Chloroplasts of symbiotic microalgae remain active during bleaching induced by thermal stress in Collodaria (Radiolaria)

Collodaria (Radiolaria) are important contributors to planktonic communities and biogeochemical processes (e.g. the biologic pump) in oligotrophic oceans. Similarly to corals, Collodaria live in symbiosis with dinoflagellate algae, a relationship that is thought to explain partly their ecological success. In the context of global change, the robustness of the symbiotic interaction and potential subsequent bleaching events are worth consideration. In the present study, we compared the ultrastructure morphology, symbiont density, photosynthetic capacities and respiration rates of colonial Collodaria exposed to a range of temperatures corresponding to natural conditions (21{degrees}C), moderate (25{degrees}C) and high (28{degrees}C) thermal stress. We showed that symbiont density immediately decreased when temperature rises to 25{degrees}C and the collodaria holobiont metabolic activity increased. When temperature reached 28{degrees}C, the collodarian host arrived at a tolerance threshold with a respiration nearly stopped and largely damaged morphological structures. Over the course of the experiment the photosynthetic capacities of remaining symbionts were stable, chloroplasts being the last degraded organelles from the microalgae. These results contribute to a better characterization and understanding of temperature-induced bleaching processes in planktonic photosymbiosis.