<i>Biecheleria tirezensis</i> sp. nov. (Dinophyceae, Suessiales), a new halotolerant dinoflagellate species isolated from the athalassohaline Tirez natural pond in Spain

<p>A new euryhaline and eurythermal dinoflagellate species, <i>Biecheleria tirezensis</i> sp. nov., is described based on samples taken from an extreme environment, the athalassohaline and particularly sulphate-rich Tirez natural pond (Spain). This species is able to survive in salinities from almost fresh water up to 56 and over a 5–25°C temperature range. Thus, the ecological characteristics of this isolate differentiate it from other species of the same genus. Its morphology, as examined by light, scanning and transmission electron microscopy, shows that the cells are almost spherical, with several series of amphiesmal vesicles. It also has a single elongate amphiesmal vesicle (EAV) observed by SEM and the eyespot has a type E structure, typical of <i>Biecheleria</i>. Spherical and smooth cysts were observed in old cultures. The pigment composition is typical of a peridinin-containing dinoflagellate. Phylogeny inferred from nuclear rDNA SSU, ITS and LSU sequences showed the isolate belongs to the genus <i>Biecheleria</i>, closest to <i>B. cincta</i> and <i>B. brevisulcata</i>. Modelling and analysis of the secondary structure of its ITS2 region, and that of other species of the same genus and some representatives of the most closely related genera, indicated that the isolate represents a new species clearly separated from but related to <i>B. cincta</i>. The criterion of the presence of Compensatory Base Changes (CBCs) in the secondary structure of the ITS2 region as an indicator of species differentiation confirmed this, supporting the establishment of the Tirez pond isolate as a new species of <i>Biecheleria</i>.</p>