Could Climate Change Boost Toxic Algal Blooms in the Oceans?

Scientific American

In 1799 about a hundred Aleut hunters working for a Russian-American trading group died in Alaska’s Peril Strait only two hours after eating black mussels collected there. Those who survived did so because they threw up after desperately consuming gunpowder, tobacco and alcohol to purge toxin from their bodies. This was the first recorded incidence of paralytic shellfish poisoning on the west coast of North America.

The Aleuts were killed by natural poisons known as toxins produced by certain algae that were trapped in the mussels’ food-gathering filters. Filter feeders like shellfish, some finned fish and other animals concentrate the toxins present in these algae.

Physical and chemical conditions cause populations of algae to wax and wane in cycles. Out of the vast diversity of plankton in the oceans, the worst offenders are a few species of diatoms, dinoflagellates and cyanobacteria, collectively called harmful algae. For example, some diatoms make domoic acid, which causes vomiting, cramping, headache and even seizures and memory loss; some dinoflagellates produce saxitoxin, which causes numbness, staggering and respiratory failure, among other symptoms.

Toxic blooms can occur naturally when deep, nutrient-rich water wells up in places like the west coasts of North and South America. They can be amplified by land runoff of fertilizers and other chemicals that provide nutrients such as phosphorus. Algal blooms have been increasing in coastal waters nearly everywhere.

In mid-December 2012 recreational mussel harvesting was closed along the entire Oregon coast because the mussels were contaminated with paralytic shellfish toxins. In 2002 razor clam harvesting was prohibited for the full season in Washington State because of high domoic acid levels. Florida’s coastline has frequent outbreaks of the toxic dinoflagellate Karenia brevis, whose toxins can escape into the air and cause severe respiratory distress. Today in the U.S. alone such incidents cause $82 million in public health costs and economic damages to fisheries and tourism annually, according to the National Oceanic and Atmospheric Administration (NOAA). These costs include emergency room visits and other medical treatment, lost work productivity, and fewer dollars reaching local businesses if beaches and sport or commercial fishing is curtailed.

Now scientists are investigating whether climate change could contribute to toxic blooms. As atmospheric carbon dioxide increases, the greenhouse gas is absorbed into ocean water, making it more acidic. The most obvious peril is that marine organisms like clams and sea snails either can’t build their calcium carbonate shells or find their housing harder to maintain. Acidifying ocean conditions could cause toxic algae to become nastier and more abundant. Conversely, the organisms might simply adapt without becoming more poisonous; their numbers could even be reduced.

Of course, researchers must assess ocean acidification as one of many simultaneous stressors in the oceanic environment. Scientists don’t fully understand the relationship between growth rates, toxin production and ocean conditions for these algae. Some species are known to ramp-up toxin production as a defense against predators, others in response to low supplies of crucial nutrients. Another possibility is that the toxins are simply a way for a diatom or dinoflagellate to store excess nutrients, such as carbon or nitrogen, rather than a stress response, says microbial ecologist William Cochlan of San Francisco State University.

To see how nutrient limitation and acidification interact, Avery Tatters, a graduate student in David Hutchins’s lab at the University of Southern California, cultured the diatom Pseudo-nitzschia fraudulenta taken from southern California waters, where it blooms frequently. The species produces domoic acid.

Tatters and colleagues varied the amount of dissolved CO2 and the availability of the silicate the diatoms use to make their shells. In a presentation at a recent ocean acidification conference, Tatters reported that the more CO2 and the less silicate, the higher the diatom’s toxin production–more than doubling at the level of dissolved CO2 scientists expect the oceans to reach by 2100. Earlier research by the Hutchins lab found a fourfold increase in toxicity under limited phosphorus and increased CO2 in a related species.

However, Cochlan cautions, what exactly triggers toxic blooms is “the million-dollar question” that hasn’t been answered. Sometimes algae produce more toxins “when they are growing very well,” he says.

Water temperature may also be a factor. Anke Kremp, a researcher at the Finnish Environment Institute, reported in a January 2012 study that eight strains of the toxic dinoflagellate Alexandrium ostenfeldii grew at very different rates under increased acidity and higher temperatures. The amount of toxin in each cell didn’t always increase, but the composition of the toxic compounds consistently changed as temperature and acidity increased.

A. ostenfeldii can make several nasty chemicals, and the overall trend in Kremp’s study was toward more saxitoxin—the most potent compound in its arsenal. Although this may be bad news for the Baltic Sea and other areas plagued by this dinoflagellate, Kremp also noted that the short duration of most lab studies limits what we can know about how toxic algae may evolve over the next century.

Further, NOAA researcher Vera Trainer says that although some species may become more toxic, there may not be a net increase in risk to humans and other consumers of seafood. If the more harmful species become less numerous, she says, “It’s sort of a moot point.” But if they become more toxic and more numerous, she adds, “you’ve got a double whammy.”

These conundrums illustrate how little we know. The different genetic heritages of diatoms, dinoflagellates and cyanobacteria will affect their survival. And in addition to temperature, other physical factors like available light and even large-scale ocean–atmosphere interactions like the El Niño–La Niña oscillation can affect plankton behavior.

“The work is really at an early stage,” says Ulf Riebesell, a professor of biological oceanography at the Helmholtz Center for Ocean Research in Kiel, Germany. But it is fair to say that as algae and other tiny ocean species solve new survival problems, they may force us to do the same.

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