I was rung up today by a journalist who wanted me to comment on whether sunscreens could cause coral bleaching. Her question was triggered by an article published this month in Environmental health Perspectives by Danovaro, R. et al. (doi:10.1289/ehp.10966.) which shows that very small amounts of sunscreen can cause corals to bleach. This is potentially interesting given the often close association of tourists and coral reefs.
Looking closely at the paper, however, I think there may be a few problems. Whereas the article talks about ‘bleaching’ (which involves the specific movement of symbiotic dinoflagellates out of the coral tissue, which remains behind), the photo that accompanies the article shows a white coral which looks as if it has lost all of its host tissue. That is, the coral looks dead in my opinion rather than bleached.
Danovaro et al. (2008) also discuss the mechanism behind the putative bleaching caused by the sun screens. The authors saw a proliferation of viral like particles or VLPs in their ‘bleached’ specimens and concluded that the VLPs were responsible. Why did Danovaro and his team conclude this? Well, there are earlier pieces of work out of Willy Wilson’s laboartory, supposedly showing that mass coral bleaching is triggered by latent VLPs are triggered by elevated water temperatures (Lohr et al. 2007). Willy is pretty straight up about it. “I’m pretty convinced that viruses are instrumental in the whole bleaching process,” says William Wilson from the Bigelow Laboratory for Ocean Sciences Sciences in Boothbay Harbor, Maine (Nature News).
The final section of Lohr et al. (2007) is entitled “Implications for the mechanism of coral bleaching”. Based on the observation of lots of VLPs can be observed in stressed symbiotic anemones and three species of stressed corals, the paper concludes that VLPs are the reason for why corals bleach. Basically, temperature triggers VLPs which then attack the host and symbionts.
While I think the observation of VLPs in corals is very interesting, I find the evidence and conclusion that they cause of bleaching (or are part of the mechanism) less than convincing. The appearance of VLPs on stressed corals, just as with bacteria that are seen associated with many coral diseases, doesn’t allow us to conclude that they cause the coral to bleach or have a disease.
Isn’t it more likely that VLPs proliferate as a consequence of the dying corals/dinoflagellates as opposed to being the cause.
After all, 30% of all people that die on our roads will have the deadly bacterium Staphylococcus aureus on their skins, yet we would be incorrect to conclude that these poor victims died of Staphylococcus aureus (or that Staphylococcus aureus caused the road accident wounds).
Perhaps the conclusion that “VLPs cause mass coral bleaching” requires a touch of Ockham’s Razor (Principle of parsimony). What do you think?