Background
In regard to juvenile colonies of massive Porites corals, the authors write that ocean acidification (OA) "challenges their capacity to produce calcareous skeletons," and that it therefore tends to "diminish their capacity to function as ecosystem engineers," citing Wild et al. (2011). And further confusing the situation, they say that pertinent laboratory studies "typically are performed under conditions that do not replicate the natural environment perfectly, notably by providing light at a constant low intensity throughout the day, and with a spectral composition differing from ambient sunlight," citing Kinzie et al. (1984) and Schlacher et al. (2007). "Also of great importance," as they describe it, "is the hydrodynamic regime of reef environments (Monismith, 2007), which is difficult to recreate in tanks and yet is critical for multiple aspects of coral physiology (Dennison and Barnes, 1988; Patterson et al., 1991)."

What was done
In an attempt to overcome these experimental deficiencies, Wall and Edmunds collected juvenile Porites spp. from 2-3-m depths on the back reef of Moorea, attached them to plastic bases with epoxy, and after allowing them to recover in a 1000-L flow-through tank, they placed them in nine 150-L reservoirs consisting of three replicates each of filtered seawater maintained at three different conditions: Treatment 1 (un-manipulated seawater), Treatment 2 (seawater equilibrated with pCO2 at 98.9 Pa), and Treatment 3 (same as Treatment 2 but augmented with baking soda to increase [HCO3^- ] to 2730 µmol kg^-1 at a pHT of 7.69. Then, they conducted three experiments on 4, 6 and 8 February 2012, where in each case the custom-cast, UV-transparent acrylic chambers they created remained on the reef for a period of 28 hours.

What was learned
There were no differences between the behaviors of the juvenile massive Porites corals in Treatments 1 and 2; but calcification was enhanced by fully 81% in Treatment 3 relative to Treatments 1 and 2.

What it means
Quoting the two U.S. researchers, "these findings indicate that juvenile massive Porites spp. are resistant to short exposures to OA in situ," and that "they can increase calcification at low pH and low Ωarg if [HCO3^-] is elevated," while this latter finding leads them to also suggest that juvenile Porites spp. may actually "be limited by dissolved inorganic carbon under ambient pCO2 condition."

References
Dennison, W.C. and Barnes, D.J. 1988. Effect of water motion on coral photosynthesis and calcification. Journal of Experimental Marine Biology and Ecology 115: 67-77.

Kinzie, R.A., Jokiel, P.L. and York, R. 1984. Effects of light of altered spectral composition on coral zooxanthellae associations and on zooxanthellae in vitro. Marine Biology 78: 239-248.

Monismith, S.G. 2007. Hydrodynamics of coral reefs. Annual Review of Fluid Mechanics 39: 37-55.

Patrterson, M.R., Sebens, K.P. and Olson, R.R. 1991. In situ measurements of flow effects on primary production and dark respiration in reef corals. Limnology and Oceanography 36: 936-948.

Schlacher, T.A., Stark, J. and Fischer, A.B.P. 2007. Evaluation of artificial light regimes and substrate types for aquaria propagation of the staghorn coral Acropora solitarvensis. Aquaculture 269: 278-289.

Wild, C., Hoegh-Guldberg, O., Naumann, M.S., Colombo-Pallotta, M.F., Ateweberhan, M., Fitt, W.K., Iglesias-Prieto, R., Palmer, C., Bythell, J.C., Ortiz, J.-C. Loya Y. and van Woesik, R. 2011. Climate change impedes scleractinian corals as primary reef ecosystem engineers. Marine and Freshwater Research 62: 205-215.