Bivalve regulation of extrapallial fluid carbonate chemistry under ocean acidification: insights from pH, DIC and trace element measurements of the extrapallial fluid

Ocean acidification (OA) decreases pH and carbonate ion concentration of seawater, thereby affecting aspects of the physiology of calcifying organisms, such as pH homeostasis and calcification. Although previous studies have revealed that bivalves are vulnerable to OA, some species demonstrate greater tolerance. Yet, the physiological mechanisms underpinning these differential responses to OA are not well understood. Bivalves produce their shells within their extrapallial fluid (EPF)—between their mantle tissue and inner shell surface. Bivalves can increase the CaCO₃ saturation state of their EPF by (1) increasing pH, (2) increasing dissolved inorganic carbon (DIC), and/or (3) increasing Ca²⁺ concentration. Previous studies show that the EPF pH of adult bivalves is lower than seawater pH; however, no studies have analyzed the full carbonate system of bivalve EPF. Here, we characterize the full carbonate chemistry of EPF within 4 marine (C. virginica, P. magellanicus, M. arenaria and A. islandica) and 1 freshwater (E. complanata) bivalve species from measurements of DIC, pH and trace element composition—revealing varying degrees of control over EPF carbonate chemistry. Additionally, we exposed two of the species (C. virginica, P. magellanicus) to OA conditions. Although the EPF pH of both species declined when exposed to OA, the difference in pH between their EPF and surrounding seawater decreased—indicating a potential mechanism for mitigating impacts of OA. Furthermore, both species elevated EPF DIC under OA scenarios, which, in combination with elevated EPF pH (relative to equilibrium pH), translates to elevated CaCO₃ saturation states. We also assess the impact of OA on elemental partitioning within the shells of these species by comparing elemental ratios of their shells, EPF, and surrounding seawater. Collectively, these results provide insight into mechanisms of bivalve calcification and help constrain bivalve energy budgets under future OA scenarios.