Evaluation of the ddPCR assay to identify and quantify Alexandrium pseudogonyaulax through eDNA

Harmful algal blooms (HABs) are pervasive worldwide, posing significant threats to marine ecosystems as well as fisheries and aquaculture. In Northern Europe, HAB occurrences are frequent, notably in the Baltic Sea and eastern North Sea. Alexandrium pseudogonyaulax, a dinoflagellate species, stands out from other species groups due to its distinct nutritional mode, phylogenetic relatedness, and toxigenic capacities. Populations can be toxic, producing the cytotoxic macrolide polyether goniodomin A (GDA), with potential impacts on fish populations. Its increasing prevalence in coastal waters of central and northern Europe raises concerns. The feeding mechanism of A. pseudogonyaulax, trapping and immobilizing prey with toxic mucus, enhances its growth and competitive advantage, likely benefiting from environmental changes such as increased river discharge and favourable temperatures attributed to climate change. This may also explain the recent increased biomass of the dinoflagellate in phytoplankton in summer in the western and southern Baltic Sea. This study aimed to explore the potential of using digital droplet PCR (ddPCR) for A. pseudogonyaulax detection and enumeration in Northern European water bodies, offering a rapid and reliable alternative to traditional microscopy methods. In 2020 an expedition was carried out in Northern European sea areas, covering 35 stations along a salinity gradient from the North Sea and Baltic Sea, where plankton net tow samples were collected from the research vessel Uthörn to detect and quantify A. pseudogonyaulax. Additionally, water samples collected from the coastal German long-term monitoring station Heiligendamm over four months in 2021 were evaluated. For each sample, ddPCR was conducted using a species-specific 28S rDNA primer. To our knowledge the ddPCR approach was not applied to this species before. Results were compared with microscopic cell counts, validating the assay's efficacy. More detailed results will be presented at the conference, but overall, it seems that the ddPCR approach is as reliable as traditional microscopy, crucial for mitigating the impacts of HABs on marine ecosystems and associated industries.