Immune Activation of Symbiotic Hexacorallia Induces the Bleaching Phenomenon

Rising seawater temperatures have negatively affected coral reefs worldwide by initiating a phenomenon called “bleaching”, in which the coral’s symbiotic algae is expelled from host tissue. The symbiotic algae reside inside the coral’s gastrodermal cells and provide nutrients to the host. Thus, their loss by bleaching is detrimental to coral health and can lead to mortality. Previous research on coral bleaching found that immune genes were differentially regulated upon heat stress, which suggested the involvement of the immune system in the process. Therefore we hypothesized that the coral immune response instigates bleaching by “rejecting” the symbiont from host cell. In order to test our hypothesis we previously discovered phagocytic immune cells in Hexacorallia, a subclass containing stony corals and sea anemones. Furthermore, these immune cells increased their activity when we exposed them to heat stress. Interestingly, we observed similar increases of phagocytic activity regardless of algae presence in the host cell, suggesting that heat stress induced immune activation is independent of symbiotic status. We then aimed to separate the two processes of phagocytosis vs. algae intake for symbiosis. To test this, we have used a Hexacorallia model system: the sea anemone Exaiptasia diaphana – which can be reared with and without symbiotic algae. We developed a cellular algae intake assay to test the effects of heat stress on symbiosis. We saw that while exposure to heat stress increases phagocytic activity, that same stress reduces algae intake. We then asked whether activation of the immune system could lead to algae expulsion. To this end, we injected immune activators into whole polyps and measured algae expulsion. We found that immune activation on its own might cause algae expulsion similar to that of heat stress. Hence, our data provides the basis for further study into heat induced immune activation in corals and sea anemones. We aim to elucidate the mechanisms in which immune activation leads to the destruction of symbiosis between the host and their symbiotic algae.