Ultrastructure and taxonomy ofActinomonas pusilla, a heterotrophic member of the Pedinellales (Chrysophyceae)

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Abstract
Actinomonas pusilla, a heterotrophic member of the Pedinellales (Chrysophyceae), was studied by light and electron microscopy. The cells are about 7 µm in diameter and usually swim around rapidly in large arcs, although they are occasionally sessile. The cells possess one emergent flagellum with tripartite hairs. Internally, the flagellum is supported by a cross-banded paraxial rod. The central pair of microtubules is shown to be twisted 360° counterclockwise (viewed from the outside) in about 8 µm, which corresponds to the wavelength of the beating flagellum. Tentacles emanate from the cell in all directions, but are particularly numerous and well-organized anteriorly. The tentacles are supported by microtubular triads, which terminate on the outer nuclear membrane. The triads are connected by cytoskeletal material situated immediately below the plasma membrane. A long trailing stalk which may exceed 120 µm in length is present posteriorly. It is connected to a vacuole and also supported by microtubular triads emanating from the outer nuclear membrane. Actinomonas pusilla is tolerant of a wide range of salinity and is probably cosmopolitan.