Collaborative Research: Biomineralization Processes and their Environmental Modulation in Marine Bivalves

牡蛎和文蛤等双壳类软体动物是生态系统工程师，也是河口重要的经济资源。双壳类贝壳是由碳酸钙(CaCO3)建造的，碳酸钙可以保护它们免受捕食者和环境压力的伤害，对生存至关重要。软体动物的贝壳是由海水中丰富的前体制成的，具有独特的机械性能，使其优于地质上的碳酸钙。然而，它们的沉积成本很高，而且在低盐度、高二氧化碳浓度(CO2)和低pH的腐蚀性条件下会被侵蚀，这是河口水域的特点。最近的研究表明，海洋双壳贝类的生物矿化作用因长期暴露于中等升高的二氧化碳水平(800-2000ppm)而受到损害，这似乎是自相矛盾的，因为在二氧化碳水平范围很大(400-10000ppm)的河口发现了蓬勃发展的双壳贝类种群。最近发现的另一个难题是，一些贝壳由更易溶的文石制成的软体动物，比那些贝壳由较难溶解的方解石制成的贝壳受二氧化碳水平升高的影响更小。这些发现挑战了目前生物矿化的范式，并表明环境与生物矿化生物控制之间的相互作用比目前所认识的要复杂得多。为了缩小这一知识差距，研究人员将确定由不同类型的CaCO3制成的贝壳的软体动物生物矿化的分子和细胞机制，评估这些机制如何受到海水化学的影响，并确定在潜在腐蚀性河口水域中生物矿化的生理和能量限制。该项目包括开发一个跨学科的BioCADRE项目，用于培养本科生，培训和指导博士后研究人员，并促进匹兹堡大学和北卡罗来纳大学夏洛特分校之间的跨学科合作。本研究重点研究了两个关键双壳类物种，长牡蛎和文石贝壳的矿化生物学机制。PIS将测试以下假设：血细胞和外套细胞利用不同的分子机制进行矿物隔离和运输，以及生物矿化机制的物种差异导致这些双壳类在低CaCO3饱和水平下建造贝壳的能力不同。PIs将评估盐度和二氧化碳水平如何影响矿物的封存、运输和沉积，以及参与基质形成和酸碱平衡的蛋白质的表达和活性。PIs将决定在低CaCO3饱和度水平下成功的生物矿化是否会导致基础维护成本的增加以及生物矿化和其他能源依赖功能之间的权衡。这些研究将阐明细胞、分子和整个生物体的过程是如何协调的，以支持在广泛的CaCO3饱和水平下活跃的矿物质沉积，确定在暴露于高碳酸血症和低盐度时，哪些机制补偿了较高的矿物质溶解度，并评估这些机制在文石沉积方面是否比方解石沉积的软体动物更有效。该项目建立在生物矿化专家和软体动物生理学家之前成功合作的基础上，并拥有成功所需的材料和智力资源。

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.