Impact of ocean acidification on Mediterranean coralline algae

Coralline algae are a major calcifying component of most Mediterranean benthic coastal ecosystems. They are of particular ecological importance, inducing settlement and recruitment of numerous invertebrates and providing habitats for a high diversity of associated organisms. They are also of significant importance in the carbon and carbonate cycles of shallow coastal ecosystems, being major contributors to CO2 fluxes through high community CaCO3 production and dissolution. However, coralline algae are among the calcifying organisms that appear to be the most sensitive to ocean acidification due to the solubility of their high magnesium calcite skeletons. We investigated the effects of ocean acidification on coralline algae both through in situ observations in a volcanic CO2 vent area off Ischia (Italy) and through a long-term (one-year) mesocosm experiment combining the effects of elevated pCO2 (lowered pH) and elevated temperature.

We focused our work on (i) epiphytic crustose coralline algae in Posidonia oceanica meadows, where coralline algae are the dominant contributors to calcium carbonate mass on seagrass blades and (ii) the crustose coralline alga Lithophyllum cabiochae, which is one of the main calcareouscomponents of coralligenous communities in the Mediterranean Sea. We observed that coralline algae were absent at pHT 7.7 (pH on the total scale) in naturally acidified seawater where other calcifiers were living (Fig. 3). We showed a strong combined effect of elevated pCO2 and temperature on L. cabiochae with a decrease in calcification in summer and an increase in mortality and tissue damage under elevated pCO2 (700 µatm, pHT 7.9) and elevated temperature (+3°C, Fig. 4). Our findings suggest that the degree of ocean acidification and global warming expected over the next 100 years may have major consequences for the biodiversity and biogeochemistry of coastal ecosystems dominated by coralline algae.

This work contributes to the EU `Mediterranean Sea Acidification under a changing climate’ project (MedSeA; grant agreement 265103) and the European Project on Ocean Acidification (EPOCA; grant agreement 211384).

References

S. Martin, J.-P. Gattuso (2009) Response of Mediterranean coralline algae to ocean acidification and elevated temperature. Global Change Biology, 15, 2089-2100.

S. Martin, R. Rodolfo-Metalpa, E. Ransome, S. Rowley, M.-C. Buia, J.-P. Gattuso, J.M. Hall-Spencer (2008) Effects of naturally acidified seawater on seagrass calcareous epibionts. Biology Letters, 4, 689-692.

J.M Hall-Spencer, R. Rodolfo-Metalpa, S. Martin, E. Ransome, M. Fine, S.M. Turner, S.J. Rowley, D. Tedesco, M.-C. Buia (2008) Volcanic carbon dioxide vents show ecosystem effects of ocean acidification. Nature, 454, 96-99.

Martin S., Hall-Spencer J. & Gattuso J.-P., 2012. Impact of ocean acidification on Mediterranean coralline algae. IMBER Newsletter Issue n°20 – May 2012. Article.


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