Advancing the Taxonomy of the Diatom Pseudo-nitzschia Through an Integrative Study Conducted in the Central and Southeastern Adriatic Sea

The marine diatom genus <i>Pseudo-nitzschia</i> comprises cosmopolitan phytoplankton species commonly present in the Adriatic Sea. Species within the genus <i>Pseudo-nitzschia</i> have been of significant concern because they produce domoic acid (DA), which can cause amnesic shellfish poisoning (ASP). In this study, we identified <i>Pseudo-nitzschia</i> species along the Central and Southeastern Adriatic Sea, where monthly sampling carried out from February 2022 to February 2024 allowed for comprehensive species documentation. <i>Pseudo-nitzschia</i> species cell cultures isolated from the study areas were morphologically and molecularly analysed. Morphological analyses were performed using a scanning electron microscope (FE-SEM/STEM), while molecular analyses were conducted, targeting the ITS1-5.8S-ITS2, LSU, and <i>rbc</i>L regions, to confirm species identity. This integrative approach led to the identification of eight species: <i>Pseudo-nitzschia allochrona</i>, <i>Pseudo-nitzschia calliantha</i>, <i>Pseudo-nitzschia delicatissima</i>, <i>Pseudo-nitzschia fraudulenta</i>, <i>Pseudo-nitzschia mannii</i>, <i>Pseudo-nitzschia multistriata</i>, <i>Pseudo-nitzschia pseudodelicatissima</i>, and <i>Pseudo-nitzschia subfraudulenta</i>. Our findings underscore the value of a combined approach for reliable species identification and contribute to the development of genetic sequence databases that support the advancement of next-generation methods such as metabarcoding. This research emphasises the importance of combined morphological and molecular methods for the differentiation of the cryptic and pseudo-cryptic <i>Pseudo-nitzschia</i> species.