How are microbes involved in coral bleaching under global warming?

Question

What will be an ideal response?

Answer 1

Coral bleaching happens when the symbiosis between the coral animal and it's photosynthetic microbial symbiont is disrupted. Symbiodinium is an endosymbiotic (symbiotic inside the host) dinoflagellate (algal protist). They enter the host cell through phagocytosis. Symbiodinium is a genus of diverse endosymbiotic algae with genus members commonly referred to as zooxanthellae. Coral are born azooxanthellate, symbiont free, and are infected with the symbiont horizontally while in the polyp stage through either feeding or phagocytosis by gastrodermal cells. Symbiodinium provide photosynthetically fixed sugars to coral while coral provides inorganic nutrients, a high light environment, and protection from the threat of aquatic herbivores

With global warming (+1-2oC), corals lose their symbionts (60-90%) or they lose the symbiont's photosynthesis pigments (50-80%). There is evidence of irreversible damage to photosystem II in heat-stressed symbiotic dinoflagellates within corals during a natural bleaching event. The dysfunction of PSII correlates with the loss of the PSII reaction-center protein. It is hypothesized that host corals expel the dysfunctional symbionts because they no longer produce fixed C. When this happens, the coral turns white because the calcium skeleton is showing through.