## 3 Results and Discussion
### PST Removal from Bivalves
#### 3.3.2 Depuration with H-Form and Na+-Form Resins  
As the identified toxins are mostly positively charged, a cation-exchange resin was used to try to accelerate the purification process. When using the H-form resin, an acceleration of the average clearance rate seems to occur after 24 h, mostly attributed to the decrease in dcSTX concentration (Figure S4). However, due to the large variation observed, this increase in acceleration is not statistically significant (\(p\) = 0.056). At 48 and 70 h no significant changes are observed, which could be related to the typical deceleration of clearance after unassimilated toxins have been released [52]. Several factors may contribute for the low effectiveness of the resin tested, namely absorption of the resin by the mussel and the presence of other water constituents (e.g., cations in saltwater, organic matter). Seawater contains various cations with different selectivity for this resin [53], which may compete with toxins for the same binding sites. Moreover, organic matter (OM) may affect the adsorption, the effect being variable with pH [15]. Three arguments have been used to explain the reduced adsorption of PST in the presence of OM, namely, pore-blocking, preferential interaction/competition by the active sites available, and the electrostatic interactions with PST [54]. Regarding the absorptive capacity of mussels, we did not observe large amounts of resin inside their digestive tract in preliminary experiments carried out (first set of experiments). Interestingly, the highest number of particles was observed in the digestive tract of the mussel after 24 h, while after 48 h few particles were observed. This could mean that the resin particles are not truly absorbed by the mussel, which renders the action of the resin more difficult. Contrary to what happened during the experiments in water, with toxins coming directly from the culture, in the resin experiments with bivalves there is no direct interaction between the resin and the PST absorbed by mussels. During the experiments with H-form resin, an interesting behaviour was observed: a sudden pH oscillation was registered after the addition of the H-form resin, stabilizinguntil the end of the experiment. To better understand what happened, a parallel in vivo experiment was carried out. To three small containers with 2 L of saltwater was added, separately: (1) H-resin (Control 1); (2) H-resin with two mussels (H\({}^{+}\) + M); and (3) two mussels, as a negative control group (M) (Figure S6). All containers had continuous aeration and a small aquarium pump was used to increase water movement. The pH was measured regularly along a period of 40 h and the results are depicted in Figure 9. The negative control group (container 3) displayed pH values in the ranges 7.75-7.93. Immediately after adding the H-resin to the containers 1 and 2 the pH decreased to 3.00. The pH in the H-resin container (container 1) remained constant over time, but the pH in the container with H-resin and mussels (container 2) gradually increased. The pH rise occurred as a steady acid-base titration, with a strong increase after 8 h and reaching a plateau at 24 h, with a pH close to 7. After 40 h, the pH reached a maximum of 7.26. This result suggests the shellfish's role in adaptability during deprivation in acidic conditions. The mussel shell is composed of calcium carbonate, CaCO\({}_{3}\), and it would have been the reaction between CO\({}_{3}\)2- and H\({}^{+}\) in the acidic aqueous medium that would have promoted the pH increase [55] in the container with mussels, unlike for the container without mussels. Although the mussels bear the ability to adjust the pH of the medium, this excessive acidification is likely to affect not only their shell, but also their well-being [56]. Moreover, due to acidification, complete closure of mussel shells was observed in tanks where resin was added. This defensive response by the organisms will likely have inhibited their clearance rate during the first hours.