Background
The authors introduce their study by noting that coralline algae have been shown to be a major contributor to the formation and stabilization of coral reefs and in enhancing coral larvae settlement, citing Chisholm (2000). And due to what they call "their crucial role in shallow water ecosystems and their worldwide distribution," they say that "understanding the impact of ocean acidification on calcifying algae is fundamental," especially since "their high-Mg calcite skeleton is the most soluble polymorph of CaCO3 (50% more soluble than calcite and 20% more soluble than aragonite)," and that coralline algae are therefore "likely to be particularly sensitive to a reduction in ?," which is the calcium carbonate saturation state of seawater, citing Ries (2011), Burdett et al. (2012) and Martin et al. (2013).

What was done
Due to the fact that "species with wide geographic ranges, such as coralline algae, are in general very plastic and able to acclimatize to a variety of habitats through morphological and functional responses (Brody, 2004)," Ragazzola et al. (2013) cultured Lithothamnion glaciale, one of the main maerl-forming species in the northern latitudes, under different elevated CO2 levels (410, 560, 840, 1120 ppm = 8.02, 7.92, 7.80, 7.72 pH ) for a period of ten months, but with initial analyses of the various parameters they measured being conducted at the three-month point of the study, as reported by Ragazzola et al. (2012).

What was learned
The six scientists report that the growth rates of the plants in the three CO2-enriched treatments after the first three months of their study were not significantly different from either each other or from those of the ambient-treatment plants. At the end of the ten-month experiment, however, the CO2-enriched plants' growth rates were approximately 60% lower than that of the ambient-treatment plants. On the other hand, they found that the individual cell wall thicknesses of both inter and intra filaments at the three-month point of the study were significantly thinner than those of the control plants, whereas at the end of the ten-month study they were equivalent to those of the control plants.

In discussing these findings, Ragazzola et al. (2013) write that a possible explanation for them is "a shift from what could be termed a 'passive' phase during the first three months to an 'active' phase by the end of ten months," whereby "during the 'passive' phase, the increased energy requirement for calcification due to higher CO2 results in a reduction in the amount of calcite deposited in each cell well," but during the 'active phase,' L. glaciale reduces its growth rate so that the cell wall structure can be better maintained.

What it means
Noting that maintaining skeletal integrity is one of the main priorities of marine organisms living in high CO2 environments, the German and UK researchers say "the results of this study indicate that seawater chemistry can drive phenotypic plasticity in coralline algae," and that "the ability to change the energy allocation between cell growth and structural support is a clear adaptive response of the organism," which they say "is likely to increase its ability to survive in a high CO2 world."

References
Brody, H.M. 2004. P. 247 in Phenotypic Plasticity: Functional and Conceptual Approaches. Oxford University Press, Oxford, United Kingdom.

Burdett, H.L., Aloisio, E., Calosi, P., Findlay, H.S., Widdicombe, S., Hatton, A.D. and Kamenos, N.A. 2012. The effect of chronic and acute low pH on the intracellular DMSP production and epithelial cell morphology of red coralline algae. Marine Biology Research 8: 756-763.

Chisholm, J.R.M. 2000. Calcification by crustose coralline algae on the northern Great Barrier Reef, Australia. Limnology and Oceanography 45: 1476-1484.

Martin, S., Cohu, S., Vignot, C., Zimmerman, G. and Gattuso, J. 2013. One-year experiment on the physiological response of the Mediterranean crustose coralline algae Lithothamnion cabiochae, to elevated pCO2 and temperature. Ecology and Evolution 3: 676-693.

Ragazzola, F., Foster, L.C., Form, A., Buscher, J., Hansteen, T.H. and Fietzke, J. 2012. Ocean acidification weakens the structural integrity of coralline algae. Global Change Biology 18: 2804-2812.

Ries, J.B. 2011. Skeletal mineralogy in a high-CO2 world. Journal of Experimental Marine Biology and Ecology 403: 54-64.