Scale and Bristle Morphology of Mallomonas tonsurata (Synurophyceae) in Cultures with Varied Nutrient Supply *

Scale‐bearing taxa of the chrysophytes are important paleoindicators because of their often restricted ecological requirements and their highly preserved extracellular siliceous structures. The diagnostic features can usually only be resolved using electron microscopy and, since these features are often shared between closely related taxa, identification in field studies remain uncertain in most cases. As the respective influences of genetics and environmental factors upon morphological variation cannot be evaluated in field studies, examinations were conducted by growing a clonal culture of Mallomonas tonsurata in different media with varying silicon, nitrogen and phosphorus supply, followed by micromorphological analyses of scales and bristles. Increased silica content resulted in significant elongation of scales with a corresponding tendency to build slender scales. In experiments with varied ratios of nitrogen and phosphorus in the media the most pronounced changes were observed under phosphorus enrichment leading to significantly larger scales and more needle‐like bristle tips. These bristles resemble those of M. areolata . This shows that bristles are more variable than scales and thus must be regarded as of little value for taxonomical description. However, the above‐mentioned changes are not sufficiently pronounced to result in a sharp distinction between two different species. This confirms that siliceous structures of chrysophytes are, in principle, a most valuable tool for taxonomic differentiation although in field studies morphological variability of taxonomically relevant characters should still be considered.