Is Your Sunscreen to Blame for Coral Bleaching?

by Dr. Hannah Sivak | June 3, 2018

Corals are among the most amazing and beautiful and complex living beings. What we see is an exoskeleton made of calcium carbonate that has been formed slowly by the whole colony, made of identical individual polyps. These polyps are marine invertebrates of the phylum Cnidaria; in this phylum, members of the class Anthozoa are the reef builders, but you may be more familiar with other members of the phylum that include the nasty, if beautiful, jellyfishes.

Corals are even more complicated than “just” the colony of tiny polyps making a beautiful exoskeleton that used to end up as beautiful earrings and necklaces (this practice is not heavily regulated).

How do these polyps feed themselves? They may use their tiny mouths to eat some passing plankton or even tinier fish, but their static position limits this source of food. The solution? They incorporate food producers into their own bodies: most of their energy and nutrients is derived from photosynthetic unicellular dinoflagellates that live within the polyps. These photosynthetic organisms are commonly known as zooxanthellae. Corals that depend on photosynthetic organisms for energy are limited to clear waters and water depths that allow enough light reaching the zooxanthellae.

What is Coral Bleaching

In some stressful environmental conditions, the polyps in the coral can expel the zooxanthellae. This happens when the temperature increases (or decreases) beyond a certain limit, or there may be a change in nutrient availability of light. A change in temperature is the kind of stress that may make the symbiosis of coral and algae unsustainable. This does not mean that the coral is dead, the polyps are still viable, but will now depend on what food they can obtain from the environment. Also, they can be re-colonized by other zooxanthellae that may be better adapted to the new conditions, or the temperature change may be transitory.

There is no doubt that a gradual increase in sea temperature has first bleached and then killed many corals. Now, however, a new possible cause has been revealed. Not surprisingly, adding an unusual chemical to the environment can damage corals, and this has lead to a new law in Hawaii that bans the use of chemical sunscreens. In one paper, it was shown that adding sunscreen to a container where a piece of coral had been placed lead to bleaching (loss of the algae). This is a crude study, unlikely to represent anything remotely like real life because we don’t apply sunscreen to corals! What we do, if we care enough about our skin health, is apply sunscreen to our skin. If the sunscreen happens to have a chemical sunscreen (not a physical one like zinc oxide) it will be washed into the sea and within minutes it will be broken down by the sunlight. In fact, one of the problems with chemical sunscreens is that they are so unstable under sunlight.

Why do scientists do bad science (because this study of effect of sunscreen on corals is bad science)? Many reasons, and the main one is that scientists are human. One of humans’ bad traits is that we may wish to be famous for the 15 minutes allocated to scary scientific news. I am not going to give a list of these scary (bad science) scientific news because 1) it will lengthen the 15 minutes of fame allocated to the bad scientists and 2) this is the way that bad science is perpetuated. Apparently, we seem to remember false news even better than we remember the true ones.

Another reason may be that false news may help hide the real ones, like the fact that the warming up of the sea, caused by global warming, definitely causes coral bleaching.

And a third one: due to the crazy rules that dictate what is a “charitable” organization in the USA, anybody can start a “not for profit” company in a day or so, and start collecting donations via a website based on any dramatic piece of bad science, especially if the news touches of something as dear to humans like corals.

Here you have the triad: dramatic scientific news (bad science usually), a precious natural treasure like corals and the prospect of making fast money out of people (i.e. the marks).

What can you do about corals? First, don’t panic. We have always been involved in doing good science, supporting sustainability, protecting the environment and participating in societies that support these aims. That’s what we do.


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