Experiments by the University of St Andrews show microscopic organisms, called foraminifera ('forams'), suffer the equivalent of tooth-decay as seawater becomes more acidic.

Foraminifera are tiny single-celled organisms that build intricate shells to protect themselves. They feed on algal cells called diatoms, which they break open using tooth-like structures on their shells.

Experiments suggest that as seawater increases in acidity (reduces pH), these 'teeth' are reduced in number and size, with many becoming deformed. These mutations are likely to make them much less effective at feeding.

Since 'forams' may number 500,000 in a square metre of sediment, other organisms further up the food chain are also likely to be affected by these changes.

"The studies of foraminifera are only a small part of the story of ocean acidification impacts. But we know that many species were lost from the fossil record the last time that CO2 rapidly rose, around 55 million years ago, with other major disruptions to marine," said professor David Paterson.

"The threat of future acidification is very real, and comes at a time when the human population depends more than ever on a healthy and productive marine environment," he said.

Too much CO2 is a great threat, being the major greenhouse gas responsible for climate change. Less well-known is the role of CO2 in increasing ocean acidity, by forming carbonic acid: this lowers pH and causes other chemical changes to seawater.

Marine scientists are seriously concerned that these changes will have a significant impact on marine life (and human life), with consequences for shellfish, cold and warm-water corals, as well as many other components of ocean food webs, researchers said.

"There have already been enough measurements around the world to conclusively demonstrate that ocean acidification is occurring. But there are regional differences that we can't fully explain," Dr Phil Williamson, Science co-ordinator of the UK Ocean Acidification research programme, said.

"We now need joined-up science to observe not only chemical changes but also ecological responses, to improve threat assessments and provide the evidence for national and international policy action," said Williamson.