Researchers from North­eastern University’s Marine Sci­ence Center and the Uni­ver­sity of North Car­olina at Chapel Hill have found that mod­erate ocean acid­i­fi­ca­tion and warming can actu­ally enhance the growth rate of one reef-​​building coral species. Only under extreme acid­i­fi­ca­tion and thermal con­di­tions did cal­ci­fi­ca­tion decline.

Their work, which was pub­lished Wednesday in the journal Pro­ceed­ings of the Royal Society B: Bio­log­ical Sci­ences, is the first to show that some corals may ben­efit from mod­erate ocean acidification.

Justin Ries, an asso­ciate pro­fessor at North­eastern and one of the paper’s co-​​authors, focuses his research on the bio­log­ical impacts of rising atmos­pheric carbon dioxide levels, which has been increasing ocean acidity since the Indus­trial Rev­o­lu­tion. One group of organ­isms that would be greatly affected by ocean acid­i­fi­ca­tion are those that build cal­cium car­bonate shells and skele­tons, such as coral, snails, and clams, which, he said, are already near the point of dis­solving in some parts of the ocean.

The authors attribute the coral’s pos­i­tive response to mod­er­ately ele­vated carbon dioxide to the fer­til­iza­tion of pho­to­syn­thesis within the coral’s algal sym­bionts, which may pro­vide the coral with more energy for cal­ci­fi­ca­tion even though the sea­water is more acidic. They pro­pose that the even­tual decline in coral cal­ci­fi­ca­tion at the very high levels of carbon dioxide occurs when the ben­e­fi­cial effects of fer­til­izing pho­to­syn­thesis are out­weighed by the neg­a­tive effects of acid­i­fi­ca­tion on the skeleton-​​forming process.

The study showed that this species of coral (Sideras­trea siderea) exhib­ited a peaked or par­a­bolic response to both warming and acid­i­fi­ca­tion, that is, mod­erate acid­i­fi­ca­tion and warming actu­ally enhanced coral cal­ci­fi­ca­tion, with only extreme warming and acid­i­fi­ca­tion neg­a­tively impacting the corals,” Ries said. “This was sur­prising given that most studies have shown that corals exhibit a more neg­a­tive response to even mod­erate acidification.”

Fur­ther­more, their work indi­cates that ocean warming is likely to threaten this coral species more than acid­i­fi­ca­tion by the end of the cen­tury, based on pro­jec­tions from the Inter­gov­ern­mental Panel on Cli­mate Change.

He noted that in the past 200 years, ocean pH level has dropped from 8.2 to 8.1 and is expected to fall even fur­ther to about 7.8 over the next one or two cen­turies. That is a sig­nif­i­cant decrease over a rel­a­tively short period of time, Ries said, when looking at the geo­logic his­tory of ocean acidification.

The amount of change that would typ­i­cally occur in about 10 mil­lion years is being con­densed into a 300-​​year period,” Ries said. “It’s not the just the mag­ni­tude of the change that mat­ters to the organ­isms, but how quickly it is occurring.”

In addi­tion to pub­lishing these find­ings, Ries has lever­aged his research in this area to secure a pres­ti­gious fel­low­ship from the Hanse Wis­senschaft­skolleg Insti­tute for Advanced Study in Del­men­horst, Ger­many and a sup­porting research award from the National Sci­ence Foun­da­tion. He will spend 10 months over the next four years there working with researchers at three promi­nent German research insti­tu­tions to use var­ious tools such as micro­elec­trodes, iso­tope ratios, and pH sen­si­tive dyes to see how ocean acid­i­fi­ca­tion affects the organ­isms’ internal cal­ci­fying processes that lead to the for­ma­tion of their shells and skeletons.

Acid­i­fi­ca­tion of the sur­rounding sea­water is cer­tainly impor­tant for marine organ­isms, but what is equally as impor­tant — per­haps even more impor­tant — is how the chem­istry of their internal cal­ci­fying fluid responds to these changes in sea­water chem­istry,” Ries said.