Photosymbiosis in planktonic foraminifera across the Palaeocene-Eocene Thermal Maximum

Under stress, corals and foraminifera may eject algal symbionts ('bleach'), which can increase mortality. How bleaching relates to species viability over warming events is of great interest given current global warming. We use size-specific isotope analyses and abundance counts to examine photosymbiosis and population dynamics of planktonic foraminifera across the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), the most severe Cenozoic global warming event. We find that, unlike modern bleaching-induced mass mortality, populations of photosymbiont-bearing planktonic foraminifera increased in relative abundance during the PETM. Multigenerational adaptive responses including flexibility in photosymbiont associations and excursion taxa evolution may have allowed some photosymbiotic foraminifera to thrive.