Studies in the Southern Ocean by Australian scientists found that the shells of tiny amoeba-like creatures called foraminifera have become thinner since the Industrial Revolution.
The scientists say this shows that increasing CO2 uptake in the ocean has a direct effect on the ability of micro-organisms to make shells.
The paper, being presented at the University of Copenhagen's International Scientific Congress on Climate Change, will add to a rising tide of scientific concern over ocean acidification.
Already, ocean acidity has increased about 32% since pre-industrial times. By 2100, it is projected to have increased by perhaps 130%, which scientists fear could have a potentially catastrophic impact on marine life.
In a study published in Nature Geoscience, William Howard, Andrew Moy and colleagues collected the shells of the organisms as they fell towards the sea floor.
They compared the mass of the shells, about the size of a grain of sand, to the mass of older shells on the sea floor.
The modern shells were 30 to 35% lighter than those that formed prior to the industrial period.
The researchers from the University of Tasmania attribute the change to the acidification of the Southern Ocean, which they say is driven by the uptake of CO2 from factories, cars and power stations.
Other scientists are wary of attributing all the blame for the acidification of the Southern Ocean on humans - there is major upwelling of more acidic water from the deep seas.
The more we look at long-term chronic effects of acidification, the more worrying it becomes
Dr Steve Widdicombe, Plymouth Marine Laboratory
Waters from the deep ocean are colder than the surface waters and contain more carbon, which mixes with the seawater to form carbonic acid.
But this will not diminish concern over the problem of ocean acidification in general.
Some of the cutting-edge work in this new field of science is being done at Plymouth Marine Laboratory (PML) in south-west England.
A blustery wind is cutting in from the English Channel when we go to meet the laboratory's Steve Widdicombe, who is gathering mussels from the estuary of the River Exe.
He will use the molluscs to test how they will respond to increasing acidity as CO2 emissions rise.
The Exe is nearly a mile wide at this point and Steve has to work quickly because the tide is about to turn.
The specimens are taken back to the lab where their blood is sampled with a needle thrust through a gap in their shell.
An enzyme test indicates the strength of their immune system.
Then they are placed in tanks where acidity has been increased by bubbling CO2 through the water.
Mussels given health check
This is a long-term test, and Dr Widdicombe suspects it will show that mussels will be seriously compromised by the levels of acidification expected by 2100.
A previous experiment at PML, published in the Proceedings of the Royal Society, showed that starfish would be killed by a pH of 7.7, which may occur by 2100 if CO2 emissions continue to rise.
Ocean pH levels have, on average, previously remained roughly constant at 8.2 for at least half a million years.
"We found that relatively small changes in pH (acid/alkali balance) for a long period cause creatures to use up energy trying to respond to the change," Dr Widdicombe says.
"The more we look at long-term chronic effects of acidification, the more worrying it becomes," he adds.
"It's a continuous stress. We can all respond to temporary stress but if we are under continuous stress we get sick."
The big question with acidification is whether calcifying organisms that need more alkaline conditions to create their shells will be able to adapt to more acidic waters.
"We need to look at evolutionary timescales," Dr Widdicombe explains.
"No-one has done the studies on what if anything would drive the ability to adapt.
"I personally think evolutionary timetables are simply too short to respond to the sort of changes we are making. I really fear the worst."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.