I keep reading endless hype about the Great Barrier Reef (GBR) getting bleached out of existence. So let’s start with some facts.

First, coral is not a single organism. Coral is a curious critter. Coral is a symbiotic partnership between an animal from the Anthozoa group and a microbial alga called Symbiodinium. The microbial algae use photosynthesis to create sugar, and the Anthozoa polyps feed off the sugar. Here’s a description from the USGS.

Next, modern-type warm-water shallow coral reefs have been around for about half a billion years … so obviously, they must be resistant to temperature changes, including the large temperature swings in the transitions between the glacial periods and the interglacial times such as the current Holocene.

Next, “bleaching”. Bleaching is the natural response of coral to excessively warm or excessively cool temperatures. Or to pollution. Or to siltation. Or to toxic chemicals. Or to the death of the parrotfish or other grazers that keep the coral from being overgrown by other plants.

When the symbiotic relationship comes under pressure due to any of those stressors, the algae may leave the coral’s tissue. And if the stress is due to pollution, siltation, or the lack of parrotfish, then the reef may die.