PHYSIOLOGICAL, BIOCHEMICAL, AND MOLECULAR CHARACTERS FOR THE TAXONOMYOF THE SUBGENERA OF SCENEDESMUS (CHLOROCOCCALES, CHLOROPHYTA)

Biochemical and physiological properties of 16 Scenedesmus species rep resenting the three subgenera Scenedesmus, Acutodesmus, and Desmodesmu s are not suitable far species differentiation. All Scenedesmus specie s studied thus far produce secondary carotenoids, e.g. astaxanthin and canthaxanthin, under nitrogen-deficient conditions. In addition, with the exception of one strain, hydrogenase activity under anaerobic con ditions is generally present, Sequence analyses of ribosomal 18S RNAs indicate that, the subgenus Desmodesmus is phylogenetically well separ ated from the other subgenera, whereas the separation of Scenedesmus a nd Acutodesmus appears less convincing and is dismissed in favour of a single subgenus, Scenedesmus. Three taxa formerly assigned to the gen us Chlorella are shown to be unicellular species of the genus Scenedes mus, ''Chlorella'' fusca var. vacuolata and ''C.'' fusca var. rubescen s, which is closely related to S. obliquus, belong to the Scenedesmus/ Acutodesmus group. ''f.'' fusca var. fusca is closely related to S. co mmunis and thus belongs to the subgenus Desmodesmus. Inclusion of Kerm atia pupukensis into the genus Scenedesmus is also strongly supported by the RNA data which furthermore indicate a relationship with the sub genus Desmodesmus.