The role of corallum morphology in mediating the response of the coral Porites rus to ocean acidification

Evaluating the response of corals to ocean acidification (OA) is crucial to understanding how coral reefs will change at low pH, yet the factors affecting coral susceptibility to OA remain unknown.  Skeletal morphology might be one factor of importance because it affects mass transfer characteristics and modulates the flux of ions necessary for calcification.  In this ongoing study, the phenotypically plastic coral Porites rus was used to test the role of branching versus plating morphologies in determining the susceptibility to high pCO₂.  Theory predicts that the two morphologies will differ in their response to mass transfer limitation, which might occur at high pCO₂ (e.g., due to low [CO₃^2-]).  Following 21 d exposure to ambient (40.5 Pa) and elevated pCO₂ (81.1 Pa) at two light intensities (251 and 1013 µmol photons m^-2 s^-1), calcification was similar in branches and plates, although morphology affected the response to light; for branches, biomass-normalized net calcification increased with light intensity, but for plates, it was unaffected by light intensity.  These initial results suggest that morphological plasticity will not affect the response of corals to OA, possible because mass transfer effects do not limit calcification at high pCO₂ (≤ 81.1 Pa).  Under high light intensities, morphology might play a stronger role in determining access to soluble metabolites, thereby favoring faster calcification in branches compared to plates.  Future studies will test the hypothesis that phenotypic plasticity in P. rus is a key feature conferring resistance to OA.