Algivorous Freshwater Protists of the Phylum Cercozoa (Rhizaria) – Structure, Life Histories and Evolution

Various elusive protists consume aquatic microalgae in a selective and highly specialised manner. During the 19th and early 20th century some of these ‘parasitoids’ were studied by naturalists in environmental samples. Difficulties in long-term maintenance of such forms strongly impeded further research, as did their convoluted taxonomic history. During the present studies, amoeboid and flagellate protists feeding on zygnematophycean and chlorophycean green algae (Viridiplantae) were isolated from natural freshwater samples. Cell morphology, feeding processes, locomotory behaviour and life histories of these isolates were studied by means of light microscopy and time-lapse photography. Feeding experiments were conducted to reveal the food range specificity and SSU rDNA sequences of the strains were obtained and subjected to molecular phylogenetic analyses to place the organisms of interest in the Tree of Life. The phenotypic and phylogenetic information resulted in a comprehensive characterisation of vampyrellid amoebae (Vampyrellida, Cercozoa) and the establishment of a novel family of algivorous amoeboflagellates, the Viridiraptoridae, which also belongs to the phylum Cercozoa. A detailed ultrastructural analysis of Viridiraptor invadens gen. nov., sp. nov. resulted in the reconstruction of the flagellar apparatus, which serves as a reference for cercozoan flagellar apparatus ultrastructure and led to a basal body triplet numbering system for flagellate cells of the Rhizaria.