Combining intact and oxidative LC-MS/MS methods to mitigate health risks from Ostreopsis cf. ovata

Ostreopsis cf. ovata is a benthic marine dinoflagellate which produces highly potent neurotoxins named ovatoxins (OVTXs), with OVTX-a being the dominant variant in the Mediterranean strains. OVTXs are associated with respiratory issues via aerosol inhalation and skin/eye irritation from direct seawater contact. Recently, FAO-IOC-IAEA guidelines indicated potential health risk when O. cf. ovata concentrations exceed 3 × 10⁴ cells L⁻¹ in seawater and 5 × 10⁵ cells g^-1 in macroalgae. Routine monitoring programs focus only on cell counts and do not measure OVTX-levels, limiting the accuracy of health risk assessment. Herein, we detected and quantified ovatoxins in two of six samples collected along Campania coast by a combination of intact and oxidative sample preparation and Liquid Chromatography tandem Mass Spectrometry (LC-MS/MS). OVTXs concentration in seaweed wash seawater was in the range 1340-1497 ng/mL for Aragonese Castle-L sample and 60-107 ng/mL for Gaiola sample. Matrix effects ranged from -31 % to 2 %, and recovery yields from 41 % to 97 %. After correction for matrix effects and procedural losses, OVTX concentrations were estimated at 13.9 nM (Aragonese Castle-L) and 2.1 nM (Gaiola), values within the cellular toxicity range. However, the abundance of O. cf. ovata cells on macroalgae (6.4 × 10⁴ and 1.6 × 10⁴ cells g⁻¹) remained below the alert threshold. These results highlight the need to integrate toxin quantification into O. cf. ovata monitoring programs. Given the current lack of OVTX reference materials, the use of both intact and oxidative methods combined to LC-MS/MS is a valid strategy for effective risk assessment.