Scanning electron microscopy of the dinoflagellate Oxyrrhis marina: a matter of method

The gain of information on the general cell surface morphology and on delicate cell surface appendages such as body and flagellar scales of protists depends on the right choice of methodological approach. This assumption was proven for Oxyrrhis marina, a primitive dinoflagellate, which has not been intensely investigated by scanning electron microscopy and, from transmission electron microscopical studies, is known to harbor scales. Oxyrrhis was subjected to different preparation protocols for scanning electron microscopy. One Way ANOVA of the means measured for cell lengths and widths revealed significant differences between most of the treatments. Best cell preservations were achieved when the preparations were carried out on filters instead of adhesive glass slides, using osmium tetroxide as fixative instead of glutardialdehyde, and when the cells were subjected to critical point drying instead of hexamethyldisilazane-mediated drying. Air-drying resulted in collapsed cells. The addition of mercury chloride or a mixture of tannic acid and uranyl acetate occasionally gave rise to highly ordered arrays of body scales. When an ionic liquid was applied for conductance, the cells became masked. Body scales were not obvious in air-or freeze-dried specimen and those prepared on glass slides or treated with an ionic liquid. In summary, best results were obtained for Oxyrrhis when the preparations were carried out on filters and using osmium tetroxide in combination with either mercury chloride or a mixture of tannic acid and uranyl acetate for fixation. This study might be taken as stimulus to apply a similar set of methodological approaches on other dinoflagellates and protists as well.