Biomineralization Pathways in Calcifying Dinoflagellates: Uptake, Storage in MgCaP-Rich Bodies and Formation of the Shell

Abstract

Little is known about the formation of the shells of calcareous dinoflagellates, despite the fact that they are one of the major calcifying organisms of the phytoplankton. Here, we investigate calcitic cyst formation in two representative members from different clades of calcareous dinoflagellates using cryo-electron microscopy (cryo-SEM and cryo-FIB-SEM) in combination with micro-Raman and infrared spectroscopy.Only calcein-AM and not calcein enters these cells, indicating active uptake of calcium and other divalent cations. Multifunctional vacuoles containing crystalline inclusions are observed in both species and we identify the crystals as anhydrous guanine in the β-form. We also observe in the same vacuolar enclosures, dense magnesium, calcium and phosphorous rich mineral bodies by cryo EDX. These bodies are secreted into the outer matrix space where calcite forms. Calcite formation occurs via multiple independent nucleation events and the different crystals grow with preferred orientation into a dense reticular network that forms the calcitic cyst. The degree of atomic order of this calcite is less than that of geological calcite. We suggest a common biomineralization pathway for these two calcifying dinoflagellates: (1) Uptake of calcium and other cations through the membranes. (2) Deposition of Mg2+ and Ca2+ ions inside a disordered MgCaP-body. (3) Secretion of these bodies to the space between the outer membranes. (4) Formation and growth of calcite bodies into a dense reticulate network that forms the mature calcitic shell. This study provides new insights into calcium uptake, storage and transport in calcifying dinoflagellates.