Impact of high CO2 on the geochemistry of the coralline algae Lithothamnion glaciale

Coralline algae are a significant component of the benthic ecosystem. Their ability to withstand physical stresses in high energy environments relies on their skeletal structure which is composed of high Mg-calcite. High Mg-calcite is, however, the most soluble form of calcium carbonate and therefore potentially vulnerable to the change in carbonate chemistry resulting from the absorption of anthropogenic CO2 by the ocean. We examine the geochemistry of the cold water coralline alga Lithothamnion glaciale grown under predicted future (year 2050) high pCO2 (589 matm) using Electron microprobe and NanoSIMS analysis. In the natural and control material, higher Mg calcite forms clear concentric bands around the algal cells. As expected, summer growth has a higher Mg content compared to the winter growth. In contrast, under elevated CO2 no banding of Mg is recognisable and overall Mg concentrations are lower. This reduction in Mg in the carbonate undermines the accuracy of the Mg/Ca ratio as proxy for past temperatures in time intervals with significantly different carbonate chemistry. Fundamentally, the loss of Mg in the calcite may reduce elasticity thereby changing the structural properties, which may affect the ability of L. glaciale to efficiently function as a habitat former in the future ocean.

珊瑚藻（Coralline algae）是底栖生态系统（benthic ecosystem）的关键组成部分。其在高能环境中抵御物理胁迫的能力，依托于由高镁方解石（high Mg-calcite）构成的骨骼结构。然而，高镁方解石是溶解度最高的碳酸钙（calcium carbonate）晶型，因此极易受到海洋吸收人为二氧化碳（anthropogenic CO2）所引发的碳酸盐化学扰动的影响。本研究采用电子探针（Electron microprobe）与纳米二次离子质谱（NanoSIMS）分析技术，对在未来预估环境（2050年，589毫大气压（matm）高分压二氧化碳（pCO2））下培养的冷水珊瑚藻石叶藻（Lithothamnion glaciale）的地球化学特征展开了研究。在自然样本与对照组样本中，高镁方解石会在藻细胞周围形成清晰的同心环带。正如预期，夏季生长层的镁含量显著高于冬季生长层。与之形成鲜明对比的是，在二氧化碳分压升高的实验组中，镁元素的环带结构完全消失，且整体镁含量大幅降低。碳酸盐骨架中镁元素的流失，会削弱镁钙比（Mg/Ca ratio）作为古温度代用指标（proxy）的准确性，尤其是在碳酸盐化学环境差异显著的地质时段。从根本层面而言，方解石中镁元素的损失可能会降低其弹性，进而改变其结构特性，这或会影响石叶藻（Lithothamnion glaciale）在未来海洋中作为栖息地构建者正常发挥功能的能力。