One of the most important and abundant calcifying organisms in several marine ecosystems is the blue mussel, Mytilus edulis. It has a wide geographic distribution (Gosling 1992 Developm. Aquacult. Fish. Sci. 25, 1-20) and tolerates a broad range of environmental conditions (Seed and Suchanek 1992 Developm. Aquacult. Fish. Sci. 25, 87-170). Blue mussel beds are also common features in the Kiel
Fjord (Baltic Sea), a habitat dominated by low salinity (10-20 PSU), low alkalinity (1900-2150 μmol kg-1), low pH (minimum values < 7.5) and high pCO2 (maximum value of 2340 ppm). The resulting calcium carbonate saturation state (min. values: Ωarag = 0.34 and Ωcalc = 0.58) is significantly lower than in the open ocean (Thomsen et al. submitted). Therefore, pCO2 in Kiel Fjord during summer is already higher than what is predicted for the future (e.g., Caldeira and Wickett 2003 Nature 425, 365). Additionally, Meier (2006 Clim. Dyn. 27, 39-68) projected an increase of temperature (2.6 to 5.0 °C) in the next 100 years for the Baltic Sea. To contribute to the understanding of the ability of calcifying organisms to live under ocean
acidification conditions and of biomineralization mechanisms, M. edulis from this naturally CO2-enriched habitat were cultured in a flow-through system. Experiments were conducted using CO2 concentrations ranging from 380 ppm to 4000 ppm and temperatures ranging from 5° to 25°C. At the end of the experiments, hemolymph and extrapallial fluid (EPF) were taken and analyzed for pH, pCO2, bicarbonate and elemental ratios. Fluids showed decreased pH and increased CO2 with increasing water pCO2. Elemental ratios (Mg/Ca and Sr/Ca) in the fluids did not show pCO2 or
temperature-related systematic changes.
Furthermore, boron isotopes ([Delta]11B), used in isotope geochemistry as a pH proxy, were investigated by LA-MC-ICP-MS in shell portions precipitated during the experimental treatment. We observed high [Delta]11B variability between different individuals, but also within single shells. Average [Delta]11B values showed a weak positive correlation with pH. When comparing our results to published studies, boron isotopes appeared to represent internal pH conditions (EPF) instead of ambient water
pH (Kasemann et al. 2009 Chem. Geol. 260, 138-147; Reynaud et al. 2004 Coral Reefs 23, 539-546; Sanyal et al. 2000 Geochim. Cosmochim. Acta 64, 1551-1555)
