Taxonomy of the genus Poterioochromonas (Chrysophyceae) based on morphological and molecular evidence

The genus Poterioochromonas is characterized by spherical cells having two unequal flagella, a greenish, yellow-brown plastid, and a hemispherical cup-shaped lorica with a long, narrow stalk and a foot anchoring to the substrate. In this genus, the only three mixotrophic species having plastids have been reported up to date: P. malhamensis, P. nutans, and P. stipitata. However, we observed heterotrophic Poterioochromonas species that had lost their plastids, and we successfully cultured them. To understand the taxonomy of Poterioochromonas species, we performed a molecular phylogenetic analysis and observed their morphological features using light and scanning electron microscopes. For the phylogenetic analysis, we used a combined dataset from five gene sequences: the nuclear small subunit (SSU) rRNA gene, the large subunit (LSU) rRNA gene, the internal transcribed spacer region (ITS rRNA region 1-5.8S-ITS2), the plastid LSU rRNA gene, and the rbcL gene. The molecular phylogeny of the genus Poterioochromonas was divided into two major clades: mixotrophic and heterotrophic lineages. The mixotrophic clade comprised three species including two new species-P. andersenii sp. nov. and P. longicaulis sp. nov.-characterized by a colonial lifestyle and the long stalk of the lorica. The heterotrophic clade included the four new species-P. amplexa sp. nov., P. communis sp. nov., P. similis sp. nov., and P. sinechrysos sp. nov.-that had lost their plastids. The species P. amplexa produced a very distinctive stomatocyst, which features a true complex collar. Here, we report six new species of Poterioochromonas that exhibit mixotrophy and heterotrophy, showing that the phylogenetic tree is distinctly divided according to nutritional modes.