Functional Characterization of Hexacorallian Phagocytic Cells, and Their Response in Heat Stress Induced Bleaching

Rising seawater temperatures have increased coral bleaching events worldwide, to the detriment of coral reefs. Previous research on coral heat stress-induced bleaching found that different immune genes were differentially regulated and suggested immune activation. Yet it is still uncertain if the immune response is a consequence of bleaching or of the heat stress itself. To test this, we first functionally characterize phagocytic cells in stony corals and sea anemones models. To test the role of the immune system in bleaching we have used two model systems: Exaiptasia diaphana – which can be reared with and without symbiotic algae, Symbiodiniaceae, and Nematostella vectensis - which lacks symbiotic algae. We examined the effect of increased temperature on phagocytic activity, as an indication of immune function, using flow cytometry. Our data shows that immune cell activity increases during heat stress, while small molecule pinocytosis remains unaffected. We observed an increase in cellular production of reactive oxygen species with increasing temperatures. We also found that the cellular immune activity was not affected by the presence of Symbiodiniaceae. This suggests that immune activity observed in heat-stress induced bleaching in corals is a fundamental and basic response independent of the bleaching effect. These results establish a foundation for improving our understanding of hexacorallian immune cell biology, and its potential role in coral bleaching.