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

Abstract

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.