A diverse community constitutes global coccolithophore calcium carbonate stocks

Abstract. Coccolithophores are diverse calcifying plankton, yet most research has focused on a single species, Gephyrocapsa huxleyi, with the global contributions of other species hitherto unexplored. Since coccolithophores account for the majority of marine calcium carbonate (CaCO₃) production, this narrow focus risks biasing our understanding of CaCO₃ cycling, as other species differ in their distributions, CaCO₃ production and response to climate change. Using a global, species-resolved machine-learning approach, we show that G. huxleyi contributes only about 7 % of estimated coccolithophore CaCO₃ stock, while a morphologically and functionally diverse assemblage dominates. Since stock contributions are a good proxy for contribution to production, our findings challenge the view that G. huxleyi underpins CaCO₃ cycling and show that lab and in situ datasets centred on this species capture only a small fraction of coccolithophore calcification. Our work identifies key species and regions to guide future laboratory, in situ, and modelling efforts, laying the groundwork for more realistic representations of CaCO₃ cycling under climate change.