Background
In introducing their study, the authors say that "natural daily variations in seawater pH on some coral reefs already create conditions that are more acidic for short periods than those created in most perturbation experiments designed to simulate pCO2 conditions expected to occur in the oceans of the future," citing Santana-Casiano et al. (2007), Bates et al. (2010) and Hofmann et al. (2011). And they go on to report that the effects of oscillations in pCO2 (i.e. constant versus oscillating treatments) on corals have been tested in only one study: that of Dufault et al. (2012), who found that "corals respond in different ways to each condition with the highest rates of calcification occurring under oscillating pCO2 conditions."

What was done
Thinking it unwise to have only one such study, Comeau et al. conducted a second one, where they "tested the effects of oscillatory pCO2 regimes corresponding to those occurring presently (280-550 ľatm), to one scenario expected by the end of the current century (550-1000 ľatm) and to one scenario of extreme diel oscillation in seawater pCO2 (400-2000 ľatm)," which they compared to "constant pCO2 treatments representing the average daily pCO2 encountered in the three oscillating treatments." This they did with branches of Acropora hyacinthus corals they collected in two habitats (upstream and downstream in a unidirectional flow) in a shallow back reef in Moorea, French Polynesia, "where different diel amplitudes of pCO2 oscillation were expected."

What was learned
The four U.S. researchers report that "calcification rates in all treatments tended to increase over time," and that "after six weeks and for both upstream and downstream corals, there was no effect of pCO2 on the calcification of corals in the oscillating treatments."

What it means
In commenting on their findings, Comeau et al. state that "the increased resistance of Acropora hyacinthus exposed to large pCO2 oscillation (particularly in the 400-2000-oscillating treatment) compared to the response of corals under constant pCO2 conditions suggests that reef corals may be more resistant to future ocean acidification conditions in locations where diel variation in seawater pCO2 is pronounced."

References
Bates, N.R., Amal, A. and Andersson, A.J. 2010. Feedbacks and responses of coral calcification on the Bermuda reef system to seasonal changes in biological processes and ocean acidification. Biogeosciences 7: 2509-2530.

Default, A.M., Cumbo, V.R., Fan, T.Y. and Edmunds, P.J. 2012. Effects of diurnally oscillating pCO2 on the calcification and survival of coral recruits. Proceedings of the Royal Society of London B Biological Science 279: 2951-2958.

Hofmann, G.E., Smith, J.E., Johnson, K.S., Send, U., Levin, L.A., Micheli, F., Paytan, A., Price, N.N, Peterson, B., Takeshita, Y., Matson, P.G., Crook, E.D., Kroeker, K.J., Gambi, M.C., Rivest, E.B., Frieder, C.A., Yu, P.C. and Martz, T.R. 2011. High-frequency dynamics of ocean pH: a multi-ecosystem comparison. PLOS ONE 6: e28983.

Santana-Casiano, J.M., Gonzalez-Davila, M., Rueda, M.J., Llinas, O. and Gonzalez-Davila, E.F. 2007. The inter-annual variability of oceanic CO2 parameters in the northeast Atlantic subtropical gyre at the ESTOC site. Global Biogeochemical Cycles 21: 10.1029/2006GB002788.