The geo-engineering scheme -- bitterly attacked by environmentalists -- is being conducted by a joint German-Indian research team aboard a German vessel, who say they are not breaking any rules or damaging the ecosystem.

The scientists aim to discharge six tonnes of iron sulphate in the South Atlantic to find out how this affects microscopic marine plants on the ocean surface.

Proponents believe iron nutrition will cause this phytoplankton to grow explosively and thus absorb more atmospheric carbon dioxide, the principal greenhouse gas, as a result of photosynthesis.

It could become an invaluable buffer against global warming, they argue.

Opponents, though, say the consequences of wide-scale iron fertilisation could be catastrophic. They fear it could cause the sea to become more acidic or trigger algal blooms that would de-oxygenate swathes of the ocean.

Environment ministry spokesman Matthias Machnig told AFP on Wednesday that the ministry had asked the German research ministry to "immediately halt" the experiment.

The test runs counter to a global moratorium on ocean fertilisation established under the UN Convention on Biological Diversity (CBD), Machnig said.

According to regional daily the Maerkische Allgemeine, Environment Minister Sigmar Gabriel has also written to Research Minister Annette Schavan, saying the experiment "destroys Germany's credibility and its vanguard role in protecting biodiversity".

However, the research ministry told AFP that it believed the German institute in question, the Alfred Wegener Institute (AWI), "had prior agreement with the environment ministry" for carrying out the experiment.

The iron-sowing expedition, named LOHAFEX, comprises 48 scientists, 30 of them from India's National Institute of Oceanography (NIO), aboard the research ship Polarstern.

The team set sail from Cape Town on January 7 and after two weeks will arrive in a target zone where the dissolved iron will be discharged over a patch of 300 square kilometers (115 sq. miles). The zone has not been identified.

After research, the ship will dock in Punta Arenas, Chile, on March 17.

In a press statement, the Bremerhaven-based AWI said the experiment "is in accordance" with the provisions of the CBD and the London Convention on ocean fertilisation "that call for further research to enhance understanding of ocean iron fertilisation".

Planning for the experiment began in 2005, and the scheme was part of a memorandum of understanding between the AWI and NIO that the two institutes signed during a trip to New Delhi in October 2007 by Chancellor Angela Merkel, it said.

"The size of the fertilized patch is considerably smaller than the impact of melting icebergs that may leave a swathe of several hundred kilometers (miles) breadth of enhanced iron concentrations," AWI added.

"LOHAFEX will contribute legitimate and much needed scientific research to the controversial discussions on ocean fertilization."

Once written off as irresponsible or madcap, geo-engineering schemes are getting a closer hearing in the absence of political progress to roll back the greenhouse gas problem.

Other, far less advanced, projects include sowing sulphur particles in the stratosphere to reflect solar radiation and erecting mirrors in orbit that would deflect sunrays and thus slightly cool the planet.

Green groups are concerned by these projects, and say they could cause more problems than they resolve.

They also say these schemes' financial cost is unknown, but possibly far more than the bill for reducing emissions that cause the problem.

"This case clearly shows why we need strong, enforceable rules to prevent rogue geo-engineers from unilaterally tinkering with the planet," said Jim Thomas of the ETC Group, an environmental watchdog based in Montreal, Canada.

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