SYMBIONT BLEACHING IN PLANKTONIC FORAMINIFERA DURING EARLY EOCENE HYPERTHERMAL EVENTS

In modern oceans, temperature stress can cause symbiont-bearing taxa, like corals and sponges, to expel their photosymbionts in ‘bleaching’ events that can be lethal. Whether or not symbiotic clades will be able to adapt to elevated temperatures for the coming millennia is hard to predict from empirical observations of living taxa alone. Instead, past global warming events, particularly the hyperthermal events of the early Eocene, can provide case studies in biotic response to somewhat similar environmental disturbances.

Planktonic foraminifera are eukaryote protists, which can host photosymbionts. A positive correlation between δ¹³C and test size is an indicator of photosymbiont activity, and a sudden absence of this relation in species which otherwise show it may indicate that the specimens expelled symbionts. We compare carbon isotope trends of planktonic foraminifera during ontogeny across early Eocene hyperthermals to evaluate whether episodes of rapid global warming triggered bleaching events. Specifically, we measured size-specific stable carbon and oxygen isotopes across up to six size fractions (150-180μm, 180-212μm, 212-250μm, 250-300μm, 300-355μm, >355μm) in three planktonic foraminiferal species in upper Paleocene – middle Eocene sediments from Ocean Drilling Program (ODP) Site 1209 (Pacific Ocean), ODP Site 690 (Southern Ocean, Weddell Sea), and Deep Sea Drilling Program (DSDP) Site 401 (NE Atlantic Ocean). Acarinina soldadoensis and Morozovella subbotinae are interpreted as photosymbiont-bearing, whereas Subbotina eocaena is interpreted as asymbiotic. By comparing across hyperthermal events of varying amplitude (i.e. Paleocene-Eocene Thermal Maximum, Eocene Thermal Maximum 2) and oceanographic locations, we can elucidate the sensitivity of foraminiferal bleaching to temperature perturbations of differing magnitudes.

Our research into the bleaching of foraminifera across hyperthermal events thus may offer new insights into the effects of rapid environmental changes and the mechanisms of photosymbiont loss today and in the future in organisms such as corals and sponges, in addition to foraminifera.