Coccolithophores, a group of unicellular calcifying phytoplankton, have been major contributors to marine carbonate production since the calcite plates that they produce (coccoliths) first appeared in the fossil record over 200 million years ago (Ma). The response of this process to changes in environment on evolutionary timescales remains poorly understood, particularly in warm climates. Here we integrate a dataset consisting of carbon isotope ratios of size-separated coccolith calcite from marine sediments with a cell-scale model to interrogate cellular carbon fluxes and \(p_{{\mathrm{CO}}_2}\) through the Eocene (~55–34 Ma), Earth’s hottest interval of the past 100 million years. We show that the large coccolithophores that rose to dominate the oceans through the Eocene have higher calcification-to-carbon fixation ratios than their predecessors while the opposite is true for smaller coccolithophores. These changes, which occurred in the context of increasing ocean alkalization, may have played a role in an apparent positive carbon cycle feedback to decreasing \(p_{{\mathrm{CO}}_2}\). Our approach also provides independent support of multiproxy-based evidence for general \(p_{{\mathrm{CO}}_2}\) decline through the Eocene in step with temperature. Together, this challenges the emerging view that a general decline in \(p_{{\mathrm{CO}}_2}\) reduces calcification on evolutionary timescales.

球石藻是一组单细胞钙化浮游植物，自从它们产生的方解石板块（球石）在 2 亿多年前（Ma）首次出现在化石记录中以来，它一直是海洋碳酸盐生产的主要贡献者。这一过程对进化时间尺度上环境变化的反应仍然知之甚少，特别是在温暖的气候中。在这里，我们整合了一个数据集，该数据集由来自海洋沉积物的大小分离的球石方解石的碳同位素比与细胞尺度模型组成，以询问细胞碳通量和\(p_{{\mathrm{CO}}_2}\)穿过始新世（~55-34 Ma），这是过去 1 亿年地球上最热的间隔。我们表明，通过始新世统治海洋的大型球石藻比它们的前辈具有更高的钙化与碳固定比率，而较小的球石藻则相反。这些变化发生在海洋碱化增加的背景下，可能在明显的正碳循环反馈中发挥了作用，以减少\(p_{{\mathrm{CO}}_2}\)。我们的方法还为通过始新世与温度同步下降的一般\(p_{{\mathrm{CO}}_2}\)提供了基于多代理的证据的独立支持。总之，这挑战了新兴观点，即\(p_{{\mathrm{CO}}_2}\)减少进化时间尺度上的钙化。