Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle

Identifying mechanisms driving the substantial dissolution of biogenic CaCO₃ (60 to 80%) in surface and mesopelagic waters of the global ocean is critical for constraining the surface ocean’s alkalinity and inorganic carbon budgets. We examine microzooplankton grazing on coccolithophores, photosynthetic calcifying algae responsible for a majority of open-ocean CaCO₃ production, as a mechanism driving shallow dissolution. We show that microzooplankton grazing dissolves 92 ± 7% of ingested coccolith calcite, which may explain 50 to 100% of the observed CaCO₃ dissolution in supersaturated surface waters. Microzooplankton grazing on coccolithophores is thus a substantial, previously unrecognized biological mechanism affecting the ballasting of organic carbon to deeper waters, the ecology and fitness of microzooplankton themselves due to buffering of food vacuole pH, and ultimately the continued ability of the surface ocean to take up atmospheric carbon dioxide.