Changing oceans breed disease

In the planet’s warming and acidifying oceans, species from corals to lobsters and fish are succumbing to pathogenic infection.

The Great Barrier Reef stretches more than 2,300 kilometers along Australia’s northeast coastline, from north of Bundaberg, Queensland, to the far corner of the continent, just south of Papua New Guinea. As the world’s largest natural structure (visible from space), the reef is bursting with a diversity and abundance of life unlike anywhere else on Earth. (…)

But today, even with that immense biodiversity, the reef is a ghostly vestige of its former self. (…) The death and destruction is overwhelming, says Terry Hughes, renowned reef researcher and director of the Australian Resource Council’s Centre of Excellence for Coral Reef Studies. “The barrier reef north of Cairns will not look again how it did [before this bleaching event] in my lifetime.” (…)

And corals are not alone. Warming and acidifying oceans appear to be contributing to an uptick in diseases among other species, too. From 2013 to 2015, an unprecedented outbreak of sea star wasting disease decimated populations of 20 different species from Mexico to Alaska, killing 90 percent of the sea stars in some areas. Since 2000, young Caribbean lobsters have been falling victim to a viral infection that leaves them with no energy to move or eat. Oysters3 and abalone4 have been plagued by Vibrio bacteria, and numerous fish species are regularly attacked by the protozoan Ichthyophonus.5 In many of these cases, the disease outbreaks have been linked to climate change.

“We have a narrow window of opportunity to quickly reduce greenhouse gas emissions before the degradation of reefs becomes irreversible,” says Hughes. (…)

Effects of increasing ocean acidity

Rising global temperatures are largely due to the increase in atmospheric carbon dioxide (CO2) that primarily stems from automobile and industrial emissions. But higher atmospheric CO2 doesn’t just warm the planet; it also lowers the pH of seawater by reacting with H2O to form carbonic acid. The ocean has become 30 percent more acidic in the last 200 years and, as with temperature, the rate of change is accelerating.

Calcifying organisms, including corals and the coralline algae that paint the reefs’ surfaces and cement their structures, are particularly vulnerable to ocean acidification. The lower pH makes it more difficult for these species to produce the calcium carbonate structures that form the foundation of the reef, and extreme acidification speeds the dissolution of existing carbonate structures, dissolving the very foundations upon which corals build their homes.

Because acidification stresses these reef-building organisms separately from temperature-related effects, it is thought that the combination of changes will present a worst-case scenario for coral and coralline algae. “I think many people assume global climate change impacts, such as ocean warming and acidification, will have additive or synergistic affects on disease impacts to reefs,” says Gareth Williams, a lecturer in the School of Ocean Sciences, Bangor University, U.K. But as scientists delve deeper into marine epidemiology, they are discovering that the reality is far more complex.

In 2009, El Niño conditions led to an outbreak of coralline fungal disease (CFD), which afflicts coralline algae, in Palmyra Atoll, southwest of Hawaii. Once it has infiltrated its host, the fungus radiates outward, leaving patches of dead coral and bare rock in its wake. Studying the outbreak’s destruction, Williams and his colleagues found that the temperature and acidification of the water worked against each other. Higher temperatures increased the disease’s prevalence and lethality, while more-acidic waters, though they stressed and weakened its algal host, also slowed the fungus’s spread.11
“Such complex, interactive effects between global climate change stressors on disease dynamics are important to consider if we are to accurately predict the response of coral reef communities to future climate change,” says Williams.

Unfortunately, for most marine diseases, the role of acidification hasn’t been well studied. “The component of climate change that we have stressed so much is temperature because it’s just such a pervasive influence,” says Harvell. “Ocean acidification is a whole other matter. We know almost nothing about its potential role or ability to affect diseases. It’s a big knowledge gap that needs to be filled.” (…)

Marine diseases also affect humans

Climate change may do more than unleash scores of diseases that attack marine organisms. Lurking in ocean waters are pathogens, such as V. parahaemolyticus and V. vulnificus, which can infect people, either through open wounds on the body or via consumption of contaminated and undercooked seafood, such as raw oysters. With an unsettling case fatality rate above 50 percent,13 Vibrio bacterial infections are the leading cause of seafood-related deaths in the United States. (…)

“You need only look at what’s happening to the Great Barrier Reef at the moment to see how scary the reality is,” says Bangor University’s Williams. “But we cannot lose sight of the fact that much of this could still be within our control.”

Christie Wilcox, The Scientist, 1 July 2016. Article.



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