Phylogenetic relationship of Chlorella and Parachlorella gen. nov (Chlorophyta, Trebouxiophyceae)

Chlorella is one of the archetypical forms of coccoid green algae and one of the best-studied phototrophic eukaryotes. However, its systematics remains enigmatic due to conflicts between morphological and molecular phylogenetic approaches. The sequences of the 18S ribosomal RNA gene of nine strains of Chlorella and related taxa, and the ITS2 region of 17 strains of Chlorellaceae were determined and included in phylogenetic analyses. The secondary structure of the ITS2 region of C. vulgaris was compared to that of Parachlorella beijerinckii. All phylogenetic analyses showed that the Chlorellaceae form a clade within the Trebouxiophyceae. The Chlorellaceae studied here are divided into two sister groups: (1) the Parachlorella-clade with Parachlorella beijerinckii gen. et sp. nov. and P. kessleri comb. nov. as well as Dicloster acuatus and Closteriopsis acicularis; and (2) the Chlorella-clade including the 'true' spherical Chlorella species C. vulgaris, C. lobophora and C. sorokiniana. The latter are intermixed with taxa that differ in morphology and were formerly classified in other families of coccoid green algae: Actinastrum hantzschii (formerly Coelastraceae), Diacanthos belenophorus (formerly Oocystaceae), Dictyosphaerium pulchellum (formerly Botryococcaceae), Didymogenes anomala and D. palatina (formerly Scenedesmaceae) and Micractinium pusillum (formerly Micractiniaceae). It was shown that morphological criteria traditionally used for classification (e.g. spines, mucilaginous envelopes or the formation of colonies or coenobia) are burdened with a high degree of phenotypic plasticity. These plastic traits represent adaptations to environmental factors in order to optimize floating in the water column and to resist grazing pressure. A genus and species concept in Chlorellaceae remains provisional, because, in most cases, there is still a lack of classical and molecular approaches to identify clear phylogenetic lineages.