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 2μm in length and 200nm in width, and composed of the characteristic features, i.e., an anterior tip and the posterior crystalline body. The tip measures approximately 440nm 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 1.5μm in length and show a regular striation of electron-dense and electron-translucent lines with a spacing of 9nm. Cross-sectioned bodies are square-shaped and show a crystalline lattice composed of particles which are 8-9nm in size. Discharge of regular trichocysts results in long rigid rods. They are square-shaped, 54nm broad, and with a regular striation of approximately 54nm along their longitudinal axes. Besides regular trichocysts, an additional type of extrusome was registered. It is not as abundant as regular trichocysts, membrane-enclosed, 2μm in length and 180nm 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, 740nm; posterior part, 590nm) and widths (anterior part, 126nm; posterior part, 117nm) and separated from each other by a gap of 30nm. 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.