Can Old Plants Learn New Tricks? Testing the Adaptation Potential of Crustose Coralline Algae along a Natural Gradient in Ocean Acidification

Anne Cohen, Geology & Geophysics
Angela Helbling, Geology & Geophysics


2013 COI Funded Project
Ocean Acidification Initiative Project


Coral reefs exist because reef calcifying organisms produce calcium carbonate (CaCO3) skeletons much faster than they can be eroded by the sea, or by boring organisms.  Reef-building corals produce massive CaCO3 skeletons that provide the basic building blocks - the bricks - from which the reef is built.  The encrusting skeletons of crustose coralline algae (CCA) provide the mortar.  By covering the skeletal remains of dead corals in a dense layer of high magnesium (Mg) calcite, they effectively cement the reef together, building a continuous, protective barrier against waves and storms that provides sheltered habitat for a myriad of marine and terrestrial communities, including humans.

High Mg calcite is the most soluble form of biogenic CaCO3 and CCA are thought to be among the most vulnerable to dissolution as anthropogenic ocean acidification progresses over this century.  If this occurs, then the majority of the world’s coral reef systems will shift from net accreting to net dissolving structures within the next several decades.

 We are characterizing the carbonate chemistry of multiple reef systems across the Pacific basin and have identified amongst them, a handful of “naturally acidic’ sites, where seawater pH and carbonate ion concentration are significantly lower than that of the open ocean.  At these sites, CCA is often abundant, thriving under conditions in which their high Mg calcite skeletons should be dissolving.  This raises the possibility of potential adaptation or acclimation of CCA to ocean acidification.

The proposed project tests the hypothesis that CCA can adapt to more acidic conditions by reducing the concentration of Mg in the calcite skeleton.  Calcite with less than 4 mol% Mg is less soluble than aragonite, and could readily accrete and persist under conditions measured at our reef study sites.  However, accurate quantification of carbonate mineral ratios (aragonite:calcite:high Mg calcite) and mineral composition (% Mg in calcite) in biogenic skeletons is not a trivial undertaking.

This pilot study exploits recent advances in technology and data processing, as well as our discovery of a strong natural gradient in ocean acidification within a single CCA-abundant reef system, to develop and apply the quantitative analytical tools that will allow us to begin to test this hypothesis.  I expect that a successful outcome will enable our and other WHOI groups to apply similar tools and approaches across other marine systems and organisms vulnerable to the effects of ocean acidification.  The new line of investigation proposed here will also open up new opportunities to access funds from foundations with interests in coral reef and marine ecosystem conservation as well as a range of federal funding sources including NOAA and EPA, and NSF.