Seawater carbonate chemistry and particulate inorganic carbon, particulate organic carbon production, and growth rates of Scyphosphaera apsteinii

Coccolithophores are unicellular marine phytoplankton and important contributors to global carbon cycling. Most work on coccolithophore sensitivity to climate change has been on the small, abundant bloom-forming species Emiliania huxleyi and Gephyrocapsa oceanica. However, large coccolithophore species can be major contributors to coccolithophore community production even in low abundances. Here we fit an analytical equation, accounting for simultaneous changes in CO2 and light intensity, to rates of photosynthesis, calcification and growth in Scyphosphaera apsteinii. Comparison of responses to G. oceanica and E. huxleyi revealed S. apsteinii is a low-light adapted species and, in contrast, becomes more sensitive to changing environmental conditions when exposed to unfavourable CO2 or light. Additionally, all three species decreased their light requirement for optimal growth as CO2 levels increased. Our analysis suggests that this is driven by a drop in maximum rates and, in G. oceanica, increased substrate uptake efficiency. Increasing light intensity resulted in a higher proportion of muroliths (plate-shaped) to lopadoliths (vase shaped) and liths became richer in calcium carbonate as calcification rates increased. Light and CO2 driven changes in response sensitivity and maximum rates are likely to considerably alter coccolithophore community structure and productivity under future climate conditions.

颗石藻（Coccolithophores）是一类单细胞海洋浮游植物，亦是全球碳循环的重要贡献者。当前针对颗石藻气候变化敏感性的研究，多聚焦于个体微小、易形成水华的优势物种赫氏颗石藻（Emiliania huxleyi）与大洋桥石藻（Gephyrocapsa oceanica）。然而，大型颗石藻物种即便丰度较低，仍可在颗石藻群落总生产力中占据重要份额。
本研究将同时考量二氧化碳（CO₂）与光照强度协同变化的解析方程，拟合至斯氏梭球石藻（Scyphosphaera apsteinii）的光合作用速率、钙化速率与生长速率数据。通过对比其与大洋桥石藻、赫氏颗石藻的环境响应特征，研究发现斯氏梭球石藻为低光照适应物种；与之相反，当处于不适宜的CO₂浓度或光照条件下时，其对环境变化的敏感性反而会显著增强。
此外，三种物种的最优生长光照需求均随CO₂浓度升高而降低。本研究分析表明，该现象源于最大生理速率的下降，而在大洋桥石藻中还伴随底物摄取效率的提升。
光照强度升高会使板状颗石（muroliths，板形）相较于瓶形颗石（lopadoliths，瓶形）的占比更高，且随着钙化速率提升，颗石片（liths）的碳酸钙含量也随之升高。
光照与CO₂驱动的响应敏感性及最大生理速率变化，或将在未来气候条件下显著改变颗石藻群落结构与整体生产力水平。