Food Supply and Seawater pCO2 Impact Calcification and Internal Shell Dissolution in the Blue Mussel Mytilus edulis

Abstract

Progressive ocean acidification due to anthropogenic CO2 emissions will alter marine ecosytem processes. Calcifying organisms might be particularly vulnerable to these alterations in the speciation of the marine carbonate system. While previous research efforts have mainly focused on external dissolution of shells in seawater under saturated with respect to calcium carbonate, the internal shell interface might be more vulnerable to acidification. In the case of the blue mussel Mytilus edulis, high body fluid pCO2 causes low pH and low carbonate concentrations in the extrapallial fluid, which is in direct contact with the inner shell surface. In order to test whether elevated seawater pCO2 impacts calcification and inner shell surface integrity we exposed Baltic M. edulis to four different seawater pCO2 (39, 142, 240, 405 Pa) and two food algae (310–350 cells mL−1 vs. 1600–2000 cells mL−1) concentrations for a period of seven weeks during winter (5°C). We found that low food algae concentrations and high pCO2 values each significantly decreased shell length growth. Internal shell surface corrosion of nacreous ( = aragonite) layers was documented via stereomicroscopy and SEM at the two highest pCO2 treatments in the high food group, while it was found in all treatments in the low food group. Both factors, food and pCO2, significantly influenced the magnitude of inner shell surface dissolution. Our findings illustrate for the first time that integrity of inner shell surfaces is tightly coupled to the animals' energy budget under conditions of CO2 stress. It is likely that under food limited conditions, energy is allocated to more vital processes (e.g. somatic mass maintenance) instead of shell conservation. It is evident from our results that mussels exert significant biological control over the structural integrity of their inner shell surfaces.

How will Ocean Acidification Affect Baltic Sea Ecosystems? An Assessment of Plausible Impacts on Key Functional Groups

“…Similar small negative effects on larval shell growth have also been reported in populations of M. edulis from the North Sea (Gazeau et al 2010;Bechmann et al 2011), and in related Mytilus species around the world (Kurihara et al 2009;Gaylord et al 2011;Sunday et al 2011). Early reports of the effects of ocean acidification on shell growth in adult M. edulis showed negative impacts (Gazeau et al 2007), a result that contrasts with those of Melzner and co-workers in the Kiel fjord (Thomsen et al 2010;Melzner et al 2011), although the latter might be expected to be a result of local adaptation to seasonally low pH-especially at such extreme levels (Melzner et al 2009b;Thomsen et al 2010).…”

“…This 'acidification stress' will operate in concert with other stressors that limit the distribution and function of species-particularly salinity, which is a major determinant of mussel size and distribution in the Baltic Sea (Tedengren and Kautsky 1987;Westerbom et al 2008). Food-dependent tolerance to ocean acidification has not only been observed in the mussel (Melzner et al 2011), but also in barnacle, Balanus improvisus, from the Baltic Sea (Pansch, unpublished results), as well as in other species elsewhere. Consequently, the availability of adequate energy reserves in the form of food may determine tolerance to ocean acidification in many species of macrozoobenthos.…”

“…M. edulis was also observed to recruit actively at 1000 latm CO 2 (&pH 7.75; Thomsen et al 2010). Thomsen et al (2010) suggest that this unusual ability to maintain physiological function despite substantial levels of acidification was due in part to an abundance of food, which provided the energy needed to offset the physiological costs of maintenance and growth at such low ambient pH (Melzner et al 2011). More sensitive early life-history stages of M. edulis from the Skagerrak have been shown to respond differently to ocean acidification: fertilization success increased at reduced pH (induced by high pCO 2 ) whereas subsequent larval shell growth was negatively affected, albeit only slightly (Renborg and Havenhand, unpublished results).…”

“…Unfortunately, there are no available published data on the effects of ocean acidification on Macoma species, and therefore it is not possible to draw meaningful conclusions on the likely sensitivity of this key component of the Baltic Sea food web. It is clear, however, that tolerance of M. edulis to ocean acidification increases with available food ration (Melzner et al 2011). Increased ocean acidification represents an additional stress on marine organisms that can cause corresponding increases in maintenance and physiological costs (Pörtner et al 2004;Pörtner 2008).…”

Ocean acidification and temperature increase impact mussel shell shape and thickness: problematic for protection?

“…The feeding regime (10 mL of ~2.8 million cells mL −1 algae culture) was equivalent to ~4666 cells mL −1 during experimental culture; this is sufficient to allow for growth under OA (Melzner et al. 2011; Thomsen et al. 2013).…”