Testing the Effectiveness of DNA Barcoding Markers and Species Delimitation Methods Within the Genus Coelastrella (Sphaeropleales, Chlorophyta), With a Description of Coelastrella polaris sp. nov. Isolated from Arctic Soils.

The species diversity in the genus Coelastrella is not yet fully unclarified, particularly with regard to algal communities in ecosystems characterized by extreme climatic conditions, such as those found in polar regions. The study objects were strains VKM Al-421, VKM Al-488, and VKM Al-489 isolated from the soils of the Far North, Russian Federation. The analysis of the 18S-ITS1-5.8S-ITS2 fragment revealed that these strains represent a unique phylogenetic lineage outside the 'core Coelastrella' group. The species status is also confirmed by morphological differences between the studied species and its sister species (absence of edges) (lack of ribs), interspecific genetic distances, the presence of compensatory base changes in the internal transcribed spacers ITS1 and ITS2, as well as the results of delimitation using various algorithms applied to both the full-length fragment and shorter barcodes. The strain VKM Al-421 of the new species exhibits a fatty acid profile characteristic of the genus Coelastrella; however, it differs from closely related strains due to the presence of Δ7,10,13-hexadecatrienoic acid and a notably high concentration of α-linolenic acid. These features may indicate an adaptation to polar regions. In addition, the studied strain has the potential to be used as a producer of α-linolenic acid, which is essential for human health. A detailed comparative analysis of the effectiveness of different DNA barcodes showed that ITS2 is the most promising for distinguishing species within Coelastrella. Among all species delimitation algorithms, GMYC is the most accurate when working with variable barcodes. At the same time, the less laborious KoT algorithm demonstrated a similar level of accuracy for ITS1.