Ocean acidification upsets the acid-base balance of corals

Researchers in Monaco and Australia have solved a key part of the mystery surrounding why ocean acidification slows down the growth of reef-building corals.

Ocean acidification, caused by seawater uptake of man-made carbon dioxide, is rapidly changing the chemistry of the world’s oceans, and current evidence suggests that ocean acidification will be a big threat to marine life in the future, particularly corals that form the structural base of vibrant reef ecosystems.

But what has always remained unclear is how and why ocean acidification is such a problem for corals.

The new research published in the journal Proceedings of the National Academy of Sciences of the USA (PNAS) shows that seawater acidification can slow down the growth of coral skeletons by upsetting acid-base balance in the coral’s skeleton-producing cells and driving down pH in the corals “internal calcifying fluid” where the skeleton is formed.

Using an innovative “bio-imaging” approach to visualize pH changes within living coral tissue, the researchers were able to get a unique insight into the acid-base physiology of corals confronted with low seawater pH.

“We exposed our corals to dramatic, long term changes in seawater pH and chemistry” said Sylvie Tambutté, manager of the team at the Centre Scientifique de Monaco who obtained the results. “Then we took a sneak-peek into their acid-base physiology to see how they were coping”.

Initial results were a surprise; because the corals showed a remarkable ability to maintain high, alkaline pH in their internal calcifying fluid and a stable pH in their cells, which favors the formation of the calcium carbonate crystals that make up their skeleton. Under extreme conditions of acidification however, skeleton growth slowed dramatically as pH decreased in both the calcifying fluid and inside the calcifying cells.

“We make the mechanistic link between CO2 driven seawater acidification and a drop in pH at the site of calcification” said Alexander Venn, first-author on the paper. “This is vital to understanding that corals have to keep maintaining acid-base balance in order continue building reefs in a future high CO2 ocean”.

Although the corals used in this laboratory study were surprisingly tolerant to acidification, the researchers don’t expect things to be so rosy on the reef. Michael Holcomb, formally based in Monaco now at the ARC Centre for Excellence in Coral Reef Studies Australia, stressed the point.  “To really predict what’s going to happen in the future we need to consider that corals experience lots of additional stressors in the natural environment. We also need to work with potentially less resistant species to see how they cope”.

The work was funded by the Government of the Principality of Monaco.

ARTICLE: “Seawater acidification impacts pH at the tissue-skeleton interface and calcification in reef corals”. Alexander A. Venn, Eric Tambutté, Michael Holcomb, Julien Laurent, Denis Allemand and Sylvie Tambutté in Proceedings of the National Academy of Sciences of the USA. Early edition.





ARC Centre of Excellence Coral Reef Studies, 10 January 2013. Press release.

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