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. Competing Interest Statement The authors have declared no competing interest. Footnotes This revision adds more explicit discussion around model assumptions and uncertainty and the implications of stock estimation vs production. It also updates some of the figures to include stippling to denote regions where model predictions are outside of the area of applicability. Finally, it has been restructured to meet Biogeosciences formatting requirements.