The first commercial venture into growing vast plankton blooms big enough to suck carbon from the atmosphere starts this month.

Tons of powdered iron will be poured into the Pacific to induce the growth of blooms big enough to be seen from space. The scheme’s backers believe that the iron seeding technique could radically reduce the carbon in the atmosphere and will open up a multimillion-pound carbon-offsetting industry. Simultaneously, they hope to reverse the decline in plankton levels, which are estimated to have fallen by at least 9 per cent in the past two decades.

Iron seeding is thought to work because it provides a crucial nutrient for plankton growth that is missing or in short supply in up to 70 per cent of the world’s oceans.

As the phytoplankton multiplies it will absorb large quantities of carbon and, if the trials are a success, much of it will sink to the seabed when the microscopic plants die and sink.

Planktos, an environmental company based in California, hopes to create a bloom of 50-60 million tonnes of which, it estimates, up to 20 per cent will sink, taking with it 3-5 million tonnes of carbon.

The trial is to be carried out in international waters 350 miles west of the Galápagos Islands. Previous research has shown that iron seeding can encourage a plankton population explosion but scientists have met with mixed results.

Planktos intends to drop up to 100 tonnes of iron, despite concerns in the scientific community. If successful the project will open up iron seeding to the carbon offsetting industry, giving firms an alternative to planting trees to compensate for the carbon emissions blamed for global warming.

Phytoplankton, the plant form of plankton, struggle to grow if there is little iron, so, with the introduction of extra supplies, they can expand into huge blooms.

As phytoplankton grow they photosynthesise and absorb carbon that, when they die and sink, will be trapped on the seabed, where it will be out of the system.

Once the microscopic plants have taken carbon out of the water, the oceans absorb more from the atmosphere, but only if the plankton, and the carbon, have sunk deep.

Other marine life should benefit from increased quantities of plankton, the basis of the food chain in the oceans.

The Planktos expedition is expected to last two years, with repeated monitoring of the 3,860 sq mile drop site, (1,000 sq km) to measure plankton levels and how much carbon they have taken.

Experiments on a much smaller scale have been carried out by scientific institutions and their findings cast doubt on the likely success of bigger carbon-removal projects.

Andrew Watson, of the University of East Anglia’s School of Environmental Sciences, said it was possible that a plankton bloom would be produced but there were serious doubts about how much carbon would be removed and how accurately it would be measured. He said that many oceanographers had concerns about the knock-on effects of man-made plankton blooms.

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