On the extrusomes of Oxyrrhis marina (Dinophyceae)

Abstract

Oxyrrhis marina was subjected to conventional transmission electron microscopy, with emphasis being laid on its extrusomes. Mainly regular trichocysts were obvious in ultrathin sections. They were highly abundant, approximately 2 μm in length and 200 nm in width, and composed of the characteristic features, i.e., an anterior tip and the posterior crystalline body. The tip measures approximately 440 nm in length and is built by an outer less electron-dense concentric layer followed by an inner electron-dense core with a translucent center in the middle. The less electron-dense layer most likely ends up in a bundle of filaments which are concentrically placed around the electron-dense core in the transition zone between the tip and body. Trichocyst bodies which are sectioned along the longitudinal axis are approximately 1.5 μm in length and show a regular striation of electron-dense and electron-translucent lines with a spacing of 9 nm. Cross-sectioned bodies are square-shaped and show a crystalline lattice composed of particles which are 8–9 nm in size. Discharge of regular trichocysts results in long rigid rods. They are square-shaped, 54 nm broad, and with a regular striation of approximately 54 nm along their longitudinal axes. Besides regular trichocysts, an additional type of extrusome was registered. It is not as abundant as regular trichocysts, membrane-enclosed, 2 μm in length and 180 nm in width, and resembles two bullets adjacent to each other with the tips facing in opposite directions. The two parts are slightly of different lengths (anterior part, 740 nm; posterior part, 590 nm) and widths (anterior part, 126 nm; posterior part, 117 nm) and separated from each other by a gap of 30 nm. The anterior part is more electron-dense than the posterior one. A faint electron-dense sheet-like structure was registered between the envelope membrane and these two inner structures. In extrusomes which had been arrested in the process of discharge, the anterior part gives rise to an oozing, amorphous, fibrous blob, while the posterior part consists of twisted filaments which most likely function as the charge of a gun for the release of the anterior part.