Occurrence of toxic microalgae and associated toxins in the western Black Sea: insights from the PHYCOB cruise in September 2021

Harmful Algal Blooms (HABs) represent a significant global challenge to human health, economies, and ecosystems, including those of the Black Sea. Despite previous reports of potentially toxic microalgae and phycotoxins in the basin, the taxonomy and occurrence of toxigenic species and their associated toxins remain poorly resolved. During the PHYCOB cruise in September 2021, the diversity and distribution of toxigenic microalgae and phycotoxins were investigated across 23 stations in the western Black Sea, covering Bulgarian and Romanian waters. Numerous potentially toxic microalgal taxa were identified using complementary morphological (light microscopy and scanning electron microscopy) and molecular (DNA metabarcoding) methods. The genus Pseudonitzschia was the only representative of potentially toxic diatoms, but no domoic acid was found. Among toxic dinoflagellates, Dinophysis spp., Protoceratium reticulatum, Lingulaulax polyedra, and Gonyaulax spp. were frequently observed, along with the related pectenotoxins and yessotoxins. Species distribution modelling indicated that the western Black Sea provides favorable conditions for Dinophysis spp., L. polyedra, and P. reticulatum. Additionally, several Alexandrium species were identified, including the first record of A. fragae in the basin, along with the detection of the associated phycotoxins (GTX-2/3 and GDA). This study provides the first integrated assessment combining light and scanning electron microscopy, DNA metabarcoding, and chemical analyses of toxigenic microalgae in field samples from the western Black Sea, contributing to an improved understanding of their region-specific profiles.