Effects of Expanded Polystyrene Particles on the Diversity and Composition of the Gut Microbiome of the Coastal Isopod Ligia cinerascens

Expanded polystyrene (EPS) is a major coastal plastic pollutant, yet its effects on gut microbiota in coastal invertebrates remain underexplored. This study investigated the impact of short-term EPS exposure on the gut microbial diversity and composition of the coastal isopod Ligia cinerascens under controlled laboratory conditions. Isopods were fed an EPS-containing diet for 7 and 14 days, and their gut microbiota was analyzed via 16S rRNA gene sequencing. Intake-excretion patterns were also evaluated to understand toxicokinetics. Results showed no significant differences in alpha and beta diversity indices between control and EPS-exposed groups. The gut microbiota was dominated by the phylum Proteobacteria and the genus Vibrio, with V. hibernica and V. algivorus being the most abundant species regardless of exposure. However, Clostridium tarantellae and Vibrio cyclitrophicus were significantly enriched in the EPS-exposed group at day 7, suggesting a potential shift toward pathogenic taxa. Intake-excretion analyses revealed a rapid elimination of EPS, which may explain the limited impact on gut microbial diversity. The findings suggest that the gut microbiota of L. cinerascens may exhibit resilience to EPS exposure, possibly due to prior adaptation to plastic-contaminated environments. Nevertheless, subtle microbial shifts and enrichment of potential pathogens indicate that EPS may pose hidden health risks to marine invertebrates. These results highlight the importance of species-specific and exposure-dependent assessments when evaluating microplastic toxicity in marine organisms.