Food uptake and the fine structure of the dinophytePaulsenella sp. an ectoparasite of marine diatoms

Abstract

Motile dinospores ofPaulsenella attach to a host diatom frustule, form a feeding tube, drive it between epi- and hypocingulum, pierce the host plasmalemma and suck up host cytoplasm gradually. This mode of endocytosis (“myzocytosis”) implies that the host plasmalemma is not ingested and that the host cytoplasm within the food vacuole is bounded only by the vacuolar membrane. The feeding tube is formed by the emergence of a preformed “microtubular basket” consisting of plates of microtubules. At its entrance into the cell body the feeding tube channel is surrounded by an electron-dense ring. Similar “sphincters” enclose the two exits through which the two flagella emerge. These sphincters are composed of microfibrils which reveal a cross striation when the fixative does not contain calcium ions. The flagellar bases as well as the internal part of the feeding tube are surrounded by a common cavity which is in open connection also with the ampullae of the pusule. The light and electron microscopical observations do not support the assumption that food uptake is driven by a flow of the membrane of the feeding tube channel caused by an interaction with the microtubular basket (as postulated for food uptake inSuctoria) but rather by an hydrostatic gradient which might be caused by rhythmical ion pumping and be based on the existence of the common cavity and the sphincters. Myzocytosis is inhibited by cytochalasin B.—The fine structure of dinospores and trophonts, especially with respect to the cell covering, the amphiesma, and the en- and excystment, is described.