Heat Stress leads to the Activation of Hexacorllia Immune Cells and Inhibits Endo-Symbiosis

Coral reefs are an important ecosystem, supporting the ocean’s biodiversity by harboring a quarter of all marine life.
Corals are marine animals in the subclass Hexacorallia. They live in a symbiotic relationship with an intracellular algae that provides most of the nutrients they need to survive. When exposed to heat stress the coral cells expel the algae living within them, a phenomenon known as coral bleaching. If the warm temperatures persist, the algae won`t re-enter the coral cells, potentially leading to its death.
There are theories that during heat stress, the algae produce reactive oxygen species (ROS), harming host tissues until being expelled. However, we found that during heat-stress induced bleaching the immune activation markers ROS and Nitric Oxide (NO) are elevated in phagocytic cells of a Hexacorallian model; symbiotic sea anemone Exaptasia diaphana (E. diaphana). These results suggest that the immune cells are at the base of ROS and NO induction, which could lead to the bleaching phenomenon.
Furthermore, with our development of cellular algae intake assay with apo-symbiotic E. diaphana cells we demonstrate that during heat stress the endosymbiosis process is being inhibited, compared to the phagocytic activity that is shown to be elevated.