The present work uncovers a potentially ancient but yet undescribed biomineralization process, in which unicellular planktonic eukaryotes produce multiple intracellular inclusions of near-amorphous calcium carbonate (ACC), called micropearls. This process is not linked to a specific habitat and occurs in multiple areas around the world in marine, brackish and freshwater environments. This unsuspected biomineralization capacity was discovered to be common in the Tetraselmis genus (Chlorodendrophyceae, Chlorophyta). In these organisms, the micropearls may be slightly enriched in strontium and their arrangement forms a pattern (alignements linked to the chloroplast shape), which is characteristic for a given species. Another organism forming micropearls highly enriched in barium was detected in Lake Geneva (Switzerland). It is still unidentified, but might belong to the zooplankton. All the micropearls seem to result of a biologically controlled process. They represent a new physiological feature for eukaryotes.