trait variation. Overall, our findings suggest that intraspecific variation in thermal tolerance may play an important role in determining how individual species of SO diatoms persist in the face of climate change.