For hundreds of millions of years, marine creatures of all shapes, sizes and descriptions have gone about the daily business of converting calcium ions dissolved in seawater into the hard shells and skeletons that are so reminiscent of a trip to the seaside. Many of these shell-makers are tiny life forms that die in their billions each day, falling to the seabed to form what will eventually become another geological layer of rock. Without them we wouldn't have the White Cliffs of Dover, Chartres Cathedral or any of the other limestone wonders of the world.

The chemistry behind the process of shell-making, called marine calcification, relies on a complex series of chemical equations kept in a state of equilibrium – balancing acts that can be tipped in either direction. The big question for science is trying to understand how rising levels of man-made carbon dioxide in the atmosphere can affect these chemical equilibria and, ultimately, the ability of these organisms to carry on making their shells and skeletons.

There is a lot at stake in being able to answer this question. Marine calcification is vital for coral reefs and fundamental to the key organisms at the base of the food chain on which all other sea creatures depend.

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But it is also important in terms of answering the wider questions of climate change. This is because the chemical process by which these organisms convert calcium ions into shells is central to knowing how the oceans will – or will not – continue to act as a "carbon sink" that helps to soak up man-made carbon dioxide in the atmosphere.

The latest effort in this field suggests that there will be no simple solution to the problem. Scientists have found that rising levels of carbon dioxide in the atmosphere may affect shell-making creatures in different ways. Some may find it harder to carry out calcification, whereas others may actually find it easier.

Scientists have already estimated that some 118 billion tons of carbon released into the air as carbon dioxide between 1800 and 1994 have been taken up by the oceans worldwide. Indeed, about a third of the carbon dioxide produced by human activities since the start of the Industrial Revolution has been absorbed by the seas. So, without the capacity of the ocean to act as a natural carbon sink, the concentration of carbon dioxide in the air today – about 380 parts per million – would be significantly, and dangerously higher.

Marine calcification actually produces carbon dioxide in the short term, but in the long term it takes carbon out of the atmosphere, for example by the formation of limestone rock deposits on the seabed. Indeed, marine calcification is estimated to be the biggest carbon sink on earth over geological timescales by forming layers of calcium carbonate, the basic ingredient of chalk, limestone and marble. And one of the most important organisms that performs the task is the coccolithophore, a microscopic plant that exudes exquisitely formed calcium "plates" around its cell wall. Coccolithophores may be among the smallest and most insignificant members of the shell-making ocean community, but these tiny photosynthetic organisms play a critical role in banking huge amounts of carbon by growing in huge numbers. Indeed, coccolithophore "blooms" are so big they can even be seen from space.

Coccolithophores need carbon dioxide dissolved in seawater for photosynthesis, and bicarbonate ions, in equilibrium with carbon dioxide, to build their calcium shells. But it was assumed that too much carbon dioxide would jeopardise the delicate balance of this two-way chemical reaction.

The oceans are naturally alkaline, but as more carbon dioxide dissolves in the sea to form carbonic acid, the water's acidity increases and there is evidence to suggest that many marine organisms find it difficult to make their shells when ocean acidity increases beyond a certain point. (The sea doesn't actually become "acidic", which means its pH is less than seven – it just becomes less alkaline.)

Not so, it seems, with the coccolithophore, or at least with the most abundant species, called Emiliania huxleyi. The latest study into this species shows that it appears to thrive on high levels of carbon dioxide. Instead of finding it difficult to make its calcium carbonate plates, as some scientists had expected, the organism can, in fact, make bigger and bigger plates as carbon dioxide concentrations are increased artificially, according to a study published in the current issue of the journal Science.

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Debora Iglesias-Rodriguez, a biological oceanographer at the National Oceanographic Centre in Southampton, carried out the experiments by bubbling carbon dioxide into tanks to see how the species would cope with rising levels of the dissolved gas. She found that the single-celled plant actually excreted bigger plates at higher concentrations. In fact, at levels of carbon dioxide in the experiment reaching 750ppm (about double of those today), the calcification rates also doubled compared with the calcification rates of coccolithophores grown at CO2 levels of 280ppm, which is to say, pre-industrial levels.

"Our widely held assumption that the acidification of the oceans causes a decrease in calcification in all coccolithophores needs to be reappraised," says Dr Iglesias-Rodriguez. "Our data reveal that these microscopic organisms have been responding to climate change by increasing the size of the cells and their calcium carbonate plates."

Previous experiments with coccolithophores suggested that as acidity levels increased, calcification would decrease. However, Dr Iglesias-Rodriguez believes this may have been due to the way the experiments were carried out. The scientists simply added acid to the water to mimic the increase in acidity due to dissolved carbon dioxide. Her method was to simulate the more natural process by bubbling the gas through the water until it dissolved. "This work contradicts previous findings and shows, for the first time, that calcification by phytoplankton could double by the end of this century," she says. "This is important because the majority of ocean calcification is carried out by coccolithophores such as Emiliania huxleyi and the amount of calcium carbonate produced at the ocean surface is known to have a direct influence on levels of atmospheric carbon dioxide."

Dr Iglesias-Rodriguez and her colleagues point out that the last time the earth experienced large increases in the levels of atmospheric carbon dioxide was 55 million years ago during a period known as the Palaeocene-Eocene Thermal Maximum. It was also a time when coccolithophores were abundant.

Paul Halloran of Oxford University, a co-author of the Science study, says that coccolithophores must have thrived during the recent increase in carbon dioxide since the start of the Industrial Revolution. "Our research has also revealed that, over the past 220 years, coccolithophores increased their mass of calcium carbonate by 40 per cent. These results are in agreement with previous observations of coccolithophores being abundant in a period of ocean acidification 55 million years ago," he adds.

One of the main conclusions of the latest study is that it is no longer easy to make simple assumptions about the relationship between rising carbon dioxide, increasing acidity and the ability of sea creatures to continue making shells and calcium carbonate skeletons. Things are more complicated than had been assumed. "Based on our research, the situation is not clear. There will be winners and losers," says Dr Iglesias-Rodriguez.

Victoria Fabry of California State University in San Marcos, an expert on marine calcification, points out that there are many studies suggesting that organisms that engage in making calcium carbonate shells and skeletons will find life difficult in the coming decades.

"As atmospheric CO2 levels continue to rise, we are embarking on a global experiment with as yet uncertain long-term consequences for many marine calcifers," she says.

The coming century could see carbon dioxide levels in the atmosphere rising to 600 parts per million and beyond – which is unprecedented in terms of the human timescale on this planet. So the question of how marine calcifers will cope with this change will be critical in terms of whether the earth's oceans will continue to help us to deal with our carbon dioxide emissions.

How shells are formed

By Jamie Merrill

*Molluscs form shells or exoskeletons by secreting calcium carbonate which forms a hardened coating. But unlike most animal structures, seashells are not made of living cells and are produced outside their host's body.

*Molluscs use a flap of tissue called a mantle, which is located under the shell, to secrete the calcium carbonate and form the shell. The shell's colour and shape varies depending on the rate of growth and the mollusc's diet. So in warmer waters where food is abundant, you'll find thousands of species in different shapes, sizes and colours. While in cold waters where food sources are lacking, most shells lack any colour at all.

*Found across all the world's oceans, seashells are common because they are very effective at converting dissolved calcium carbonate which is abundant in seawater. They vary in size from the massive Tridacna clam of the southwest Pacific which weighs 226kg to the Pythina, a tiny, smooth, translucent clam the size of a grain of rice.