Sunscreen can corrode corals
Corals are under threat internationally, most notably through the effects of climate change that warm the water and cause the coral to bleach and die. But there's potentially another threat: sunscreen. And what Stanford researchers Djordje Vuckovic and Bill Mitch have discovered is that some suncreams contain molecules that, when they get into corals, are chemically altered into forms that can react with sunlight and become toxic. We had some inkling that this might be happening, but we didn't know how. Now we do. Bill first...
Bill - There had been some anecdotal evidence that certain sunscreen components could be toxic to corals, but the mechanisms by which this was occurring were unclear. We were trying to figure this out, and particularly to figure out if there were a mechanism by which sunlight could activate these molecules to be toxic to corals, counterintuitively, since after all they are supposed to be sunscreens.
Chris- Well, what was the evidence that sun cream was bad?
Bill - Well, there was some observational evidence from the US Virgin islands by a different set of researchers. Certain coral reefs that were located in areas of high recreational use were in much poorer shape and were unable to keep generating new elements to the coral reef, and that was correlated with high levels of this particular sunscreen component called oxybenzone in the water column.
Chris - So, Djordje, how did you actually go and test this? Because obviously there's the distinction between an association, actually people going and visiting the coral and having this stuff on their skin, and then causation, this stuff on their skin is what is damaging the coral. It's not something else?
Djordje - We actually used the sea anemone, at first, as a model for corals because they are very similar and they're much easier to work with in the lab. We exposed sea anemones to this sunscreen, oxybenzone, and we did this under a simulated sunlight. Now, in order for something to be phototoxic, it would actually need to interact with the sunlight. So, some of the anemones, we actually took out the spectrum of light that sunscreen can interact with, and then exposed the other anemones to full sunlight. The anemones that were in the dark section, they didn't die for 21 days, but the ones that were under full spectrum sunlight, they died on average in about 10 days.
Chris - So that would suggest that the sunlight is necessary for something to happen to the sunscreen in order to become toxic for the anemones in the experiment.
Djordje - That's right. And that seemed like a very paradoxical result, actually.
Chris - Why?
Djordje- Well, you would expect the sunscreen to actually protect you from sunlight, not to make sunlight kill an organism.
Chris - Bill, does this actually prove, then, that it is this that is causing the problem for corals?
Bill - There are two types of sunscreens out there. Some are mineral based, titanium dioxide or zinc oxide, and others are organic molecules. We were studying the latter and, in particular, a chemical called oxybenzone. These molecules are very hydrophobic, meaning they don't want to be in water, so they readily accumulate in tissue, including coral or sea anemone tissue. They absorb sunlight, particularly the UV wave bands, but when molecules absorb sunlight, they enter an excited state. Many molecules in that excited state can go on to damage biomolecules. Sunscreens also absorb sunlight, enter this excited state, but they have a portion of the molecule that's designed to deactivate this excited state and return it to heat. Essentially, to prevent damage to neighbouring biomolecules. However, these corals and sea anemones, after they've absorbed this molecule, one of the strategies to get rid of it is to alter the molecule and make it more water loving so it can get back into the water column. Unfortunately, what they do is they add the sugar to the portion of the molecule that's designed to deactivate this excited state. They convert what is a sunscreen into a phototoxic.
Chris - And on that note, although you have focused on oxybenzone in this case, a constituent of sunscreen, there are many molecules that are pretty similar, chemically speaking, aren't there? Could the same chemistry be happening with other things that are out there that we chuck away, that we use cosmetically or industrially. Could this be a bigger problem than just a sunscreen problem?
Bill - Well, we believe that to be the case. In fact, we're currently looking at alternative sunscreen components that share similar structures to the oxybenzone, the molecule we looked at in this study, but it's too early to say.
Djordje - We should also be careful about potential implications to other species, including even humans, for instance, because we do know that humans make a very similar compound from this oxybenzone sunscreen. We did test this out in the lab, since this is The Naked Scientists podcast. I should point out that I did actually get, in one experiment, naked and put on some sunscreen. Then, later on, measured it in my urine and then isolated this compound and actually found that it has potential to be a phototoxin.
Chris - But, Djordje, who we were hearing from there, is it pains to emphasise that, unlike corals and sea anemones, humans are not, thankfully, usually transparent and therefore sunlight shouldn't get inside us to turn those molecules into toxins like it does in other species like the corals. Nevertheless, we are very impressed that he got naked in the name of science.