Iron fertilization in ocean nourishes toxic algae

Carbon sequestration efforts could trigger harmful algal blooms

A plan to combat global warming by fertilizing the ocean may backfire by triggering toxic algal blooms, a new study suggests.

MICRO MUG SHOT The exoskeleton of a toxin-producing diatom Pseudonitzschia turgidula. The phytoplankton species proliferates when fed dissolved iron, a finding that reveals a potential downside of attempts to sequester carbon by fertilizing the ocean. Brian Bill, NOAA/SFSU

Scientists have known for decades that dumping iron in the ocean, especially in areas where that nutrient is in short supply, stimulates the growth of algae and other phytoplankton. These tiny organisms pull carbon dioxide from the atmosphere as they grow, prompting research on the potential of iron fertilization to pull the greenhouse gas from the atmosphere and scuttle it in the deep sea as sunken algae.

But some researchers, besides wondering about the long-term effectiveness of such efforts, have questioned whether such schemes might have unintended side effects, says Charles Trick, a biological oceanographer at the University of Western Ontario in London, Canada. Now, Trick and his colleagues report online the week of March 15 in Proceedings of the National Academy of Sciences that fertilizing the ocean with iron can stimulate algae that make a neurotoxin called domoic acid.

The amounts of domoic acid produced don’t rise to levels known to be toxic to krill and other species that feed on Pseudonitzschia, Trick notes. And the areas where iron fertilization would typically take place are relatively barren zones far from fisheries. Nevertheless, he notes, the effects of long-term exposure to low levels of the neurotoxin are unknown.

The new study is “less a prediction of ecological doom than it is a lesson about not knowing the consequences of our actions,” Trick adds.

In nearshore areas where nutrients are plentiful, algae of the genus Pseudonitzschia — diatoms that release domoic acid as they proliferate — sometimes undergo harmful blooms, Trick says. But open-ocean species of Pseudonitzschia have previously been considered nontoxic.

Sensitive chemical analyses in the new study reveal that open-ocean species of Pseudonitzschia may not be totally harmless. Two species of the diatom produced domoic acid in shipboard experiments.

In 2006, the researchers collected water samples at a site in the Gulf of Alaska where scientists had in previous years conducted iron fertilization experiments. In this nutrient-poor swath of the ocean, unsurprisingly, algae and other plankton were sparse, Trick says. Two species of Pseudonitzschia accounted for only a few percent of the plankton in the water samples and showed up at concentrations of less than 1,000 cells per liter.

But adding dissolved iron to those water samples fueled explosive growth among the Pseudonitzschia. Populations doubled in about nine days, and at the end of the experiments the genus made up as much as 80 percent of the plankton in some water samples.

Results of the experiments bolster the notion that Pseudonitzschia release domoic acid not to poison other creatures but to aid their own uptake of iron. Experiments suggest the neurotoxin binds with iron and then is reabsorbed only by Pseudonitzschia, a technique that helps these diatoms outcompete other plankton for the metal.

Domoic acid production by open-ocean species of Pseudonitzschia “had been anticipated but has not been previously published,” says Kenneth Coale, a biogeochemist at Moss Landing (Calif.) Marine Laboratories. He and his colleagues have also seen increased domoic acid production in their iron fertilization experiments, Coale says. The new findings point out potential consequences should anyone consider the technique as a serious option for carbon sequestration, he adds.

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