How does rising atmospheric CO2 affect marine organisms?

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Corals Forced to Live in a CO2-Enriched and Warmer Environment
Schoepf, V., Grottoli, A.G., Warner, M.E., Cai, W-J., Melman, T.F., Hoadley, K.D., Pettay, D.T., Hu, X., Li, Q., Xu, H., Wang, Y., Matsui, Y. and Baumann, J.H. 2013. Coral energy reserves and calcification in a high-CO2 world at two temperatures. PLOS ONE 8: e75049.

The authors write that "since scleractinian corals are calcifying organisms that already live close to their upper thermal tolerance limits, both ocean warming and acidification severely threaten their survival and role as reef ecosystem engineers." Yet they further state that "no studies to date have measured energy reserve pools (i.e., lipid, protein, and carbohydrate) together with calcification under ocean acidification conditions under different temperature scenarios," which omissions inspired them to conduct an experiment that actually did what was needed to be done in this regard.

What was done
Schoepf et al. studied the single and interactive effects of pCO2 (382, 607 and 741 ppm) and temperature (26.5 and 29.0°C) on coral calcification, energy reserves (i.e., lipid, protein, and carbohydrate), chlorophyll a and endosymbiont concentrations in four species of Pacific coral having different growth morphologies (Acropora millepora, Pocillopora damicornis, Montipora monasteriata and Turbinaria reniformis).

What was learned
The thirteen researchers discovered, as they described it, that coral energy reserves were largely not metabolized "in order to sustain calcification under elevated pCO2 and temperature," in harmony with the fact that "maintenance of energy reserves has been shown to be associated with higher resistance to coral bleaching and to promote recovery from bleaching (Rodrigues and Grottoli, 2007; Anthony et al., 2009)." In fact, they reported that lipid concentrations actually increased under ocean acidification conditions in both A. millepora and P. damicornis, and that they "were fully maintained in M. monasteriata and T. reniformis," while protein, carbohydrate and tissue biomass were also "overall maintained under ocean acidification conditions in all species." And, therefore, they found that "only one of the four corals species studied [Acropora millepora] decreased calcification in response to average ocean acidification levels expected by the second half of this century (741 ppm), even when combined with elevated temperature (+2.5°C)."

What it means
As a result of their several findings, Schoepf et al. concluded that "some corals could be more resistant to combined ocean acidification and warming expected by the end of this century than previously thought," such that "the immediate effects of rising seawater temperature and ocean acidification may be tolerable for some species," possibly because the increased availability of CO2(aq) under ocean acidification conditions may enhance algal productivity, especially in Symbiodinium phylotypes with less efficient carbon-concentrating mechanisms that rely to a greater extent on the passive, diffusive uptake of CO2(aq) and its fertilization effect, citing in this regard the work of Herfort et al. (2008) and Brading et al. (2011).

Anthony, K.R.N., Hoogenboom, M.O., Maynard, J.F., Grottoli, A.G. and Middlebrook, R. 2009. Energetics approach to predicting mortality risk from environmental stress: a case study of coral bleaching. Functional Ecology 23: 539-550.

Brading, P., Warner, M.E., Davey, P., Smith, D.J., Achterberg, E.P. and Suggett D.J. 2011. Differential effects of ocean acidification on growth and photosynthesis among ,phylotypes of Symbiodinium (Dinophyceae). Limnology and Oceanography 56: 927-938.

Herfort, L., Thake, B. and Taubner, I. 2008. Bicarbonate stimulation of calcification and photosynthesis in two hermatypic corals. Journal of Phycology 44: 91-98.

Rodrigues, I.J. and Grottoli, A.G. 2007. Energy reserves and metabolism as indicators of coral recovery from bleaching. Limnology and Oceanography 52: 1874-1882.

Reviewed 1 January 2014