## Materials and Methods
### Toxin Removal Studies in Live Bivalves  
In this work, three sets of experiments were carried out using live mussels (_Mytilus edulis_): (1) with the H-form resin and healthy mussels; (2) with the H-form resin and mussels contaminated with _G. catentatum_; (3) with the Na+-form resin and mussels contaminated with _G. catentatum_. All tanks had continuous aeration and a small aquarium pump (3 W) was used to increase water flow. All mussels used in these experiments were harvested in Ria Formosa, Algarve (southern Portugal), depurated (\(\geq\)48 h) and transported in thermal containers from the depuration centre to the laboratory (approximately 20 min). In addition to the mussels used in the feeding and depuration protocols, uncontaminated mussels were collected in each set of studies to subsequently evaluate the matrix effect in the quantification method.  
Four small tanks were filled with 10 L of seawater (salinity: 37.86 \(\pm\) 0.02), prior to the feeding experiments with toxic microalgae and the addition of resin, to assess the purification process's acceleration. At this stage, ten previously purified mussels were randomly distributed in each tank, with aeration, and allowed to condition for approximately 19 h. Following this period H-form resin was added to two tanks, while the other two remained resin-free. The objectives of this first set of experiments were to investigate possible signs of acute toxicity of the resin for the mussels studied and to assess the mussels' ability to ingest the resin. Observations were carried out at 24 and 48 h. After each time, the mussels, exposed or not to the resin, were removed from the tanks, washed outside with fresh water, then opened to analyse their digestive tract. The mussels used in this study had an average size of 6.2 (\(\pm\)0.6) \(\times\) 3.9 (\(\pm\)0.3) cm.  
Since the preliminary test revealed no signs of acute toxicity (no mortality), a second set of experiments was performed in triplicate, using six rectangular tanks with 18 L of saltwater each. The experimental design is illustrated in Figure 3. Twenty live mussels were randomly placed in each tank. The average size of mussels in each tank is shown in the Supporting Information, SI (Table S1). The mussels were acclimatized to the new medium overnight (about 18 h) and there was no mortality throughout this period.  
The third set of experiments (Figure 4) was carried out under similar conditions as the second. Briefly, 18 mussels were placed in each tank containing 18 L of saltwater. acclimation occurred overnight, followed by feeding of _G. catentatum_ as described below. Table S2 shows the average size of the mussels used in this experiment. These experiments were performed in duplicate and no mortality was observed during the entire experiment.  
Several physicochemical parameters (percentage of dissolved oxygen, % DO; temperature, T \({}^{\circ}\)C; salinity; pH) were regularly monitored throughout the experiments with live mussels, using a multiparameter (H198194, HANNA Instruments, Cluj Napoca, Romania) and the corresponding probes. DO remained over 70% throughout the experiments. Laboratory biosafety guidelines were considered for all experiments involving toxic microalgae and live bivalves to avoid human and/or environmental contamination.