Multiple mechanisms of calcification and heterotrophic capacity may determine the response of calcifying cnidarians to ocean acidification

Ocean acidification (OA) decreases the CaCO₃ saturation state of seawater, making it thermodynamically challenging for calcifying cnidarians to precipitate mineral skeletons.  In this study we tested two hypotheses: 

1. Taxonomically diverse cnidarians display dissimilar responses to OA
2. Differential exploitation of heterotrophy modulates the response to OA

We contrasted the hydrocoral Millepora platyphylla with three scleractinians -- massive Porites spp., Acropora pulchra and Pocillopora meandrina -- to contrast organisms that independently evolved the capacity to calcify in seas with different chemical conditions.  To test the first hypothesis, taxa were maintained at 400 µatm, 750 µatm, and 900 µatm pCO₂ and crossed with 28.0°C and 30.1°C; to test the second hypothesis, M. platyphylla and P. meandrina were exposed to 380 µatm and 700 µatm pCO₂ crossed with three levels of heterotrophy.  In the first experiment, calcification of P. meandrina was reduced 55% by 900 µatm pCO₂ at 30.1°C, but the three other taxa were unaffected and none responded to pCO₂ at 28.0°C.  M. platyphylla calcified 104% faster when augmented with zooplankton, but remained unaffected by pCO₂; P. meandrina calcified 35% slower in filtered seawater (i.e., depleted of zooplankton) at 700 µatm pCO₂.  These results show that different species of calcifying cnidarians are affected unequally by OA, but the effects are greater among species than between classes, and perhaps independent of the seawater chemistry prevailing when calcification first evolved in these taxa.  The capacity for heterotrophy may drive the aforementioned effects and help support resilience to OA.