What is the major cause of coral bleaching? 

Understanding Coral Bleaching Corals have a symbiotic relationship with microscopic algae called zooxanthellae that live inside their tissues. These algae provide the coral with food (via photosynthesis) and give them their vibrant colors. When water temperatures rise even by $1\text{--}2°\text{C}$ above the normal maximum, the corals become stressed and expel the algae. Without the algae, the coral loses its food source and turns completely white, which is why it is called "bleaching."

In the context of competitive examinations, it is a critical part of:

Marine Biology: The study of marine organisms and their behaviors.

Climate Change & Global Warming: Analyzing the impact of rising temperatures on biodiversity.

Conservation Biology: Focusing on the protection of coral reefs, often referred to as the "rainforests of the sea."