Massive accumulation of strontium and barium in diplonemid protists

SUMMARY Barium (Ba) and strontium (Sr) are often used as proxies for the reconstruction of past marine productivity and global climate. The ability to accumulate Ba 2+ and Sr 2+ in the form of crystals is rare among eukaryotes. Here we report that unicellular heterotrophs called diplonemids (Euglenozoa), one of the most abundant groups of marine planktonic protists, accumulate conspicuous amounts of these trace elements in the form of intracellular barite (BaSO 4 ) and celestite (SrSO 4 ) crystals, in concentrations greater than in other known Ba/Sr-accumulating organisms. Moreover, these flagellates can uptake Sr 2+ exclusively or together with Ba 2+ and form (Ba,Sr)SO 4 . One species, Namystinia karyoxenos , is naturally capable of intracellular accumulation of Ba 2+ and Sr 2+ 42,000 and 10,000 times relative to the surrounding medium. Altering the amounts of both elements in the medium resulted in corresponding changes in the quantity and composition of crystals. Planktonic copepods fed with diplonemids produce celestite-rich fecal pellets, which facilitate deposition of these minerals on the seafloor. We propose that diplonemids, which emerged during the Neoproterozoic era, qualify as impactful players of Ba 2+ /Sr 2+ cycling in the ocean that possibly contributed to sedimentary rock formation over long geological periods.