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In vitro assessment of microplastic degradation by pepsin and lipase as selected fish digestive enzymes
Summary
In vitro experiments simulating fish digestion showed that pepsin caused limited degradation of polyamide microplastics with up to 3.6% weight loss at acidic pH, while PET was chemically inert to both pepsin and lipase, indicating fish digestive enzymes cannot substantially break down most ingested plastics. This means microplastics persist largely intact after ingestion by fish, retaining their potential to transfer up the food chain and accumulate in tissues consumed by humans.
The widespread presence of microplastics (MPs) in aquatic environments has raised concerns about their interactions with biotic systems, including potential degradation during digestion by aquatic organisms. This study explores the susceptibility of two common polymers, polyamide (PA, Nylon 6,6) and polyethylene terephthalate (PET), to enzymatic degradation by two representative fish digestive enzymes, pepsin and lipase. An in vitro protocol was developed to simulate the gastric and intestinal conditions of sardine (Sardinops sagax) and yellowfin tuna (Thunnus albacares), varying pH, enzyme activity, and exposure time. The degradation extent was assessed through mass loss and surface/molecular alterations analyzed by stereomicroscopy and FTIR-ATR spectroscopy. Pepsin promoted partial degradation of PA, with up to 3.6% weight loss at pH 2 and high enzymatic activity, together with the degradation of an additive-related carbonyl band (1740 cm⁻¹) from FTIR spectra. In contrast, lipase induced only minor changes, and PET remained chemically and morphologically unaltered under all tested conditions. These findings indicate that fish digestive enzymes alone cannot cause extensive polymer breakdown within typical gut residence times but can trigger limited surface modifications and additive removal. The results provide new insights into enzyme-specific microplastic transformations and contribute to understanding their fate in aquatic organisms.