## 3 Results and Discussion
### Surface Morphology and Elemental Analysis Using SEM-EDS
The surface chemical composition of MPs recovered from seawater and fish guts was determined by EDS analysis, which is a technique that provides critical information on the surface state of the materials when combined with SEM, allowing the distinction between MPs and inorganic structures or proxy organic matter [91]. From the EDS spectra of all the different samples recovered from seawater (Figure 10), it becomes apparent that the carbon peak (C Ka, 0.277 KeV), as expected, is the most intense, certifying that these are indeed carbonaceous MPs [73; 83]. The second most intense peak, which was also present in the EDS spectra of all the six different types of MPs, was attributed to the presence of oxygen (O Ka, 0.525 KeV). Figure 9: SEM images of MPs recovered from sea bass guts after digestion that have either (**a–d**) fibrous (F1–F4) or (**e,f**) fragment (F5 and F6) structure. Images were acquired by applying an accelerating voltage of 15 kV under high vacuum (0.1 Pa), using both backscattered and secondary electron detectors (20:80). On average, the atomic concentration of oxygen on the surface of MPs ranged from 8.8% to 24.7% (Table 1), coinciding with the findings of similar studies [81, 92]. Interestingly, oxygen was detected even on the surface of polymers that do not normally contain it in their structure (PE, PP, PS), suggesting that the MPs have possibly undergone photo-oxidation and/or biodegradation in the marine environment [73, 81]. These findings are in agreement with the results of micro-Raman and FT-IR spectroscopies since both techniques verified the existence of carbonyl groups on the MPs. In some cases, other elements common to the marine environment (Cl, Mg, Si) [93] were also detected, albeit in very low atomic ratios (\(<\)1%), as in similar works on MPs recovered from seawater [73, 83, 94]. Therefore, it appears that MPs, possibly due to their large, rough surfaces, are able to adsorb and transfer various elements [81]. Figure 10: EDS spectra of selected MPs recovered from seawater, which were identified as (**a**) polyethylene (PE), polypropylene (PP), and polystyrene (PS) and (**b**) ethylene-co-acrylic acid (EAA), ethylene oxide (PEO), and ethylene-co-vinyl acetate (EVA), respectively. Spectra were acquired using an accelerating voltage of 15 kV. Through the combined use of SEM and EDS, the MPs were easily distinguished from other materials recovered from fish guts after the digestion process that had similar external morphology [54; 91; 95]. The analysis (Figure 11) revealed that all the selected MPs are mainly composed of carbon (55.7-84.5%) (Table 2), which verifies their plastic nature [31; 73]. Interestingly, the surface oxygen content of these MPs, on average, was slightly higher than that of those recovered from seawater. This could be attributed to the fact that the gastrointestinal tract of both the sea bass and the sea bream maintains an acidic environment [96; 97], which could facilitate the oxidation of MPs as well as the applied digestion process, which is also an oxidative technique. Apart from carbon and oxygen, other elements were also detected on the surface of MPs (Figure 11) but in lower atomic concentrations (Table 2). In particular, sodium and chlorine were present in all samples in a ratio of approximately 1:1, suggesting that sodium chloride had been deposited on the MPs [31]. Also, two of the six samples, namely F5 and F6, contained small amounts of iron on their surfaces, which coincides with the findings of similar studies on MPs recovered from fish and other marine organisms [87; 94; 98]. Although the most likely explanation is that iron is a residue of the digestion process, the possibility that it was previously adsorbed from the marine environment cannot be ruled out, as MPs tend to retain metals [31; 83; 86]. Interestingly, of all the selected MPs, only F4 contained aluminum as well as silicon. [PERSON] et al.