Heterosigma akashiwo is the only raphidophyte described for Chilean waters. A recent 2021 fish-killing bloom event of this raphidophyte ignited scientific research, but the ichthyotoxic mechanism and environmental conditions that promote its growth are still unclear. This is the first study confirming the occurrence of H. akashiwo in Chilean waters on the basis of the region D1/D2 of the 28S ribosomal gene. The pigment signature of the CREAN_HA03 strain revealed chlorophyll-a, fucoxanthin, and violaxanthin as the most abundant pigments, but profiles were variable depending on culture and field conditions. A factorial temperature–salinity growth experiment showed a maximal growth rate of 0.48 d⁻¹ at 17 °C and 35 in salinity, but reached a maximal cell abundance of ~50,000 cells mL⁻¹ at 12 °C and 25 in salinity. The fatty acid profile included high levels of saturated (16:0) and polyunsaturated (18:4 ω3; 20:5 ω3) fatty acids, but superoxide production in this strain was low (~0.3 pmol O^2– cell⁻¹ h⁻¹). The RTgill-W1 bioassay showed that the H. akashiwo strain was cytotoxic only at high cell concentrations (>47,000 cells mL⁻¹) and after cell rupture. In conclusion, salmon mortality during H. akashiwo bloom events in Patagonian fjords is likely explained by the high production of long-chain PUFAs at high cell densities, but only in the presence of high ROS production.

Hydrogen peroxide (H₂O₂) has been shown to efficiently remove toxic microalgae from enclosed ballast waters and brackish lakes. In this study, in vitro experiments were conducted to assess the side effects of mitigating toxic and non-toxic dinoflagellates with H₂O₂. Five H₂O₂ concentrations (50 to 1000 ppm) were used to control the cell abundances of the toxic dinoflagellates Alexandrium catenella and Karenia selliformis and the non-toxic dinoflagellates Lepidodinium chlorophorum and Prorocentrum micans. Photosynthetic efficiency and staining dye measurements showed the high efficiency of H₂O₂ for mitigating all dinoflagellate species at only 50 ppm. In a bioassay carried out to test cytotoxicity using the cell line RTgill-W1, control experiments (only H₂O₂) showed cytotoxicity in a concentration- and time- (0 to 24 h) dependent manner. The toxic dinoflagellates, especially K. selliformis, showed basal cytotoxicity that increased with the application of hydrogen peroxide. Unexpectedly, the application of a low H₂O₂ concentration increased toxicity, even when mitigating non-toxic dinoflagellates. This study suggests that the fatty acid composition of toxic and non-toxic dinoflagellate species can yield toxic aldehyde cocktails after lipoperoxidation with H₂O₂ that can persist in water for days with different half-lives. Further studies are needed to understand the role of lipoperoxidation products as acute mediators of disease and death in aquatic environments.

As a part of a monitoring program of harmful microalgae in the coastal waters of Buenos Aires Province, two strains of chain-forming Gymnodinium species (Dinophyceae), LPCc043 and LPCc044, were isolated from samples collected in Samborombón Bay. This study aims to provide a morphological, phylogenetic and toxicological characterization of the strains. Among marine chain-forming Gymnodinium species, only G. catenatum and G. impudicum are known to form harmful algal blooms (HAB). Morphological analysis with light and electron scanning microscopy (LM, SEM) revealed that strain LPCc043 had larger cells, a higher number of cells per chain, and an acrobase independent of the sulcus entering the epicone, whereas strain LPCc044 had smaller cells, a lower number of cells per chain, and an acrobase as an extension of the sulcus. Phylogenetic analysis based on partial LSU rDNA sequencing placed the sequence corresponding to strain LPCc043 in the G. catenatum clade and that corresponding to strain LPCc044 in the G. impudicum clade with a 99–1.0 and 100–1.0 bootstrap and Bayesian posterior probability, respectively. Toxicological analysis by hydrophilic interaction liquid chromatography with tandem mass spectrometry (HILIC-UHPLC-MS/MS) detected paralytic shellfish toxins (PSTs) in G. catenatum, predominantly N-sulfocarbamoyl-11-hydrosulfate toxin 1/2 (C1/C2) and decarbamoyl gonyautoxin 2/3 (dcGTX2/3). In contrast, no PST were found in G. impudicum. This study contributes to the knowledge of harmful algal species diversity along the Argentinean coast and highlights the importance of monitoring these species for public health and environmental management.