

Crustose Coralline Algae in a CO2-Enriched Ocean

Nash, M.C., Opdyke, B.N., Troitzsch, U., Russell, B.D., Adey, W.H., Kato, A., Diaz-Pulido, G., Brent, C., Gardner, M., Prichard, J. and Kline, D.I. 2012. Dolomite-rich coralline algae in reefs resist dissolution in acidified conditions. Nature Climate Change 3: 268-272.
In a paper published in Nature Climate Change, Nash et al. (2012) write that "coral reef ecosystems develop best in high-flow environments," but that "their fragile frameworks are also vulnerable to high wave energy." And that is likely why they say that the wave-resistant algal rims, which surround many shallow coral reefs and are predominantly made of crustose coralline algae (CCA), are critical structural elements for the survival of such coral reefs. On the other hand, they also indicate that "concerns have been growing about the susceptibility of CCA to ocean acidification, because CCA Mg-calcite skeletons are more susceptible to dissolution under low pH conditions than are coral aragonite skeletons." But they further state, in this regard, that the recent discovery by Nash et al. (2011) of the stable carbonate known as dolomite in the CCA Porolithon onkodes necessitates a reappraisal of the impacts of ocean acidification on it and other CCAs, such as P. pachydermum.

Taking their own advice, the eleven researchers, as they describe it, "carried out dissolution experiments on fragments of CCA that were collected fresh, but then dried, from the Heron Island reef front (Great Barrier Reef, Australia), after which they were exposed to ambient sea water as a control and an enriched CO2 treatment, where "pH ranged from 7.85 to 8.55 (control) and 7.69-8.44 (treatment), tracking natural diurnal changes measured in the lagoon water."

So what did they find?

Nash et al. (2012) determined, in their words, that "dried dolomite-rich CCA have 6-10 times lower rates of dissolution than predominantly Mg-calcite CCA in both high-CO2 (~700 ppm) and control (~380 ppm) environments." And they say that they found this stabilizing mechanism to be due to "a combination of reduced porosity due to dolomite infilling and selective dissolution of other carbonate minerals."

And what is the real-world significance of this finding?

Due to the fact, as they put it, that "the prevailing theories that Mg-calcites with higher Mg content will undergo greatest dissolution, we were surprised to find a trend in the opposite direction." And since dolomite-rich CCA frameworks are common in shallow coral reefs globally, they conclude "it is likely that they will continue to provide protection and stability for coral reef frameworks as CO2 rises."

Additional Reference
Nash, M.C., Troitzsch, U., Opdyke, B.N., Trafford,,J.M., Russell, B.D. and Kline, D.I. 2011. First discovery of dolomite and magnesite in living coralline algae and its geobiological implications. Biogeosciences 8: 3331-3340.

Archived 31 July 2013