Taxonomic reassessment of the genus Chlorella (Trebouxiophyceae) using molecular signatures (barcodes), including description of seven new species

After the description of Chlorella vulgaris by Beijerinck, 120 years ago, members of the genus Chlorella belong to the best studied green algae worldwide. However, numerous open questions remained regarding their systematics. Recent molecular studies showed the polyphyly of the genus within the Chlorophyceae and Trebouxiophyceae. Chlorella-species were traditionally characterized by spherical to oval cell shape, solitary life form and the absence of mucilaginous envelopes. The challenge in the past was how to distinguish species due to their high phylogenetic diversity combined with a limited amount of morphological characters. Using a polyphasic approach of SSU- and ITS rDNA phylogeny, secondary structure of the ITS and light microscopic observations, we were able to detect six lineages with Dictyosphaerium-like strains in close relationship to C. vulgaris, here described or combined newly as C. coloniales sp. nov., C. pituita sp. nov., C. pulchelloides sp. nov., C. singularis sp. nov., C. elongata comb. nov. and C. chlorelloides comb. nov. Furthermore, three new species without mucilage were described as C. lewinii sp. nov., C. rotunda sp. nov. and C. volutis sp. nov. Using the 5.8S rRNA and part of the ITS-2 as molecular signature (barcode), we were able to distinguish not only the five already known species of Chlorella, C. vulgaris, C. sorokiniana, C. heliozoae, C. lobophora and C. variabilis but the seven new species and two new combinations as well. CBCs and hemi-CBCs within the secondary structure of the ITS-2 confirmed the separation of the species. Our study led to a new understanding of the evolution of morphology within the genus Chlorella and to an emendation of the generic description.