Corals are most elegant bio-geochemical engineers, building their colorful homes out of sunlight and air. But why do they bleach when stressed and how can we help them relax?
Modern, shallow (sub) tropical reefs are made by hermatypic stony ‘scleractinian’ corals. They are colonies of polyps with stingy tentacles, like the related sea anemone, secreting calcium carbonate cups; the corallite, an exoskeleton in which they can retract.
Hermatypic corals are colored by pigmented unicellular algae that they host in their tissue at concentrations of several million per square centimeter. These algae are dinoflagellates known as zooxanthellae which photosynthesize, using sunlight and carbon dioxide to produce oxygen and organic compounds, which they share with corals in exchange for nutrients and protection.
When corals are stressed, for example by extreme water temperature, salinity and light, they may expel their colored zooxanthellae and turn white (Coles & Jokiel, 1978). As the corals bleach, they become brittle and more prone to disease. When conditions return to normal, corals may incorporate the zooxanthellae again and regenerate after a while (Buddemeier & Fautin, 1993). The extreme circumstances may change the zooxanthellae from beneficial symbionts into harmful parasites that need to be expelled and if, for instance by climate change, such conditions occur too often, without time for regeneration, coral bleaching becomes permanent and entire reefs will die (Baker et al., 2018).
Coral reefs are the most diverse ecosystems on earth. They are not only a wonderful underwater world of great beauty, but they protect vast stretches of coast against wave erosion and ultimately feed billions of people with hundreds of billions of dollars’ worth of seafood. Coral reefs deserve protection and we have to:
- Learn how to diminish environmental stress for coral reefs
- Teach corals how to deal better with their environmental stress
We ask ourselves, for improving the well-being of coral hosts and their symbiotic guests, what bio-geochemical technology is best to use?