Multi-level approach to investigate sublethal effects caused by synthetic and natural microfibers on <i>Daphnia magna</i>

Microplastic (MP) contamination is pervasive across all the ecosystems worldwide. Among MPs, fibers, specifically microfibers, represent the main shape commonly identified in diverse environmental matrices. In the environment, these fibers are not uniquely made of synthetic polymers, but several studies have highlighted that fibers made of natural polymers are predominant with respect to those made of synthetic ones. Despite these findings, whilst the potential toxicity of microfibers made of synthetic polymers has been explored, to date, little is known concerning the potential negative effect induced by natural microfibers, nonetheless they could pose the same threat to organisms than their synthetic counterpart. Thus, the aim of this study was to explore the potential sub-lethal effects induced by the exposure to two concentrations (0.05 microfibers/mL and 0.1 microfibers/mL) of microfibers made of a synthetic (i.e., polyester) or a natural (i.e., cellulose) polymer on the freshwater cladoceran Daphnia magna. Sub-lethal effects were investigated at the sub-individual and individual level. At the sub-individual level, a suite of oxidative stress biomarkers was applied, while at the individual level, effects on body growth and swimming behavior were assessed. Our results showed that the exposure to synthetic microfibers induced a modulation in antioxidant defenses and in swimming behavior of individuals exposed to microfiber treatments with respect to control. In contrast, cotton-based microfibers slightly modulated the antioxidant enzyme activity. These findings suggest that the exposure to synthetic and natural microfibers can affect the health status of D. magna, with synthetic microfibers potentially inducing more relevant effects than natural ones.