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

Date Published: September 16, 2011

Publisher: Public Library of Science

Author(s): Frank Melzner, Paul Stange, Katja Trübenbach, Jörn Thomsen, Isabel Casties, Ulrike Panknin, Stanislav N. Gorb, Magdalena A. Gutowska, John Murray Roberts.

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.

Partial Text: Progressive ocean acidification due to anthropogenic CO2 emissions will impact marine ecosytems [1]–[4]. Calcifying organisms may be particularly vulnerable to ocean acidification, as elevated seawater pCO2 shifts the carbonate system speciation towards a decreased concentration of carbonate ions. This leads to a reduced CaCO3 saturation state for aragonite and calcite (Ωarag or Ωcalc), which can negatively impact calcification rates in several marine heterotrophic taxa [2], [3]. Bivalve molluscs, particularly their early life stages, can react with decreased rates of growth and calcification, as well as decreased shell strength towards elevated seawater pCO2[5]–[9]. Changes in growth and calcification performance encountered in bivalve molluscs most likely are related to an altered energy budget allocation, with more energy potentially being consumed by homeostatic processes [10], [11]. Exoskeleton dissolution can be observed in some gastropod and bivalve mollusk species when Ω drops ≪1 [12]–[14]. Species vulnerability to external shell dissolution is probably strongly related to the presence or absence of a protective organic cover on the external shell side [15], [16] and, potentially, the protein and carbohydrate organic matrix that surrounds CaCO3 crystals within the shell [17]–[19]. While Ω<1 is rare in the contemporary global surface ocean [20], low Ω is a common feature of temperate coastal habitats, where seasonal hypoxia goes along with dissolved inorganic carbon production. Wind driven upwelling can then bring CO2 enriched waters in contact with shallow water habitats [15], [21]. We have recently measured very high and fluctuating summer seawater pCO2 (>100 Pa, >1000 µatm, Ωarag≪1) in habitats in the Western Baltic that are dominated by the blue mussel Mytilus edulis. We could also demonstrate that mussels from this population can calcify at high rates even when Ωarag<0.5 when food supply is abundant [10], [15]. While high calcification rates in seawater under saturated with CaCO3 already seem remarkable, it needs to be emphasized that the extracellular environment at the inner shell interface is even less favorable for biomineralization: as all heterotrophic marine ectothermic animals maintain pCO2 values in their extracellular fluids (i.e. hemolymph, extrapallial fluid) between 100–400 Pa (ca. 1000–4000 µatm) in order to drive diffusive excretion of metabolic CO2[22], the extracellular carbonate system is shifted further towards decreased [CO32−]. While it cannot be excluded that the precise site of incipient biomineralization is occluded by an organic matrix sheath or gel to create a microenvironment that is characterized by higher Ωarag[19], [23], [24], inner shell regions that are not actively being expanded are in contact with an extracellular fluid that is most likely highly corrosive (low pH and [CO32−], high pCO2). Some protection from dissolution may be provided by the chitin and protein layer that covers the uppermost (mantle facing) nacre [18]. During exposure to high seawater pCO2, the extracellular carbonate system shifts to even lower carbonate concentrations, as M. edulis and other bivalves do not perform an extracellular pH (pHe) compensatory reaction [11], [15]. While these results indicate strong biological control over the biomineralization process in mussels, they also suggest that a continuous energetic effort may be necessary to maintain inner shell integrity. It has been previously shown that during stress (e.g. aerial exposure, environmental anoxia), the inner shell surface is corroded in several bivalve species due to proton generation through anaerobic production of succinate [25], [26]. While this may be adaptive, as CaCO3 helps to buffer the developing acidosis, it also indicates that the inner shell might be more endangered by elevated seawater pCO2 than the outer shell surface, which is covered by a chemically resistant periostracum [27]. The situation may be different in those calcifiers (e.g. cephalopods, teleost fish, decapod crustaceans) that actively modulate the extracellular carbonate system speciation in order to stabilize pHe: these organisms accumulate [HCO3−]e significantly above seawater levels (ca. 2 mM). This leads to high calcium carbonate saturation states and could be one reason for the observed occurrence of increased rates of calcification or ‘hypercalcified’ skeletal structures [3], [28]–[30]. Following seven weeks of acclimation to 2 food and 4 CO2 treatment levels, experimental animals were sampled. Shell length growth was significantly reduced by low algae cell density and by high seawater pCO2, with no significant interaction between factors (Fig. 1, Table 1, 2). With respect to controls at each food level, shell length growth was significantly reduced in the highest pCO2 treatment (405 Pa, p<0.001). Shell mass growth and somatic growth were significantly reduced in the LF treatment (Table 2). However, no significant differences in shell mass and somatic growth were found with respect to seawater pCO2. It needs to be mentioned that absolute rates of growth were low in this experiment in comparison to previous experiments at higher temperatures (see Table 3). While shell length growth was based on direct initial and final measurements, initial shell and somatic mass were interpolated (see Methods). Owing to large variability of shell length vs. shell mass relationships (see e.g. [15]), shell and somatic growth rates estimated in this experiment should be viewed with caution. We acclimated blue mussels to four different seawater pCO2 values and two food concentrations at low temperatures and found effects of both factors on shell length growth. In addition, we were able to demonstrate internal shell corrosion in bivalves as a consequence of high seawater pCO2. We also found that internal shell corrosion is highly dependent on food concentration. Our results demonstrate the high degree of control that bivalves can exert over the structural integrity of the inner shell surface and suggest that shell corrosion is related to energy budget reallocations. Source: