Fertilising the oceans with iron to
absorb carbon dioxide could increase concentrations of a chemical that
can kill marine mammals, a study has found.
Iron stimulates growth of marine algae that absorb CO2 from the air, and has been touted as a "climate fix".
Now researchers have shown that the algae increase production of a nerve poison that can kill mammals and birds.
Writing in Proceedings of the National Academy of Sciences, they say this raises "serious concern" over the idea.
The toxin - domoic acid - first came to notice in the late 1980s as the cause of amnesiac shellfish poisoning.
It is produced by algae of the genus
, with concentrations rising rapidly when the algae "bloom".
Now, its presence in seawater often requires the suspension of
shellfishing operations, and is regularly implicated in deaths of
animals such as sealions.
Domoic acid poisoning may also lie behind a 1961
incident in which flocks of seabirds appeared to attack the Californian
town of Capitola - an event believed to have shaped Alfred Hitchcock's
interpretation of Daphne du Maurier's The Birds in his 1963 thriller.
Over the last decade, about 10 research projects have investigated iron fertilisation, with mixed results.
But only two of them measured domoic acid production, and only then as
an afterthought, explained William Cochlan from San Francisco State
University, a scientist on the new project.
"We had a number of major aims in this work; but one of
them was to ask 'do you normally find the species of algae that produce
domoic acid, are they producing domoic acid, and will production be
enhanced by iron?'," he said.
In studies conducted around Ocean Station Papa, a
research platform moored in the north-eastern Pacific Ocean, the
answers to all three questions turned out to be "yes".
algae were present naturally; they were producing domoic acid, and
experiments showed that production increased during fertilisation with
iron and copper.
Also, under iron-rich conditions, the
algae bloomed at a rate faster than other types.
The levels of domoic acid in iron-enriched water samples were of the
same order as those known to cause poisoning in mammals in coastal
Ailsa Hall, deputy director of the Sea Mammal Research
Institute at St Andrews University in Scotland, said that domoic acid
poisoning was already becoming a regular occurrence in some parts of
"Ever since 1998 we've seen regular episodes of mass mortality and seizures in sea lions on the US west coast," she said.
The toxin accumulates in animals such as fish that are themselves immune.
"We've seen it in seals, pelicans and harbour porpoises; it does depend
on how much they eat, but if a sea lion or a pelican eats its way
through a school of contaminated anchovies, then that would be enough,"
Dr Hall told BBC News.
Domoic acid's effect on other species was unknown, she
said, but it would be reasonable to think it would also affect marine
mammals such as whales.
Whether iron fertilisation ever will be deployed as a "climate fix" is unclear.
The last major investigation - last year's Lohafex expedition - found
that despite depositing six tonnes of iron in the Southern Ocean,
little extra CO2 was drawn from the atmosphere.
Nevertheless, one company - Climos - aims eventually to deploy the technique on a commercial basis.
A Climos spokesman agreed that further research on domoic acid production was needed.
"Moving forward, we need to understand exactly how deep-ocean
phytoplankton respond to iron, be it naturally or artificially
supplied; whether and in what situations domoic acid is produced, and
how the ecosystem is or is not already adapted to this," he said.
For William Cochlan's team, the potential impact on sea
life is something that regulators and scientists must take into account
when deciding whether to allow further studies or deployment.
"We saw some literature going around with claims like
'there is no indication of toxicity to sea life' - well, if you don't
measure it, of course there's no indication, and we have to keep that
kind of legalese out of science," he said.
"If the end goal is to use it to fight climate warming, then we have to understand the consequences for marine life."