The response of coral skeletal nano structure and hardness to ocean acidification conditions.

Ocean acidification typically reduces coral calcification rates and can fundamentally alter skeletal morphology. We use atomic force microscopy (AFM) and microindentation to determine how seawater pCO2 affects skeletal structure and Vickers hardness in a Porites lutea coral. At 400 µatm, the skeletal fasciculi are composed of tightly packed bundles of acicular crystals composed of quadrilateral nanograins, approximately 80-300 nm in dimensions. We interpret high adhesion at the nanograin edges as an organic coating. At 750 µatm the crystals are less regular in width and orientation and composed of either smaller/more rounded nanograins than observed at 400 µatm or of larger areas with little variation in adhesion. Coral aragonite may form via ion-by-ion attachment to the existing skeleton or via conversion of amorphous calcium carbonate precursors. Changes in nanoparticle morphology could reflect variations in the sizes of nanoparticles produced by each crystallization pathway or in the contributions of each pathway to biomineralization. We observe no significant variation in Vickers hardness between skeletons cultured at different seawater pCO2. Either the nanograin size does not affect skeletal hardness or the effect is offset by other changes in the skeleton, e.g. increases in skeletal organic material as reported in previous studies.

The role of aspartic acid in reducing coral calcification under ocean acidification conditions.

Biomolecules play key roles in regulating the precipitation of CaCO3 biominerals but their response to ocean acidification is poorly understood. We analysed the skeletal intracrystalline amino acids of massive, tropical Porites spp. corals cultured over different seawater pCO2. We find that concentrations of total amino acids, aspartic acid/asparagine (Asx), glutamic acid/glutamine and alanine are positively correlated with seawater pCO2 and inversely correlated with seawater pH. Almost all variance in calcification rates between corals can be explained by changes in the skeletal total amino acid, Asx, serine and alanine concentrations combined with the calcification media pH (a likely indicator of the dissolved inorganic carbon available to support calcification). We show that aspartic acid inhibits aragonite precipitation from seawater in vitro, at the pH, saturation state and approximate aspartic acid concentrations inferred to occur at the coral calcification site. Reducing seawater saturation state and increasing [aspartic acid], as occurs in some corals at high pCO2, both serve to increase the degree of inhibition, indicating that biomolecules may contribute to reduced coral calcification rates under ocean acidification.