Pirsonia, phagotrophic nanoflagellates incertae sedis, feeding on marine diatoms: attachment, fine structure and taxonomy

After being chemotactically attracted by its host, marine diatoms of the genus Coscinodiscus, the Pirsonia flagellates are guided by the topography of the host cell surface to the rimoportulae through which the trophosome, a basally formed pseudopodium, penetrates into the diatom. Cytochalasin D does not disturb the chemotaxis of the flagellates but inhibits their attachment. Pirsonia diadema and P. guinardiae, feeding on Coscinodiscus and on Guinardia flaccida, respectively, are very similar in their fine structure. The anterior flagellum has tubular hairs which seem to belong to the tripartite, stramenopile type. They are synthesized within the ER and exocytosed by the Golgi apparatus. In certain developmental stages the posterior flagellum and the cell body bear „knotted hairs“ which differ in length. The mode of their formation is unknown. The mitochondria have unique non-contorted tubular cristae which are parallel to each other. The few dictyosomes have a parabasal position. The flagellar root apparatus consists of fibrillar bands which connect the basal body of the anterior flagellum with the nucleus and with the basal body of the posterior flagellum. Other fibrillar bands and a few microtubules run from the basal bodies to the cell surface. The organization of the flagellar root apparatus thus differs from that of typical stramenopiles. The taxonomic position of Pirsonia is discussed.