Morphological, molecular and toxinological characterization of potentially toxigenic microalgal strains from the western Black Sea

Harmful algal blooms (HAB) of various toxic microalgae negatively affect the Black Sea but there is limited data on the sources of phycotoxins in this area. During the PHYCOB cruise in September 2021, two strains of one diatom and thirty strains of ten dinoflagellate species of potentially toxigenic plankton were characterized in terms of morphology, phylogeny, toxin profiles, and toxin cell quotas. Twenty strains of potentially yessotoxins (YTXs) producing dinoflagellate species were investigated. All six strains of Protoceratium reticulatum contained yessotoxin (YTX), with cell quotas between 1.9 and 5.4 pg cell^-1. Additionally, several YTX variants were detected in minor amounts in nine of twelve strains of Lingulaulax polyedra, whereas no YTXs were detected in two strains of other gonyaulacoids (Gonyaulax sp. and Sourniaea diacantha). All strains of Alexandrium spp. (four strains of A. tamutum, one strain each of A. andersonii, A. ostenfeldii, and A. pseudogonyaulax) were analyzed for all toxins known to be produced by the genus. None of the strains contained detectable levels of paralytic shellfish toxins (PSTs) or gymnodimines. A. pseudogonyaulax produced goniodomine A (GDA) and GDA-seco acid at cellular levels of 14.0 and 0.33 pg cell^-1, respectively. Moreover, four previously unreported spirolide analogues (SPX) were detected in the A. ostenfeldii strain, with cell quotas between 1.0 and 1.6 pg cell^-1. All strains of potentially ichthyotoxic species (Karlodinium sp., Polykrikos hartmanii) did not show extracellular lytic activity. In conclusion, these findings improve our understanding of the potential sources, diversity and dynamics of phycotoxins in the Black Sea.