The sea has soaked up nearly half of the CO2 that has been emitted by fossil fuels since the start of the Industrial Revolution.

The gas dissolves into surface waters and is then transported around the oceans.

But a key role is played by plant micro-organisms called phytoplankton, which take in the dissolved gas at the ocean's sunlit surface as part of the process of photosynthesis. This plankton dies and eventually sinks to the ocean floor, thus storing the carbon for potentially millions of years.

One of the big questions is how much more of CO2 the sea can absorb.

If, like a saturated sponge, the oceans cannot take up any more, atmospheric concentrations of CO2, the principal greenhouse gas, would sharply rise and stoke global warming.

Another concern is that rising levels of dissolved CO2 also causes acidification of seawater. Wildlife such as coral, which secretes a skeletal structure, are known to be affected by acidification but the impact on other marine species is largely unknown.

In an innovative experiment reported on Sunday in Nature, researchers closed off part of Raune fjord in southern Norway to see how plankton reacted to different levels of CO2.

They used nine large enclosed tanks of seawater that were exposed to CO2 concentrations likely to prevail over the next 150 years.

These three levels were today's concentrations of CO2; double that concentration, to simulate the air in 2100; and triple, replicating the air in 2150).

To feed the plankton, the researchers added nutrients to simulate food usually brought up by ocean currents and upwelling, and then monitored plankton levels over the next 24 days.

The investigators found that, the higher the CO2 level, the more the plankton bloomed.

The organisms were able to gobble up to 39 percent more dissolved carbon compared with today, but did not need any additional nutrients to achieve this.

The findings "underscore the importance of biologically-driven feedbacks in the ocean to global change," say the authors, led by Ulf Riebesell of the Liebniz Institute of Marine Sciences in Kiel, Germany.

The paper, though, warns that taking the carbon out of the air and placing it into the sea could cause problems.

Algal blooms could inflict oxygen depletion in some parts of the ocean while rising carbon levels may cause an imbalance in primary nutrients, with implications that could ripple across the marine food web.

Supporters of so-called geo-engineering -- unconventional projects aimed at easing global warming -- have been closely looking at plankton, seeing in it fantastic potential as a carbon sponge.

Their schemes entail sowing the sea with iron filings and other nutrients to encourage plankton growth and thus suck up more of the atmospheric CO2.

Mainstream scientists say such experiments are unjustified, given the uncertainty surrounding the environmental impact and the many knowledge gaps that persist about ocean topography and currents.

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