For more than a decade, researchers have been fertilising small patches of the world's oceans with iron to see if they could make phytoplankton flourish into massive blooms that would absorb CO2, one of the worst planet-warming greenhouse gases in the atmosphere.
The theory behind the experiments is that the absorbed carbon would sink to the ocean bottom and remain there essentially forever. Many experts have hailed the idea as one surefire solution to the problem of global warming.
Now a vexing new issue has risen to challenge the whole concept: A major species of plankton that produces an environmental poison known to threaten fish life as well as humans would pose a greater environmental threat when fertilised with iron, according to a team of American and Canadian scientists.
Their findings, published in the online edition of the Proceedings of the National Academy of Sciences, suggest that any massive effort to fertilise the oceans should be done with extreme caution, the researchers say.
"It is an indication that we are not masters of nature when it comes to large-scale ecological manipulations," said Charles G. Trick, a microbial biologist at the University of Western Ontario Medical School, who led the research. "Any positive carbon sequestration must be balanced against the evident and unforeseen environmental consequences."
The first experiment
The international effort at fertilising ocean plankton with iron to absorb CO2 from the atmosphere began nearly 20 years ago with seagoing experiments led by Kenneth Coale, director of the Moss Landing Marine Laboratories, and Kenneth Johnson of the Monterey Bay Aquarium Research Institute.
The marine biologists and an international team of researchers sailed to the northwestern Pacific and spread some 450kg of a liquid iron compound over phytoplankton growing across 30 square miles of the open ocean. The iron quickly created a thick mat of one-celled diatoms that spread widely and absorbed 2 million kg’s of carbon from the CO2 in the atmosphere, their calculations showed.
The experiment's success, reported at the time in four articles in the journal Nature, has been followed with a dozen more large-scale experiments by international teams of researchers who also reported similar success.
The scientists raising the new issue sailed their research ship into the northeastern Pacific where the ocean is rich in nutrients but poor in iron, and pumped gallons of the ocean water into shipboard laboratory tanks containing a widespread species of the phytoplankton variety called Pseudonizschia. The species is known to produce domoic acid, a toxin that can cause fatal nerve damage to seabirds and marine mammals. It is also a major cause of human shellfish poisoning in coastal waters.
Adding an iron compound to the plankton caused the single-celled diatoms to bloom heavily, but also greatly increased the amount of the toxin produced in every single cell, the researchers found. And that, in turn, means the toxin would spread more widely through the food web of the ocean, the scientists maintained.
The research was funded by the National Science Foundation and the Department of Energy.
"This possibility of producing a widespread toxin into the plankton of the open ocean needs to be looked at very carefully before any large-scale iron enrichment starts," said William P. Cochlan, a biological oceanographer and marine ecologist at San Francisco State University's Romberg Tiburon Center for Environmental Studies, who was on the research team.
In interviews, both Coale and Johnson, who led the first experiment two decades ago, agreed that the new findings were valid and important.