A PHAGOMYXA-LIKE ENDOPARASITE OF THE CENTRIC MARINE DIATOM BELLEROCHEA-MALLEUS - A PHAGOTROPHIC PLASMODIOPHOROMYCETE

In September 1993 the marine centric diatom, Bellerochea malleus (Brig htwell) Heurck, collected in the Wadden Sea near List/Sylt, was parasi tized by a Phagomyxa algarum-like organism. Karling (1944) reported Ph agomyxa algarum Karling in North Carolina as a parasite of the filamen tous brown algae Ectocarpus and Pylaiella. The Bellerochea parasite de velops an endocytoplasmic plasmodium and incorporates host cytoplasm i nto a large, central digestion vacuole, by a form of phagocytosis. Lat er on, the plasmodium cleaves to form a zoosporangiosorus. Each zoospo rangium is surrounded by a thin wall. It releases zoospores (2.5 x 4 m u m) with two unequal flagella, an anterior (4 mu m long) and a poster ior (8 mu m long). Cystosori and cysts could not be detected. The ultr astructure of the zoosporangia and zoospores was investigated, with pa rticular attention to the flagellar apparatus and its rearrangement du ring zoospore release. This process is very similar to that recorded f or zoospores of the plasmodiophoromycete Polymyxa graminis Ledigham (B arr and Allan, 1982). The Bellerochea parasite is closely related to o r identical with Phagomyxa algarum. The taxonomic position of Phagomyx a is discussed. In spite of its phagotrophic nutrition and the possibl e lack of cystosori and cysts, Phagomyxa should be regarded as a membe r of the Plasmodiophoromycota (or Plasmodiophorida) but not included i n a separate order Phagomyxida as proposed by Cavalier-Smith (1993a).