Opto‐Electrochemical Dissolution Reveals Coccolith Calcium Carbonate Content

Coccoliths are plates of biogenic calcium carbonate secreted by calcifying marine phytoplankton; annually these phytoplankton are responsible for exporting >1 billion tonnes (10(15) g) of calcite to the deep ocean. Rapid and reliable methods for assessing the degree of calcification are technically challenging because the coccoliths are micron sized and contain picograms (pg) of calcite. Here we pioneer an opto‐eletrochemical acid titration of individual coccoliths which allows 3D reconstruction of each individual coccolith via in situ optical imaging enabling direct inference of the coccolith mass. Coccolith mass ranging from 2 to 400 pg are reported herein, evidencing both inter‐ and intra‐species variation over four different species. We foresee this scientific breakthrough, which is independent of knowledge regarding the species and calibration‐free, will allow continuous monitoring and reporting of the degree of coccolith calcification in the changing marine environment.