More on sunscreen and coral bleaching

Here is an excerpt from a recent news article (click below for full story):

Sunscreen may be killing corals

Cosmos, Monday 4th October

Some experts are yet to be persuaded by the findings, however.

“Any contaminant can experimentally damage a coral under artificially high concentrations. The amount [in the wild] must be tiny due to dilution,” commented Terry Hughes, director of the Australian Research Council’s Centre of Excellence for Coral Reef Studies at James Cook University in Queensland.

“Imagine how much water a tourist wearing one teaspoon of sunscreen swims through in an hour-long snorkel. Compared to real threats like global warming, runoff and overfishing, any impact of sunscreen is unproven and undoubtedly trivial,” he said.

However, Pusceddu argued that the coral response to sunscreen exposure was not dose dependent, “The mechanism appears to be on-off: thus once the virus has been switched on by [the chemicals in] sunscreen, toxicity is irrelevant.”

Ove Hoegh-Guldberg, director of marine studies at the University of Queensland in Brisbane, said the study is interesting, but notes that many factors are likely to be responsible. “Bleaching is like a runny nose: there are lots of things that could cause it.”

Though sunscreens may contribute to coral death, virus-caused bleaching is only a small part of the big picture, he said: “Climate related bleaching is a direct consequence of heat stress and does not involve viruses or bacteria.”

Sunscreens trigger VLPs that cause mass coral bleaching? The case of the blunt razor.

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.Nature News

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). Continue reading

Is there a case for the “bacterial bleaching hypothesis’? Oculina patagonica / Vibrio shiloi revisited

The idea that mass coal bleaching is caused by bacteria bleaching as opposed to temperature stress has been quite a contentious issue over the past decade. Despite the fact that massive amounts of evidence that have accumulated that support the idea that thermal stress has driven the mass episodes of coral bleaching (pigment loss associated with the dissociation of the symbiosis between coral and their dinoflagellates) over the past 30 years, some oxygen has been given to the alternative idea that mass coral bleaching is triggered by the presence of bacteria from the genus Vibrio.The idea that bacteria were involved in triggering coral bleaching originated from work done on the temperate coral Oculina patagonica from the Mediterranean sea (an invading species from the Atlantic coast of South America) seemed to suggest that the annual coral bleaching was due to a bacterial infection from a putative pathogen, Vibrio shiloi.

This discussion focuses on recent work done by my laboratory and which was published in the Nature journal ISME (download here) and which suggests that bacteria are not responsible for triggering mass coral bleaching (in O. patagonica or anything else). We are interested in your thoughts and hope that you will comment in the hope of resolving debate over this issue for once and for all.

The Bacterial Bleaching Hypothesis is based on a series of eloquent microbiological investigations conducted by Eugene Rosenberg and his group revealed the intra-cellular penetration, multiplication and subsequent cellular lysis of zooxanthellae by V.shiloi resulted in the annual bleaching of O.patagonica in temperatures above 28ºC. Since then, the bacterial bleaching hypothesis has been extrapolated as an explanation for global mass coral bleaching, and more recently, the coral ‘probiotic hypothesis’ (link). To some (including Eugene Rosenberg), the workplace emphasis onto bacteria rather than climate change as the cause for mass coral bleaching – the implications for the future of coral reefs would be highly significant if Rosenberg were correct.

Continue reading