The antarctic diatom: Stellarima microtrias (Ehrenberg) Hasle & Sims cell structure and vegetative cell enlargement in culture

Summary On the whole, the ultrastructure of the vegetative cell of Stellarima microtrias (Ehrenberg) Hasle & Sims, is similar to that of other diatoms. The less common features are non membrane-limited pyrenoids crossed by two discs formed of three thylakoids and non perinuclear dictyosomal units. The enlargement of the cell is not related to sexuality and is the result of a purely vegetative process. Either one or two new vegetative cells are formed in a spherical cell depending on whether the parent cell, from which the spherical cell emerges, contains one or two vegetative nuclei (in mitotic cells before cytokinesis). Only one mitosis occurs with formation of a pycnotic nucleus, this being extruded. The ecological significance of the process is discussed.