Table_1_A Competitive Advantage of Middle-Sized Diatoms From Increasing Seawater CO2.PDF

Diatoms, one of the most important phytoplankton groups, fulfill their carbon demand from seawater mainly by obtaining passively diffused carbon dioxide (CO2) and/or actively consuming intracellular energy to acquire bicarbonate (HCO3–). An anthropogenically induced increase in seawater CO2 reduces the HCO3– requirement of diatoms, potentially saving intracellular energy and benefitting their growth. This effect is commonly speculated to be most remarkable in larger diatoms that are subject to a stronger limitation of CO2 supply because of their smaller surface-to-volume ratios. However, we constructed a theoretical model for diatoms and revealed a unimodal relationship between the simulated growth rate response (GRR, the ratio of growth rates under elevated and ambient CO2) and cell size, with the GRR peaking at a cell diameter of ∼7 μm. The simulated GRR of the smallest diatoms was low because the CO2 supply was nearly sufficient at the ambient level, while the decline of GRR from a cell diameter of 7 μm was simulated because the contribution of seawater CO2 to the total carbon demand greatly decreased and diatoms became less sensitive to CO2 increase. A collection of historical data in CO2 enrichment experiments of diatoms also showed a roughly unimodal relationship between maximal GRR and cell size. Our model further revealed that the “optimal” cell size corresponding to peak GRR enlarged with the magnitude of CO2 increase but diminished with elevating cellular carbon demand, leading to projection of the smallest optimal cell size in the equatorial Pacific upwelling zone. Last, we need to emphasize that the size-dependent effects of increasing CO2 on diatoms are multifaceted, while our model only considers the inorganic carbon supply from seawater and optimal allocation of intracellular energy. Our study proposes a competitive advantage of middle-sized diatoms and can be useful in projecting changes in the diatom community in the future acidified high-CO2 ocean.

硅藻（Diatoms）是最重要的浮游植物类群之一，其主要通过两种途径获取海水碳源以满足碳需求：被动扩散吸收二氧化碳（CO₂），或主动消耗胞内能量以摄取碳酸氢根离子（HCO₃⁻）。人为活动引发的海水CO₂浓度升高，会降低硅藻对HCO₃⁻的需求，潜在节省胞内能量并利于其生长。此前学界普遍推测，该效应在大型硅藻中最为显著——这类硅藻因表面积与体积比更小，面临更强的CO₂供应限制。但我们通过构建硅藻理论模型，揭示了模拟生长速率响应（GRR，即高CO₂与环境CO₂条件下的生长速率比值）与细胞尺寸间的单峰关系，GRR在细胞直径约7 μm时达到峰值。最小型硅藻的模拟GRR较低，这是因为环境CO₂水平下其CO₂供应已近乎充足；而当细胞直径超过7 μm后GRR出现下降，模拟结果显示这是由于海水CO₂对总碳需求的贡献大幅降低，硅藻对CO₂浓度升高的敏感性随之减弱。现有硅藻CO₂富集实验的历史数据汇总同样显示，最大GRR与细胞尺寸间大致呈单峰关系。我们的模型进一步发现，对应GRR峰值的“最优”细胞尺寸，会随CO₂升高幅度增大而变大，随胞内碳需求提升而缩小，据此预测赤道太平洋上升流区存在最小的最优细胞尺寸。最后需要强调，CO₂浓度升高对硅藻的尺寸依赖性效应是多方面的，而我们的模型仅考虑了海水提供的无机碳供应以及胞内能量的最优分配。本研究提出中型硅藻具备竞争优势，可为未来酸化高CO₂海洋中的硅藻群落变化预测提供参考。