## Materials and Methods
### Resins Preparation and Their Characterization  
To evaluate the ability to remove toxins in _G. catenatum_ cultures and in contaminated bivalves, a strong acidic resin was used: Ambechrom\({}^{\circledR}\)50 WX8 hydrogen form (100-200 mesh, Thermo Fisher Scientific, Waltham, MA, USA, CAS 69011-20-7). It is a cation-exchange resin of styrene-divinylbenzene (ST-DVB) with sulfonic acid functional groups as aryl substituents (Figure 2). For studies of toxin removal in culture and in bivalves, the resin concentration was 1 g/L.  
Before its use, the resin was cleared and activated as follows [27]. Deionized water--10 times by volume (mL) of the resin mass value (g)--was added to the resin, in a beaker. The mixture was stirred with a glass rod and allowed to settle for 20 min. Then, the fine particles were decanted. To activate the resin, the same volume of 1 M H\({}_{2}\)SO\({}_{4}\) (95%, Fisher Chemical, Fisher Scientific UK, Loughborough, UK) was added, and the mixture was left for 15-20 min, after which the fine particles were again decanted. This activation process was repeated three times. Then, the resin was rinsed with deionized water until pH > 5, using a Buchner funnel and PVDF membrane filter paper 43-48 \(\upmu\)m (Filter-Lab, Filtros Anoia S.A., Barcelona, Spain). Additionally, a sodium-form (Na\({}^{+}\)-form) of the resin was prepared from the acidic form (H-form) as follows: deionized water--10 times by volume (mL) of the resin mass value (g)--was added to the resin in a beaker. The mixture was stirred with a glass rod and was allowed to settle for 20 min. Then, the fine particles were decanted. To convert the resin to the sodium form, the same volume of 4% NaOH (p.a., Chem-Lab NV, Zedelgem, Belgium) was added, and the mixture was left for 15-20 min, after which the fine particles were again decanted. Then, a solution of 4% NaCl (p.a., \(\geq\)99.8 %, Chem-Lab) was added, the mixture was left for 15-20 min and later decanted. This final step was repeated twice. The resin was then rinsed with deionized water until pH = 7, using a Buchner funnel and PVDF membrane filter paper 43-48 \(\upmu\)m (Filter-Lab). Both forms of the resin (H and Na\({}^{+}\)) were dried in an oven at 50 \({}^{\circ}\)C for further characterization by Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), and optical microscopy (OM).  
Powder X-ray diffractograms were recorded on a PANalytical X\({}^{\prime}\)Pert Pro diffractometer using Cu K\(\alpha\) radiation (\(\uplambda\) = 0.15418 nm) filtered by Ni and an X\({}^{\prime}\)Celerator detector, operating at 50 kV and 30 mA. The patterns of the resins were recorded in the range 3-70\({}^{\circ}\) (2\(\diameter\)), with a step size of 0.334225\({}^{\circ}\) and 400 s per step. Plots were generated with manual R scripts in RStudio (Version 2022.07.2), using ggplot2 libraries for the graphic figures.  
FTIR tests were performed to analyse the functional groups present in the resins. The samples were previously grounded with KBr in a mortar and compressed into discs. FTIR spectra were recorded on an FTIR spectrophotometer (Bruker, model Tensor 27, MA, USA). For each spectrum, a 25-scan interferogram was collected in transmittance mode in the 4000-600 cm\({}^{-1}\) region, with a 4 cm\({}^{-1}\) resolution. The results were obtained using the OPUS software (V6.5, Bruker Optik GmbH 2009, Ettlingen, Germany). Plots were generated with Microsoft Excel (Microsoft 365 Apps).  
To evaluate particle shape and size, an optical microscope (Microscope Zeiss ESTATIVO AXIOSCOPE 5-TL/FL6 KH DIC, coupled with Zeiss Axiocom 305 (Carl Zeiss  
Figure 2: Structure representation of the acidic cation-exchange resin derived from the ST-DVB co-polymer.  
Microscopy GmbH, Jena, Germany) with particle diameter measurement on the Zen Blue Image processing software was used.