Scientists Discover the Mechanism Behind Coral Bleaching

Coral bleaching is a huge cause for concern along tropical coral reefs. Corals have trouble recovering after one of these events and can be more susceptible to disease and other environmental impacts. Now, scientists have taken a closer look at bleaching and have found the mechanisms responsible for the expulsion of zooxanthella from coral-the cause of bleaching.

When temperatures become warm, corals expel symbiotic zooxanthellae into the surrounding waters. Zooxanthellae are the tiny organisms that give corals their hues and also allow them to use sunlight to photosynthesize. Without zooxanthellae, corals are "bleached" white and can die from starvation if they don't recover these organisms.

But why do corals expel zooxanthellae in the first place? It's part of a regulatory mechanism that maintains zooxanthellal density and a stable concentration with expulsion of digested or normal forms of symbionts. When things heat up, zooxanthellae are damaged, and corals selectively digest the damaged cells or immediately expel them without digestion by exocytosis.

The researchers actually spotted that more photosynthetically damaged zooxanthellae were released undigested when there was prolonged exposure to warmer temperatures. This response may actually be an adaptive strategy to moderate stress to ensure survival. However, the accumulation of damaged zooxanthellae may occur when this response cannot come with the heat's duration. This, in particular, might be a possible mechanism underlying coral bleaching.

The findings reveal a bit more about coral bleaching and show the mechanisms behind it. This, in turn, may tell researchers a bit more about how corals adapt as temperatures continue to rise.

The findings are published in the journal PLOS One.

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