The hidden effects of ocean acidification on reef corals

Recent research by the Centre Scientifique de Monaco (CSM) published in Nature Communications shows that the effects of ocean acidification on reef corals aren’t always obvious to the naked eye, and lie hidden within the coral skeleton.

Seawater uptake of man-made CO2 is causing ocean acidification, changes in ocean chemistry that impair the growth of reef-building corals and other calcifying organisms.  While the threat is well documented, how and why coral growth gets effected by acidification isn’t well understood.

To tackle this knowledge gap, a research team headed by Dr Sylvie Tambutte at CSM conducted a long-term investigation on corals cultivated at low seawater pH in the lab in Monaco.

Authors of the publication : Dr D. Zoccola, Prof. D. Allemand, N. Techer, Dr S. Tambutté, Drs E. Tambutté et A. Venn, N. Segonds. Absent : Dr M. Holcomb.

At first glance, the impacts of acidification weren’t obvious – corals kept at low seawater pH seemed to grow at normal extension rates and visually resembled corals grown under control conditions.  This led the team to initially believe that their study species, Stylophora pistillata, was totally resistant to ocean acidification.

However, when the study’s first author, Dr Eric Tambutte, used a X-ray micro-CT scanner more typically used for scanning human bone to produce a 3D image of the inside of the coral skeletons, the results showed that skeletons formed at low seawater pH were much more porous than their counterparts kept under normal conditions.

The team ruled-out the role of corrosive seawater in causing this porosity.  Instead, they discovered that under seawater acidification, the corals changed the design of their skeletal architecture, enlarging its interior and exterior cavities, becoming what the team called a “porous phenotype”.

“The novelty of these findings lies in the demonstration that ocean acidification not only decreases the rate at which coral skeletons grow, but it can also trigger a biologically-controlled shift in the morphology of the coral skeleton to a more porous, less dense form”, said Dr Sylvie Tambutte.  “While the size of our corals appeared unaffected by ocean acidification, our analysis suggests that acidification causes corals to build skeleton of lower quality, which is potentially much more vulnerable to erosion and physical damage”.

Dr Tambutte went on to say “Our study was conducted in the lab on a single species, so we should be cautious before generalizing our observations to other corals in the field.  However, our lab-based study has the advantage of being able to single-out the effects of acidification against all the other factors that can also influence coral biology.  This is much harder to achieve in the field where the environment can be so variable.  More controlled lab studies like this are needed to improve our understanding of how coral reefs will respond to future ocean acidification”.

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

Monaco Scientific Centre, CSM News, 14 June 2015. Press release.

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