Hidden Hazards in the Central Eastern Arabian Sea: Metabarcoding Reveals Co-occurrence of ASP, PSP, and NSP Vectors in the Coastal Waters of Goa

Harmful Algal Blooms (HABs) represent a growing threat to marine ecosystems, aquaculture, and public health in the Central Eastern Arabian Sea (CEAS). This study utilized 18S rRNA metabarcoding to characterize the absolute abundance and community composition of potentially toxigenic diatoms and dinoflagellates in the coastal waters of Goa. The analysis reveals a distinct and alarming prevalence of multiple genera associated with diverse toxin syndromes.

The dataset was dominated by a massive proliferation of the dinoflagellate Karenia (linked to Neurotoxic Shellfish Poisoning and ichthyotoxicity), which reached extreme abundances exceeding 51,000 reads per sample at the most impacted sites. Co-occurring with this bloom, spatially distinct hotspots of Paralytic Shellfish Toxin (PST) producers were identified, specifically Alexandrium and Gymnodinium spp., with Alexandrium counts peaking at over 5,200 reads. Notably, the potent PST producer Alexandrium tamiyavanichii was positively identified, alongside detections of Gymnodinium catenatum.

The diatom community also exhibited significant toxicity potential; the Amnesic Shellfish Poisoning (ASP) genus Pseudo-nitzschia displayed high relative abundance (up to ~3,700 reads), including the presence of P. pungens. Furthermore, vectors for Diarrhetic Shellfish Poisoning (DSP), including Dinophysis spp. and Phalacroma rotundatum, and Yessotoxin producers (Lingulodinium polyedra, Gonyaulax spinifera) were ubiquitously present at lower background levels.

These findings highlight a complex, multi-risk scenario where ASP, PSP, NSP, and DSP vectors coexist within the same coastal system. The distinct spatial separation observed between peak Karenia, Alexandrium, and Pseudo-nitzschia events suggests that heterogeneous environmental drivers are influencing specific HAB assemblages. This data underscores the critical need for broad-spectrum toxin monitoring beyond single-species surveillance in the region.