Representatives of the genus Phalansterium have single anterior flagellum
surrounded by cytoplasmic collar. They belong to the class Variosea, which
includes both typical amoeboid organisms (e.g., Flamella, Filamoeba) and some
flagellates (e.g., Phalansterium, Multicilia). A recent study revealed Phalansterium
arcticum as a very polymorphic species with both sedentary and swimming
cells. It also clarified the mode of phagocytosis, and showed a cell division, but
the ultrastructure of the cell is still not sufficient. Here, we present for the first
time a thorough ultrastructural investigation of the soil flagellate Phalansterium
“solitarium” strain Ph185 of SCCAP. We demonstrate that Phalansterium uses
pseudopodia to take up bacteria inside its collar, and suggest that the inner cell
asymmetry of Phalansterium cytoskeleton is associated with its mode of feeding.
On the base of morphology and molecular phylogeny, we describe a new species
Phalansterium foulumis with the type strain SCCAP-Ph185.