Ultrastructure and 3D reconstruction of a diplonemid protist (Diplonemea) and its novel membranous organelle.

Although diplonemid protists (Diplonemea, Euglenozoa) are among the most species-rich microeukaryotes in the ocean, many ultrastructural features of this lineage remain to be clarified. Using serial block-face scanning and transmission electron microscopy, we provide the whole-cell three-dimensional model of a previously undescribed diplonemid Lacrimia vacuolata sp. nov. For the first time, we were able to reconstruct a diplonemid cell in detail, including all its organelles and the flagellar and feeding apparatuses. By analyzing various stages of the cell cycle, we provide the first description of a diplonemid cell division, which is characterized by the disassembly of flagellar axonemes and the feeding apparatus and their further construction in the daughter cells. Moreover, we have identified a novel, ultrastructurally complex organelle, herein named the colv ( C enter for O rganization of L ayered V esicles). We suggest that the colv is involved in food processing and membrane trafficking, and describe its close association with other components of the cellular digestive system. IMPORTANCE The knowledge of cell biology of a eukaryotic group is essential for correct interpretation of ecological and molecular data. Although diplonemid protists are one of the most species-rich lineages of marine eukaryotes, only very fragmentary information is available about the cellular architecture of this taxonomically diverse group. Here, a large serial block-face scanning electron microscopy data set complemented with light and fluorescence microscopy allowed the first detailed three-dimensional reconstruction of a diplonemid species. We describe numerous previously unknown peculiarities of the cellular architecture and cell division characteristic for diplonemid flagellates, and illustrate the obtained results with multiple three-dimensional models, comprehensible for non-specialists in protist ultrastructure.