## 4 Discussion
In particular, inorganic salts and bacteria have been reported to play a fundamental role in the degradation of PCL either through slow non-enzymatic bulk hydrolysis in aseptic water bodies or by rapid enzymatic degradation in bacteria-containing water bodies [52]. These authors hypothesized that the enzymatic degradation occurred, in particular at increasing degradation time, at the sample surface, while the inner part remained unaffected. On the other hand, if hydrolysis happens at the expense of starch, the reduction in chain size brings about the formation of smaller polysaccharides, with no formation of significant further carbonyl groups (Figure 6), but with an increase in hydroxyl terminal groups (Figure 5). Therefore, in this scenario, the IR signal at 1700 cm\({}^{-1}\) remains unaltered. However, when a polyester group is hydrolyzed, the C=O group is retained, but a shift should be observed from the characteristic frequencies of the esters and those of the carboxylic acid. In this study, the shape of the peak at about 1700 cm\({}^{-1}\) does not significantly change, reflecting the fact that the polyester moiety is not involved in the degradation process. Therefore, IR data point towards a preferred degradation of the starch component of the blends analyzed, with the polyester part remaining unaltered. The preferential microbial degradation of starch in blends exposed to the marine environment reported in the literature [52; 53; 53], clearly depended on microbial abundance and composition. According to the trend we have observed, the preferential removal through degradation of isotopically heavier corn starch (medium value of \(-11.45\%\)) from the analyzed compostable blends lead to polyester enrichment in the remaining matrix. This results in the decrease of the \(\delta^{13}\)C values over time, reaching lighter values characteristic of polyesters such as PCL. The degradation time of starch estimated in this study (180 and 124 days for ULB and LSB, respectively) during the present degradation experiment, is in accordance with observations of [PERSON] et al. (2019) [54], who reported that the hydrolysis of the starch component and loss of plasticizer after 8 months of seawater exposure. These authors also reported a severe loss of tensile properties of Mater-Bi(r) film. However, it is not possible to accurately estimate how long it would take the polyester fraction to decompose. In this regard, [PERSON] et al. (2015) [25] reported, using Scanning Electron Microscopy analysis, that the holes that appeared after 15 and 33 days of exposure to the marine environment in the plastic's surface might have developed due to dissolving or mineralization of the starch fraction. In this regard, the experiment conducted on PCL degradation under different environmental and laboratory conditions [52] showed that, after a one-year immersion in seawater, a weight loss of about 30% was observed, principally due to the superficial microbial enzymatic degradation, but no significant molecular weight changes were recorded, indicating that, under these conditions, the internal non-enzymatic hydrolysis is negligible. [PERSON] et al. (2000) [55] reported that in composting conditions with controlled temperature and moisture, pure PCL degrades in a few tens of days, while its blend with starch at 50% disappears in about 3 weeks. However, under marine conditions, PCL items exposed to seawater were still recoverable after 12 months, even if numerous cracks in the surface and the complete loss of tensile strength were observed [56]. All of these findings led us to hypothesize that biodegradable polymers can contribute to increasing the pool of microplastics in the marine environment. In fact, even though PCL-degrading microorganisms were found in a wide range of marine habitats, like bacteria belonging to the genus _Pseudomonas_[57], some PCL fragments were identified as micro- and meso-plastics polluting the Mediterranean Sea ([58] and literature therein).