## Materials and Methods
### HPLC-FLD Analysis  
An HPLC-FLD from Shimadzu (Prominence-i LC-2030C Plus) was used to conduct the PST analyses. This equipment comprises a quaternary pump, a refrigerator autosampler, a column oven and a spectrofluorometric detector (RF-20A XS). The temperatures used were 10 \({}^{\circ}\)C, 25 \({}^{\circ}\)C and 30 \({}^{\circ}\)C, in the autosampler, column oven and cell detector, respectively. A reversed-phase C18 column (Mediterranean Sea18, Teknokroma), 25 cm \(\times\) 0.46 cm (5 \(\upmu\)m), and an ultrapawardTM column (Sea18 10 \(\times\) 3.2 mm, Teknokroma) were used. Ammonium formate 0.1 M (adjusted to pH 6 with acetic acid 0.1 M) and acetonitrile (HPLC grade, Honeywell) were applied as mobile phase A and B, correspondingly. All eluents and solvents for HPLC were filtered, using PVDF membrane filters (0.22 \(\upmu\)m, 47 mm, Teknokroma). The gradient elution conditions were as follows. First 6 min: 1-5% phase B; between 6 and 13 min: 5-28% phase B; between 13 and 16 min: 28-1% phase B; between 16 and 19 min: 1% phase B. The flow rate was 1.5 mL/min. The injected volume was 30 \(\upmu\)L or 100 \(\upmu\)L, for standards and samples oxidised with peroxide or periodate, respectively. Three injections of each sample were performed. Fluorescence detection was achieved at 340 nm (excitation) and 395 nm (emission).  
To quantify PST, calibration solutions ranging 0.03 and 2.00 \(\upmu\)M were prepared from certified reference materials (CRM): dcGTX2&3, C1&2, GTX2&3, GTX5 (or B1), GTX6 (or B2), GTX1&4, C3&4, NEO, dcNEO, STX (from CIFGA laboratory S.A., Lugo, Spain) and dcSTX (from National Research Council of Canada, Halifax, Canada). To prepare the solutions, the following mixtures were considered: (I) dcGTX2&3, C1&2 and dcSTX (for peroxide oxidation); (II) GTX2&3, GTX5 and STX (for peroxide oxidation); (III) GTX1,4 and NEO (for periodate oxidation); (IV) C3&4 and GTX6 (for periodate oxidation); (V) dcNEO (for periodate oxidation); (VI) dcGTX2&3 and dcSTX (for periodate oxidation). The quantification of each toxin in the samples was performed by interpolation of data in the corresponding calibration curve. The contribution of the chromatographic peaks corresponding to the matrix modifier were subtracted (before interpolation) from the chromatograms of the samples and standards pre-oxidized with periodate in the presence of the matrix modifier. The limits of detection and quantification (LOD and LOQ) were determined as (3\(\sigma\))/m and (10\(\sigma\))/m, where \(\sigma\) is the residual standard deviation of the regression line and m is the slope of the calibration curve [33; 34]. For the several calibration curves, all determination coefficients (\(R^{2}\)) were above 0.9981. LOD and LOQ ranged from 0.003 \(\upmu\)M (1 \(\upmu\)STX.2 HCl eqv/Kg) to 0.03 \(\upmu\)M (169 \(\upmu\)STX.2 HCl eqv/Kg) and from 0.01 \(\upmu\)M (3 \(\upmu\)STX.2 HCl eqv/Kg) to 0.11 \(\upmu\)M (615 \(\upmu\)STX.2 HCl eqv/Kg), respectively (Table S3). The least concentrated standard solution of each calibration curve was always equal to or greater than the corresponding LOQ. To avoid misinterpretation, the following solutions were also analysed under the same conditions as the standards and samples: a chemical blank of the entire procedure--extraction, cleaning, fractionation, and oxidation (1% acetic acid), samples without the oxidizing agent (using the same volume of ultrapure water instead of the peroxide or periodate oxidant), the matrix modifier oxidised with periodate (the same volume of ultrapure water instead the sample/standard) (some examples in SI, Figures S1-S3).