Stimulating algae growth might be an efficient way to remove excess carbon dioxide from the atmosphere after all. Detailed analysis of data collected during an ocean fertilization experiment eight years ago in the Southern Ocean indicates that more than half of the carbon-rich algal biomass sank to depths where it will be stored for centuries.

In February 2004, researchers involved in the European Iron Fertilization Experiment (EIFEX) fertilized 167 square kilometres of the Southern Ocean in a stable eddy of the Antarctic Circumpolar Current with several tens of tonnes of iron sulphate. For 37 days the team on board the German research vessel Polarstern monitored the bloom and demise of single-cell phytoplankton in that iron-limited but otherwise nutrient-rich ocean region.

Each atom of iron added pulled at least 13,000 atoms of carbon out of the atmosphere by encouraging algal growth which, via photosynthesis, captures carbon. In a paper in Nature today the team reports that much of the captured carbon was transported to the deep ocean where it will remain sequestered for centuries1.

“At least half of the bloom was exported to depths greater than 1,000 metres,” says Victor Smetacek, a marine biologist at the Alfred Wegener Institute of Polar and Marine research in Bremerhaven, Germany, who led the study.

The team used a light turbidity-meter – a device that measures the degree to which water loses transparency due to the presence of suspended particles - to establish the amount of biomass that rained down the water column towards the sea-floor as dead algae and fish excrement. Data collected outside the eddy showed substantially less deep carbon export.

A subsequent Indio-German fertilization experiment, LOHAFEX, conducted in 2009 in the Southern Ocean, had cast doubts on whether phytoplankton blooms sequester any substantial amounts of carbon. Excessive grazing from fish, and the lack of sufficient dissolved silicic acid which diatom plankton need to build their cell walls, is thought to have restricted the growth of algae during LOHAFEX.

The EIFEX results confirm a hypothesis by the late oceanographer John Martin who first reported in 1988 that iron deficiency limits phytoplankton growth in parts of the subarctic Pacific Ocean2. Martin later proposed that vast quantities of iron-rich dust from dry and sparsely vegetated continental regions may have led to enhanced ocean productivity and thus contributed to the drawdown of atmospheric carbon dioxide during glacial climates3 - an idea to which the new findings also lend weight.

Some advocates of geoengineering think this natural cooling mechanism might help mitigate present-day climate change. However, the idea of deliberately stimulating plankton growth on a large scale is highly controversial. Referring to gaps in scientific knowledge, the parties to the London Convention, the international treaty governing ocean dumping, agreed in 2007 that ‘commercial’ ocean fertilization is not justified (see Convention discourages ocean fertilization).

The finding that ocean fertilization does work, while promising, is not enough to soothe concerns over potentially harmful side effects on ocean chemistry and marine ecosystems, says Smetacek. Some scientists fear that massive ocean fertilization might produce toxic algal blooms or deplete mid-waters of oxygen. Given past controversy over the LOHAFEX experiment, which critics said should not have been approved in the first place, the AWI will not conduct any further artificial ocean seeding studies (see Ocean fertilization experiment draws fire).

“We just don’t know what might happen to species composition and so forth if you were to continuously add iron to the sea,” says Smetacek. “These issues can only be addressed by more experiments including longer-term studies of natural blooms that occur around some Antarctic islands.”

But environmentally harmless studies of the type conducted in the past by AWI and other groups should not be abandoned, experts argue. “We are nowhere near the point of recommending ocean fertilization as a geo-engineering tool,” says Ken Buesseler, a geochemist at the Woods Hole Oceanographic Institution in Massachusetts. “But just because we don't know all the answers, we shouldn't say no to further research.”