Symbiont bleaching in fossil planktonic foraminifera

Size restricted carbon isotopes (δ13C) are used to track changes in the ontogenetic life strategies of two species of extinct planktonic foraminifera and demonstrate that the species Morozovelloidescrassatus lost their photosymbiotic association prior to their extinction in the latest middle Eocene. M.crassatus exhibit a strong positive correlation between test size and δ13C between 39.5 Ma and 38.7 Ma and a Δδ13C shift of 1.0‰/100 μm, this is analogous with modern species that possess an association with algal photosymbionts. Turborotaliacerroazulensis is interpreted as an asymbiotic, thermocline dweller and consistently shows no size related δ13C trends and greater δ18O values in comparison to Morozovelloides. We show a long-term (1.5 million year) deterioration of Morozovelloides ecology that culminated in their extinction at 38.021 Ma. The Δδ13C /100 μm in M. crassatus is dramatically reduced from 1.0‰ at 39.53 to only 0.2‰ at 38.026 Ma, 5 kyr before their extinction. The decline in ontogenetic δ13C suggests diminished photosymbiotic activity (bleaching) and disruption of foraminiferal ecology in the interval preceding their extinction. We conclude that the demise of Morozovelloides was directly related to the deterioration of photosymbiotic partnerships with algae.