Bivalve mollusks such as oysters and clams are ecosystem engineers and important economic resources in estuaries. Bivalves build shells from calcium carbonate (CaCO3), which protect them from predators and environmental stressors and are essential for survival. Mollusks' shells, produced from precursors abundant in the seawater, have unique mechanical properties that make them superior to geological CaCO3. However, they are energetically costly to deposit and can be eroded under corrosive conditions of low salinity, high concentrations of carbon dioxide (CO2) and low pH, characteristic of estuarine waters. Recent studies show that biomineralization of marine bivalves is impaired by prolonged exposure to the moderately elevated CO2 levels (800-2000 ppm), which appears paradoxical since thriving bivalve populations are found in estuaries with a broad range of CO2 levels (400-10,000 ppm). Another recently discovered conundrum is that some mollusks with shells made of the more soluble aragonite are less affected by elevated CO2 levels than those with the shells made of less soluble calcite. These findings challenge the current paradigm in biomineralization and indicate that interactions between the environment and biological controls of biomineralization are more complex than currently recognized. To close this gap in knowledge, the investigators will determine the molecular and cellular mechanisms of biomineralization in mollusks with shells made of different types of CaCO3, assess how these mechanisms are affected by seawater chemistry and identify physiological and energetic constraints on biomineralization in potentially corrosive estuarine waters. The project involves development of an interdisciplinary BioCADRE program for training of undergraduate students, training and mentorship of post-doctoral researchers, and fostering of interdisciplinary collaborations between the University of Pittsburgh and University of North Carolina at Charlotte.
This study focuses on the biological mechanisms of mineralization in two keystone bivalve species, Crassostrea gigas and Mercenaria mercenaria with calcitic and aragonitic shells, respectively. The PIs will test the hypotheses that hemocytes and mantle cells utilize different molecular mechanisms of mineral sequestration and transport and that species-specific differences in biomineralization mechanisms contribute to different abilities of these bivalves to build shells at low CaCO3 saturation levels. The PIs will assess how salinity and CO2 levels affect sequestration, transport and deposition of the mineral as well as expression and activity of the proteins involved in matrix formation and acid-base balance. The PIs will determine whether successful biomineralization at low CaCO3 saturation levels leads to elevated costs of basal maintenance and trade-offs between biomineralization and other energy-dependent functions. These studies will elucidate how cellular, molecular and whole-organism processes are orchestrated to support active mineral deposition in a wide range of CaCO3 saturation levels, determine what mechanisms compensate for higher mineral solubility during exposure to hypercapnia and low salinity and assess whether these mechanisms are more effective in aragonite-depositing than calcite-depositing mollusks. This project builds on previous successful collaborations between a biomineralization expert and a mollusk physiologist and has the material and intellectual resources required for its success.