Coral bleaching is a critical environmental phenomenon where corals expel their symbiotic algae, zooxanthellae, from their tissues, causing them to turn white. These zooxanthellae provide corals with up to 90% of their energy through photosynthesis and give them their vibrant colors. The primary cause of bleaching is environmental stress, predominantly elevated sea surface temperatures (SSTs) associated with climate change, but also ocean acidification, pollution, disease, extreme cold, or changes in salinity. When stressed, the photosynthetic machinery of zooxanthellae malfunctions, producing reactive oxygen species that are toxic to the coral host, leading to their expulsion or degradation. If the stress is prolonged or severe, corals can starve and die, leading to significant reef degradation.

Coral recovery from bleaching is possible but depends on several factors. If the stress event is short-lived and mild, corals can re-acquire zooxanthellae from the water column or through division of remaining algae, and gradually regain their color and health. Factors influencing successful recovery include the coral species' resilience, the availability of healthy coral larvae for recruitment, the absence of further stressors, and good water quality. However, recovery is a slow process, often taking years to decades for a severely bleached reef to regain its previous state, and many reefs may never fully recover.

A significant consequence of widespread coral mortality due to bleaching is a macroalgal regime shift. When corals die, the competitive balance on the reef shifts dramatically. Fast-growing macroalgae (seaweeds) can quickly colonize the dead coral skeletons, outcompeting any remaining live corals or new coral recruits. This shift is often exacerbated by other anthropogenic factors such as overfishing of herbivorous fish (which naturally graze on algae) and nutrient pollution (which fuels algal growth). The proliferation of macroalgae creates an 'algal-dominated' reef state, fundamentally altering the ecosystem's structure and function. This regime shift is often difficult to reverse, as the dense algal cover can prevent coral larvae from settling and growing, further entrenching the algal dominance. Such shifts lead to a significant loss of biodiversity, reduced habitat complexity, and diminished ecosystem services, including fisheries productivity and coastal protection, transforming vibrant coral reefs into degraded, less productive systems.