How does rising atmospheric CO2 affect marine organisms?

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The Effects of Ocean Acidification on Octocorals
Gabay, Y., Benayahu, Y. and Fine, M. 2013. Does elevated pCO2 affect reef octocorals? Ecology and Evolution 3: 465-473.

The authors write that octocorals possess "an internal calcium carbonate skeleton comprised of microscopic sclerites embedded in their tissue," citing Fabricius and Alderslade (2001), Jeng et al. (2011) and Tentori and Ofwegen (2011). They also note that they are "the second most important faunistic component in many reefs, often occupying 50% or more of the available substrate." And in light of these facts, they say "it is important to predict their response to a scenario of increased pCO2."

What was done
Gabay et al. studied three species of octocorals from two families found in the Gulf of Aqaba at Eilat, including the zooxanthellate Ovabunda macrospiculata and Heteroxenia fuscens (family Xeniidae) and Sarcophyton sp. (family Alcyoniidae), which they maintained for five months under normal (8.2) and reduced (7.6 and 7.3) pH conditions, while they assessed their pulsation rate, protein concentration, polyp weight, density of zooxanthellae and chlorophyll concentration per cell.

What was learned
In the words of the three Israeli scientists, their results indicated "no statistically significant difference between the octocorals exposed to reduced pH values compared to the control."

What it means
Quoting once again the words of Gabay et al., "these findings indicate that octocorals may possess certain protective mechanisms against rising levels of pCO2," and in this regard they suggest that "their fleshy tissues act as a barrier, maintaining a stable internal environment and avoiding the adverse effects of the ambient elevated pCO2," in line with the similar thinking of Rodolfo-Metalpa et al. (2011), while noting that "this suggestion is further supported by our finding that the ultrastructural features of O. macroscipulata sclerites are not affected by increased ambient seawater acidity." And so it is that they ultimately conclude that "octocorals might be able to acclimate and withstand rising levels of ocean acidification, even under conditions that are far beyond what is expected to occur by the end of the present century (pH 7.9)."

Fabricius, K.E. and Alderslade, P. 2001. Soft Corals and Sea Fans: A Comprehensive Guide to the Tropical Shallow Water Genera of the Central-West Pacific, the Indian Ocean and the Red Sea. Australian Institute of Marine Science, Townsville, Australia, and New Litho, Melbourne, Australia.

Jeng, M.S., Huang, H.D., Dai, C.F., Hsiao, Y.C. and Benayahu, Y. 2011. Sclerite calcification and reef-building in the fleshy octocoral genus Sinularia (Octocorallia: Alcyonacea). Coral Reefs 30: 925-933.

Rodolfo-Metalpa, R., Houlbreque, F., Tambutte, E., Boisson, F., Baggini, C., Patti, F.P., Jeffree, R., Fine, M., Foggo, A., Gattuso, J.P. and Hall-Spencer, J.M. 2011. Coral and mollusk resistance to ocean acidification adversely affected by warming. Nature Climate Change 1: 308-312.

Tentori, E. and van Ofwegen, L.P. 2011. Patterns of distribution of calcite crystals in soft corals sclerites. Journal of Morphology 272: 614-628.

Reviewed 28 August 2013