Nitzschia excavata sp. nov. (Bacillariaceae), a new diatom species from a post-mining reservoir revealed by morphology, molecular phylogeny, and metabarcoding-based biogeography.

The Bogdałów post-mining reservoir (Poland) represents a slightly alkaline, moderately mineralised ecosystem formed by flooding a former lignite pit. Its anthropogenic origin and stable physicochemical conditions have enabled the development of species-rich diatom assemblages, particularly numerous Nitzschia (Bacillariaceae) species. To explore this diversity, an integrative approach combining microscopy and DNA-based analyses was employed. Morphological examinations were performed using light and scanning electron microscopy, as well as confocal laser scanning microscopy. Molecular phylogenetic analyses were based on the sequencing of the nuclear SSU rDNA and the chloroplast rbcL and psbC gene markers. This comprehensive study led to the discovery and formal description of Nitzschia excavata sp. nov., distinguishable by unique morphological features and a phylogenetically distinct lineage. Furthermore, environmental DNA metabarcoding and metagenomic database searches revealed sequences identical or closely related to the N. excavata sp. nov. lineage in freshwater habitats across Europe and China, indicating that this taxon has an unexpectedly broad distribution. These findings underscore the value of integrating classical morphological analysis with multi-marker molecular data in diatom taxonomy and demonstrate that anthropogenic habitats may support taxa with broader distributions than previously recognized. The study highlights the important role of metabarcoding and metagenomics in revealing cryptic diversity and clarifying the biogeographic patterns of newly described species.