The genus Prototheca (Trebouxiophyceae, Chlorophyta) revisited: Implications from molecular taxonomic studies

The only algae which are able to inflict disease on humans and other mammals through active invasion and spread within the host tissues belong to either of two genera: Chlorella and Prototheca. Whereas Chlorella infections are extremely rare, with only two human cases reported in the literature, protothecosis is an emerging disease of humans and domestic animals, especially dairy cows. The genus Prototheca, erected by Krüger in 1894, has undergone several significant revisions, as more phenotypic, chemotaxonomic, and molecular data have become available. Due to this, a large number of Prototheca strains have been accumulated in public culture collections, over the years, where they still exist under outdated or invalid infraspecific or species names. In this study, the partial cytb gene was used as a marker to revise the taxonomy and nomenclature of a set of Prototheca strains, preserved in major algae culture repositories worldwide. Within the genus, two main lineages were observed, with a dominance of typically dairy cattle-associated (i.e. P. ciferrii, formerly P. zopfii gen. 1, the here validated P. blaschkeae, and one newly erected species, namely P. bovis, formerly P. zopfii gen. 2) and human-associated (i.e. P. wickerhamii, P. cutis, P. miyajii) species, respectively. In the former lineage, three newly described species were allocated, namely P. cookei sp. nov., P. cerasi sp. nov., and P. pringsheimii sp. nov., and the lecto- and epitypified P. zopfii species. The second, or so-called P. wickerhamii lineage, incorporated a newly proposed species of P. xanthoriae sp. nov. These protothecans were shown as the closest relatives of the photosynthetic genera, Chlorella and Auxenochlorella. The environmental species P. ulmea was synonymized with the lecto- and epitypified P. moriformis species. For circumscription and differentiation of Prototheca spp., the use of phenotypic characters, and morphology in particular, is of limited value and should rather be auxiliary to molecular marker-based approaches. As demonstrated in our previous study and corroborated in the present one, the cytb gene provides higher resolution than the conventional rDNA markers, and currently represents the most efficient barcode for the Prototheca algae.