Background
The authors observe that "most ocean acidification studies so far have been simplistic," in that they have not "jointly considered physical, chemical and biological interactions." They note, however, that "the emerging discipline of marine ecomechanics (Denny and Helmuth, 2009; Denny and Gaylord, 2010) provides a valuable framework in which such inter-disciplinary research can be conducted." The old experimental approach, as they describe it, "overlooks the existence of a discrete micro-layer (i.e., diffusion boundary layer, DBL) at the surface of many aquatic organisms that buffers them from the surrounding mainstream seawater (Vogel, 1996)." This feat is achieved by metabolic processes that alter the water chemistry within the DBL, with photosynthesis increasing pH, and calcification and respiration reducing pH (Hurd et al., 2009). Therefore, as they continue, "the chemical environment within the DBL differs from that in the mainstream seawater just micrometers away, with implications for both the dissolution of, and formation of, calcium carbonate (Borowitzka and Larkum, 1976; Ries et al., 2009)."

What was done
In a study employing the still-evolving ecomechanic approach, Hurd et al. used pH micro-electrodes and oxygen micro-optodes to directly measure the DBL thickness at the surface of the coralline seaweed Sporolithon durum, the sea urchin Evechinus chloroticus and the abalone Haliotis iris, at a range of seawater velocities (0-10 cm/sec) that reflected those found within a temperate reef in Southern New Zealand (45.38°S) that may be vulnerable to ocean acidification (OA). For S. durum, they also determined whether or not DBL thickness would be affected when mainstream seawater pH was reduced to 7.5, which is the projected worst-case scenario for the year 2215 as calculated by Caldeira and Wickett (2003). In addition, they measured pH fluctuations at the surface of S. durum on a timescale of hours at ambient seawater pH and pH 7.5 at two different flows (1.5 and 6.3 cm/sec), while for the invertebrates they measured surface pH fluctuations at ambient pH and a flow of 1.5 cm/sec.

What was learned
The seven scientists determined that coralline seaweeds encounter a wide range of pH values over each daily cycle; but they found that they are able to increase their pH substantially due to photosynthesis and to successfully withstand periods of very low pH (relative to the present day and comparable to values predicted for coming centuries) under low flows. In the case of sea urchins, they found that they are currently subjected to -- and readily survive -- very low pH values (7.5) at their surfaces in slow seawater flows, which values are also akin to those that are predicted to occur in times to come. And in the case of abalone, they say they "have a very thin DBL and hence their outer surface is subjected to the pH in the mainstream seawater, in all flow conditions," yet they too persist, probably because they are "internal calcifiers" and "the reduced pH predicted for future oceans may not directly alter their rates of calcification."

What it means
In concluding their report, Hurd et al. say their findings "support the view that although the role of chemistry on OA is well understood, the biological responses to OA will be complex," citing their own work and that of Fabry et al. (2008), while noting that "both the site of calcification and the ecomechanics of the biota, i.e., the interactions between their morphology, physiology and the surrounding hydrodynamic environment, must be considered." And it would appear that that consideration suggests that earth's marine calcifiers are much more robust to OA than most people had originally thought.

References
Borowitzka, M.A. and Larkum, A.W.D. 1976. Calcification in the green alga Halimeda. Journal of Experimental Botany 27: 879-893.

Denny, M.W. and Gaylord, B. 2010. Marine ecomechanics. Annual Review of Marine Science 2: 89-114.

Denny, M. and Helmuth, B. 2009. Grand challenges. Confronting the physiological bottleneck: a challenge from ecomechanics. Integrated Comparative Biology 49: 197-201.

Fabry, V.J., Seibel, B.A., Feely, R.A. and Orr, J.C. 2008. Impacts of ocean acidification on marine fauna and ecosystem processes. ICES Journal of Marine Science 65: 414-432.

Hurd, C.L., Hepburn, C.D., Currie, K.I., Raven, J.A. and Hunter, K.A. 2009. Testing the effects of ocean acidification on algal metabolism: considerations for experimental designs. Journal of Phycology 45: 1236-1251.

Ries, J.B., Cohen, A.L. and McCorkle, D.C. 2009. Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology 37: 1131-1134.

Vogel, S. 1996. Life in Moving Fluids: the Physical Biology of Flow. Princeton University Press, Princeton, New Jersey, USA.