Fertilizing the oceans with iron has been proposed as a way of fighting climate change. The idea is that iron will promote blooms of phytoplankton that will remove carbon dioxide from the atmosphere through photosynthesis. When the phytoplankton dies and sinks, the carbon will effectively be sequestered in the deep ocean.

Enthusiasm for the idea has waned, in part because of concerns about large-scale manipulation of ocean ecosystems. Now, a study in The Proceedings of the National Academy of Sciences points out a specific risk: by promoting the growth of certain organisms, iron enrichment may result in the harmful production of a neurotoxin.

Charles G. Trick of the University of Western Ontario and colleagues studied several species of diatoms of the genus Pseudonitzschia. These organisms produce domoic acid, which they use to help grow but that is toxic to many organisms, including marine mammals and humans.

Large blooms of Pseudonitzschia in coastal waters have led to poisonings of sea lions that eat tainted shellfish. But studies had suggested that in midocean, the diatoms did not produce the toxin.