A US company plans to carry out an experiment in the Pacific Ocean with the aim of removing carbon dioxide (CO2) from the atmosphere.
Phytoplankton are the basis for most oceanic food webs
It plans to deposit iron particles in the sea in the hope CO2-absorbing plankton blooms will form.
But the proposal by California-based Planktos has drawn concern from the US government and some environmentalists.
Research shows that seeding the oceans with iron can create favourable nutrient conditions for plankton.
Planktos believes "ocean fertilisation" may be a way to mitigate the climate changes driven by human-produced greenhouse gases.
But this process does not have universal support in the scientific community.
The Intergovernmental Panel on Climate Change (IPCC), the scientific body set up by the United Nations to assess the risk of human-induced climate change, says it regards the process as "unproven".
In its Working Group Three report, released this year, it said: "Geo-engineering options, such as ocean fertilisation to remove CO2 directly from the atmosphere, or blocking sunlight by bringing material into the upper atmosphere, remain largely speculative and unproven, and with the risk of unknown side-effects."
According to documents passed by the US government to the International Maritime Organization (IMO), the company planned to deposit 100 tonnes of iron ore powder this month in a 100 sq km area of ocean hundreds of kilometres west of the Galapagos Islands.
The Canada-based ETC environmental campaign group has asked the US Environmental Protection Agency (EPA) to launch an immediate and full investigation into Planktos' activities.
Russ George, president and chief executive of Planktos, countered that its work built on many years of study, and that the company would collaborate with "scores of scientists and engineers from international ocean science institutions both aboard ship and ashore to develop this form of ocean stewardship in a scientifically, environmentally, and economically viable form".
He added: "This is work that must be done if we are to reverse the apocalyptic collapse of the ocean ecosystem as well as the climate crisis it is helping to accelerate.
"We are the first responders to a planetary medical emergency."