Nanoplankton protists from the western Mediterranean Sea. I. Occurrence, ultrastructure, taxonomy and ecological role of the mixotrophic flagellate Ollicola vangoorii (Chrysomonadidae = Chrysophyceae p.p.)

Abstract

We give an account of the loricate mixotrophic nanoflagellate Ollicola vangoorii (basionym Codonomonas Van Goorii, synonym Calycomonas Van Goorii) (Chrysomonadidae = Chrysophyceae p.p. ) in the plankton of the western Mediterranean Sea. Using digital imaging and field-emission scanning electron microscopy (SEM), we describe the lorica morphology and ultrastructure based on some 150 specimens occurring either as dispersed (unattached) cells or cells attached to suspended particulate matter in coastal waters off Barcelona (Catalonia, Spain) in July 1998. The spectrum of morphological variation includes that of Calycomonas gracilis sensu Wulff non Lohmann, Calycomonas gracilis sensu Espeland & Throndsen non Lohmann, Calycomonas wulffii Conrad et Kufferath, Ollicola cylindrical (Conrad et Kufferath) Vørs (= Calycomonas cylindrical (Conrad et Kufferath) Lund, Codonomonas cylindrica Conrad et Kufferath), Ollicola dilatata (Conrad et Kufferath) Vørs (= Calycomonas dilatata (Conrad et Kufferath) Lund, Codonomonas dilatata Conrad et Kufferath), suggesting that those taxa are conspecific with Ollicola vangoorii. By contrast, Ollicola pascheri (van Goor) Vørs (basionym Codonomonas pascheri van Goor) is best maintained as a separate species and retained in the genus Codonomonas van Goor. From an ultrastructural viewpoint, the basic architecture of the lorica is that of a fibrillar ribbon coiled around the cell and eventually hardened as a result of inorganic mineralization from the environment. The appearance of the fibrillar component and the degree of mineralization may vary considerably. The cell apex is modified in a newly documented apical cytostome, substantiating an existing observation that cells are capable of ingesting bacteria. Although the statistical correlation between SEM-based nanoflagellate cell counts and epifluorescence-based bacterial cell counts is at best only weakly significant, it is possible that in the Barcelona populations O. vangoorii relies partly on bacterivory and therefore its ecological role is partly that of a carbon consumer. We also hypothesize that the presence of an external lorica increases the probability of intercepting bacterial prey, ready to be ingested by the cell in the apical cytostome.