A Critical Look at Sunscreen and Coral Bleaching

National Geographic News Podcast reported last week that sunscreen has been implicated as a cause for coral bleaching. National Geographic News online also had a brief article about it. A group of Italian researchers studied the effects of some common sunscreen ingredients on corals from the Caribbean Sea (Atlantic Ocean), the Celebes Sea (Pacific Ocean), the Andaman Sea and the Red Sea (Indian Ocean). Their results were published in Environmental Health Perspectives on January 3, 2008 [1]. You can access the article here as a pdf. They found that sunscreen concentrations of 10 μl/L caused complete coral bleaching within 96 hours.

*Note: I edited this post at 10 PM, 2/18/08 to address a comment by ecogeofemme which resulted in a closer, more critical look at the paper on my part. As a result, I split the original post in two and changed the title of this one.*

Here's what you need to know to understand how it happened: Corals are animals in the same phylum as jellyfish, but they also have photosynthetic unicellular alage (zooxanthellae) that live inside the individual coral polyps. The zooxanthellae are what give coral color with their photosynthetic pigments. Corals provide protection and carbon dioxide for the zooxanthellae, and the corals get energy in the form of photosynthetic products (carbon compounds) from the zooxanthellae. Corals are mostly restricted to shallow waters because the symbiotic zooxanthellae require light for photosynthesis. Coral bleaching happens when the zooxanthellae are expelled from the coral polyps, usually due to stress such as rising sea temperatures. This can either kill the coral, or it may be able to recover by recolonizing polyps with a new or the same type of zooxanthellae.

Here's what the authors think happened with the sunscreen: Due to the presence of numerous VLPs (virus-like particles) in the water around the sunscreen-treated corals, they suggest that some of the sunscreen ingredients induced the lytic cycle in latent viral infections in the zooxanthellae. What does that mean? The unicellular zooxanthellae were burst open (lysed) by a virus that had been 'dormant' inside the cells until 'activated' by the sunscreen. Once lysis occurs, the virus can spread to infect new cells. With the zooxanthellae dead, the corals lose their color and a major energy source.

Considering the authors' estimates that 4,000-6,000 tons of sunscreen will be washed off in coral reefs each year, I think this research is unique and important. However, I have concerns about their methods and results.

1. They describe a reasonable procedure for estimating the amount of sunscreen that comes off the body after 20 minutes and say that "The percentage of sunscreen released into the seawater was estimated by HPLC analyses on the sunscreen and seawater samples" but do not report the results of this in their paper. It is not clear if the resulting concentrations were the ones used in the coral experiments.

2. The authors say, "We tested sunscreen (10 μL L-1) containing concentrations of UV filters higher than those reported in most natural environments." After looking through the literature cited, there appears to be only one paper that measured concentrations of UV filters in situ [2]. Poiger et al. (2004) found concentrations of UV filters in the range of 2-125 ng per liter in Swiss lakes. To spare you the math, 125 ng=.125 µg, which means that Danovaro et al. used concentrations ten times those reported in the (sparse) literature. We really need to know what the concentrations are at heavily visited coral reefs for perspective.

3. Based on their results, they conclude, "...the coral response to sunscreen exposure was not dose dependent, as the same effects were observed at low and high sunscreen concentrations. Therefore, we hypothesize that UV filters can have potentially negative impacts even at concentrations lower than those utilized in the present study." This may be true, but there must be some threshold of sunscreen expose that does not cause bleaching. I hope that further research will be able to determine what it is.

4. It was unnatural that the corals were exposed to constant concentrations of sunscreen or isolated ingredients for the duration of the study since the corals were in plastic bags, but this did allow the authors to control level of exposure which would otherwise fluctuate.

I have seen surprisingly little critical discussion of this research, so I'm hoping to start some here. I would really love to hear the authors' responses.

Personally, I do think that excessive amounts of parabens and chemical sunscreens in our ocean is detrimental. However, I'm not convinced by this one paper that sunscreen is a threat in the wild, although this new research certainly presents the distinct possibility. I wrote this post and the next one for people who are interested in making better choices for their environment when it comes to sunscreen.

[1] Danovaro R, Bongiorni L, Corinaldesi C, Giovannelli D, Damiani E, Astolfi P, Greci L, Pusceddu A. 2008. Sunscreens Cause Coral Bleaching by Promoting Viral Infections. Environmental Health Perspectives doi:10.1289/ehp.10966
[2] Poiger T, Buser HR, Balmer ME, Bergqvist P-A, Muller MD. 2004. Occurrence of UV
filter compounds from sunscreens in surface waters: regional mass balance in two swiss lakes. Chemosphere 55(7):951-963; doi:10.1016/j.chemosphere.2004.01.012.