Seawater carbonate chemistry and biomineralization pathways and shell material properties of Magallana gigas and Mytilus spp.

Molluscs are among the organisms affected by ocean acidification (OA), relying on carbon for shell biomineralization. Metabolic and environmental sourcing are two pathways potentially affected by OA, but the circumstances and patterns by which they are altered are poorly understood. From previous studies, mollusc shells grown under OA appear smaller in size, brittle and thinner, suggesting an important alteration in carbon sequestration. However, supplementary feeding experiments have shown promising results in offsetting the negative consequences of OA on shell growth. Our study compared carbon uptake by δ13C tracing and deposition into mantle tissue and shell layers in Magallana gigas and Mytilus species, two economically valuable and common species. After subjecting the species to 7.7 pH, +2 °C seawater, and enhanced feeding, both species maintain shell growth and metabolic pathways under OA without benefitting from extra feeding, thus, showing effective acclimation to rapid and short-term environmental change. Mytilus spp. increases metabolic carbon into the calcite and environmental sourcing of carbon into the shell aragonite in low pH and high temperature conditions. Low pH affects M. gigas mantle nitrogen isotopes maintaining growth. Calcite biomineralization pathway differs between the two species and suggests species-specific response to OA.

软体动物（Molluscs）是受海洋酸化（Ocean Acidification, OA）影响的生物类群之一，其贝壳生物矿化（shell biomineralization）过程依赖碳源供给。代谢碳获取与环境碳摄取是两类可能受OA干扰的途径，但目前学界对其受影响的具体情境与变化模式仍知之甚少。既往研究表明，在OA环境中培育的软体动物贝壳尺寸更小、质地更脆且更薄，这提示碳封存过程发生了显著改变。不过，补充投喂实验已展现出抵消OA对贝壳生长负面影响的潜在积极效果。本研究通过δ13C同位素示踪技术（δ13C tracing），对比了长牡蛎（Magallana gigas）与贻贝属（Mytilus）这两类兼具经济价值与野外常见性的物种的碳摄取过程，以及碳在其外套膜组织与贝壳层中的沉积情况。研究将受试物种暴露于pH为7.7、水温升高2℃的海水环境，并辅以强化投喂处理。结果显示，两种受试物种在OA环境下均能维持贝壳生长与代谢通路，且未从额外投喂中获得增益，由此展现出对快速短期环境变化的有效适应能力。在低pH与高温条件下，贻贝属物种会将代谢来源的碳更多分配至方解石（calcite）结构，同时将环境来源的碳更多沉积于贝壳的文石（aragonite）层中。低pH环境会改变长牡蛎外套膜的氮同位素特征，以此维持其生长速率。两种物种的方解石生物矿化通路存在显著差异，这提示它们对OA的响应具有物种特异性。