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If you try to stay up to date on science news, you may have notice a number of recent stories about coral reefs being damaged by sunscreen. After seeing several headlines to this effect, I decided to investigate. My initial assumption was that the likely proposed mechanism for this problem would be that dissolved sunscreen in the oceans was preventing sunlight from reaching the coral. It turns out that I was wrong.

Before discussing the potential ill effects of sunscreen on coral, let’s review some key features of coral itself. Coral are small invertebrates, similar to clams or oysters, which secrete calcium carbonate exoskeletons. Unlike clams or oysters, coral join their exoskeletons together to form the large monolith that we call a reef. Most species of coral don’t actually “eat” but rather get their energy from a symbiotic relationship with a type of algae known as zooxanthellae. The algae generate sugars via photosynthesis and share some of this bounty with the coral. In exchange, the coral provides place for the algae to latch on and await the sunlight. When sunlight is limited due to polluted water, the algae suffer so the coral suffer. A healthy coral reef will look greenish-brown due to the algae and a dead coral reef will be bright white, the color of algae-free calcium carbonate.

The reproductive habits of coral are fascinating. (There is a sentence that you don’t encounter every day!) They reproduce both asexually and sexually. In asexual reproduction, an adult coral splits in two, calving off a new coral that then forms a new exoskeleton. This is the process by which an existing reef grows in size. Coral also reproduce sexually by releasing eggs and sperm into the water with the hope that some of the eggs will be fertilized while floating about in the sperm-infused water and then settle to the ocean floor to start a new reef. The fascinating part of the sexual reproduction of coral comes from the timing. Even when there are multiple species of coral are present, on exactly the same night on or near the full moon, all of the coral release their eggs and sperm into the water. (All of a sudden I am wondering if I have ever been swimming in the ocean on a full moon.) Given that humans and coral have common evolutionary ancestors, it is by no means coincidental that the period of the reproductive cycle of coral and the human female are the same. One of the many reasons that I enjoy writing Common Science® is stumbling across fascinating echoes of evolution such as this. I have to imagine that Ben Carson’s life must be a bit less satisfying by excluding the joy of pondering the wonder of these types of phenomena, but I digress.

Coral reefs are incredibly important to the ocean ecosystem. They support a staggeringly high density of sea life and a wide diversity of species and thus are often referred too as the rain forests of the oceans. Therefore, a reduction in the number of viable coral reefs has a disproportionately negative and impact on the overall food chain of the oceans. If you are a reader of this column, you are likely already aware that our coral reefs are in trouble. It is estimated that 10% of the world’s coral reefs are already dead and other 60% are at risk. Many reasons for this are already well understood; coral suffer when the oceans warm, and increased carbon dioxide concentration in the atmosphere makes the ocean water more acidic, which dissolves the corals’ calcium carbonate exoskeletons.

Let’s return now to the question of sunscreen. As I mentioned above, I figured the problem arose from sunscreen in the water preventing the sun light from reaching the symbiotic algae. But even though something like 14,000 tons of it enters the oceans each year, the concentration of sunscreen in the water is too low for this to be a problem. What appears to be happening is that a common component of many sunscreens called benzophenone – also referred to as oxybenzone – is harming the coral through one or both of two other mechanisms. The first proposed mechanism is that the benzophenone stimulates dormant viruses in the algae, killing the algae and, thus, depriving the coral of sustenance. The second is that benzophenone is causing damage to the DNA of the coral. While we may not understand all of the details of the mechanism as this point, it does seem clear that benzophenone is very damaging to coral even at low concentrations. As I was doing the research for this column I was disappointed to see that National Geographic reported this problem at least as early as 2008, but that nothing has been done about it. Earlier this year the National Aeronautic and Oceanic Administration (NOAA) published a report that may have more of an impact and inspire some action.

So what should we do? We can’t expect people to just stop using sunscreen. However, there is no reason that we need to allow continued use of benzophenone. There are many other compounds that can be used just as effectively in sunscreens. In particular, zinc oxide and titanium oxide are very effective. In addition, since they are essentially just little bits of rock dust, they are environmentally benign. In the past, these mineral-based sunscreens had a bad reputation for making your skin look pasty white, but manufacturers now grind the particles to a much smaller size, this “problem” has been greatly reduced. The other key benefit of titanium and zinc oxides is that, unlike other active ingredients in sunscreens, their effectiveness does not decline over time during exposure to the sun. So as long as they are not rinsed or sweated off, they will continue to work. So, especially the next time you head for the coast, consider giving them a try.

Jeff Danner discussed this week’s column with Aaron Keck on WCHL.


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