A multimethod approach was used to characterize unicellular green algae that were traditionally assigned to the genus Chlorella Beijerinck and to resolve their phylogenetic relationships within the Chlorophyta. Biochemical, physiological, and ultrastructural characters, together with molecular data such as DNA base composition and DNA hybridization values, were compared with a molecular phylogeny based on complete 18S rRNA sequences. Our results show that Chlorella taxa are dispersed over two classes of chlorophytes, the Trebouxiophyceae and the Chlorophyceae. We propose that only four species should be kept in the genus Chlorella (Chlorophyta, Trebouxiophyceae)

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Biochemical and physiological properties of 16 Scenedesmus species representing the three subgenera Scenedesmus. Acutodesmus. and Desmodesmus are not suitable for species differentiation. All Scenedesmus species 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 conditions is generally present. Sequence analyses of ribosomal 185 RNAs indicate that the subgenus Desmodesmus is phylogenetically well separated from the other subgenera, whereas the separation of Scenedesmus and Acutodesmus appears less convincing and is dismissed in favour of a single subgenus, Scenedesmus. Three taxa formerly assigned to the genus Chlorella are shown to be unicellular species of the genus Scenedesmus. "Chlorella" fusca var. vacuolata and "C." fusca var. rubescens. which is closely related to 5. obliquus. belong to the Scenedesmus/Acutodesmus group. "C." fusca var. fusca is closely related to 5. communis and thus belongs to the subgenus Desmodesmus. Inclusion of Kermatia pupukensis into the genus Scenedesmus is also strongly supported by the RNA data which furthermore indicate a relationship with the subgenus Desmodesmus.

The species of the genus Chlorella exhibit considerable biochemical and physiological differences. Therefore, it is important to select for and utilize in research or biotechnology correctly identified strains of the species having the most favorable properties for the respective project. We examined the Chlorella strains of the University of Texas collection at Austin, Texas, according to species‐specific chemotaxonomic characters and assigned 58 strains to 10 well‐established species (only 17 of these strains were correctly named before).

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