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Although phytoplankton may prove an unlikely ally in the effort to reduce the impact of climate change, enlisting these microorganisms to sequester carbon could have deadly consequences.

One proposed method to combat climate change is to dump iron in regions of the ocean where the growth of marine phytoplankton - tiny organisms that grow via CO2-absorbing photosynthesis - is limited by the amount of iron available.

Adding iron is intended to cause a bloom of phytoplankton growth, sucking up CO2 in the process.

But new findings, published today in the Proceedings of the National Academy of Science, show that one type of phytoplankton that thrives under such circumstances makes domoic acid, a potent neurotoxin.

This neurotoxin can move up the food chain as other animals eat the phytoplankton, harming sea life. It can kill or weaken birds, fish, sea mammals or even humans who eat seafood that contains the toxin.

In coastal waters, blooms of Pseudonitzschia, the organism that produces the toxin, have occasionally closed coastal shellfish harvests. In a few instances, people have died from consuming contaminated seafood.

Coastal waters typically contain much more iron, which encourages Pseudonitzschia's growth, says Professor Charles Trick of the University of Western Ontario, who led the new study.

The researchers gathered samples of seawater from the eastern Pacific during an expedition designed to test the effect of adding iron to the ocean to stimulate plankton growth. They added extra iron to the seawater samples on board their ship and measured the amount of neurotoxin that was produced and what kind of phytoplankton grew.

"If we added the normal amount of iron that one would add for these fertilization experiments, the level of toxins in each of the cells goes higher," says Trick. "It allows (Pseudonitzschia) to grow faster. And as they grow, they stop the other species from growing. They become dominant."

"The surprising part was not just that it made toxin," says Trick, "but that it made lots of toxin, and it stopped the other species from getting the nutrients."
Growing concerns

Proposals to use large-scale iron fertilisation to combat climate change have been met with concern about the unintended consequences they could bring.

One company that had hoped to sell carbon offsets by seeding the ocean with plankton, Planktos, has put its efforts on hold. But other companies are still pursuing the possibility, says Kenneth Coale of the Moss Landing Marine Laboratory in California.

"We too have measured domoic acid production in our enrichment experiments, but find a much larger response than those reported by Trick et al.," says Coale.

"Together these results suggest a wrinkle in the notion that iron fertilisation could simply draw down atmospheric carbon dioxide. Nature is always more complex, and the curve she has thrown us needs to be carefully considered before iron fertilisation should be seriously considered as a carbon sequestration option."

Trick agrees. "Now we've dealt with one of the uncertainties, and we recognise that we don't know as much as we think," he says. "Modifications of nature at this big scale are kind of a fickle process. We'd like to think that we're smart enough to understand exactly what's going to happen. But in reality, the possibility that something unsuspected like this could happen is large, and we might not want to take that risk."
Weapons of last resort?

"That's one of the concerns with all of these geoengineering schemes," says Mak Saito of the Woods Hole Oceanographic Institution in Massachusetts.

"If we're going to actively change the planet's chemistry or biology to actively reverse global warming, what are the unintended consequences? Here he's already documented one of the concerns. What are the ones we don't even know about?"

"None of these strategies will really be effective without conservation and reduction of carbon dioxide emissions," says Saito. "I kind of see them as weapons of last resort. To what extent are we on a trajectory that is so bad that we have to use these, and at what point do we say we have to accept these consequences?"

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