Coral bleaching, which causes high coral mortality, has been occurring widely in coral reefs. coral bleaching actually refers to the death of symbiotic dinoflagellates living within the corals. why does coral bleaching cause the corals to die? \

Dinoflagellates are a type of algae that live in a symbiotic relationship with the coral. The algae are autotrophic, meaning they can convert the energy of the sun into chemical energy that organisms can use. The algae share some of the nutrients they produce with the coral in exchange for coral's protection. 
if the algae die off, the coral loses an important source of nutrients and dies too.

Could you elaborate?