To help cool a warming world, some scientists have suggested fertilizing the oceans with iron. The idea is to stimulate vast blooms of phytoplankton, which sequester carbon dioxide. But such an approach could have deadly consequences. Experiments in the northern Pacific Ocean show that phytoplankton in waters far from land produce a molecule called domoic acid, a neurotoxin that has killed wildlife and people in coastal areas.
About 20% of Earth's oceans are strangely bare of greenery. Despite abundant nitrogen and phosphorous, these areas—known as high-nutrient, low chlorophyll zones—have very small populations of phytoplankton, single-celled algae that photosynthesize. That's because phytoplankton also need iron, and these zones tend to be too far from the river deltas and land runoff that supply it. So-called geoengineers have proposed sprinkling these regions with iron to foster phytoplankton blooms big enough to consume vast amounts of the greenhouse gas carbon dioxide and thus cool the globe.
But too many phytoplankton can be a bad thing, especially when it comes to members of the genus Pseudonitzschia. This alga produces domoic acid, which it spews into the surrounding seawater to help it ingest iron. Domoic acid also happens to be a potent neurotoxin that travels up the food chain into shellfish and small fish. In 1987, three people died and 107 fell ill from amnesic shellfish poisoning after eating mussels that fed on Pseudonitzschia blooms off Prince Edward Island in Canada. The poison has also killed sea lions off the coast of California, and coastal regions such as Seattle, Washington, and Vancouver, Canada, often close beaches and fisheries because of Pseudonitzschia blooms.
Still, researchers haven't found domoic acid in phytoplankton-poor zones seeded with iron, suggesting that geoengineering efforts would be safe. But that may be because phytoplankton samples in these experiments were preserved and tested on shore, which may have affected their production of domoic acid, says Charles Trick, an oceanographer at the University of Western Ontario in London.
So in the new study, Trick and colleagues tested seawater in the open ocean west of British Columbia. They detected small amounts of domoic acids—about 30 trillionths of a gram per liter—in the samples. After they added iron, the phytoplankton quintupled, and domoic acid concentrations within the algae doubled. Concentrations of domoic acid in the water also increased. Seeding a batch with both iron and copper, which is often found in low-grade iron, amplified the effects, the team reports online today in the Proceedings of the National Academy of Sciences.
The implications could be serious for any would-be phytoplankton farmers, Trick says. When given an iron boost, the domoic acid concentrations were in the range of concentrations seen in toxic coastal blooms, meaning they could be deadly to wildlife.
Trick speculates that humans wouldn't have the chance to eat contaminated organisms from these regions, but the toxin could travel up the food chain into crabs and small fish and even higher into sea mammals and birds. Further studies are needed to determine just what the effects would be, he says. "It's a warning that we don't know very much about how nature will really respond."
"They've done a very thorough job," says Philip Boyd, an oceanographer at the University of Otago in Dunedin, New Zealand, who has run two large-scale fertilization experiments in the sub-arctic Pacific and the Antarctic oceans. On top of pointing out potential risks of iron, Boyd says, the results challenge the notion that iron fertilization strategies would be cheap, as they would require expensive, analytical-grade iron rather than low-grade copper-containing iron. "One of the attractions of iron, and the way they sell it to venture capitalists, is that it can do it quite cheaply—for the carbon you sequester, it's not going to cost that many dollars," Boyd says. "But there may be hidden costs, and this may be one of them."