Tracking microbial diversity associated to microplastic distribution in the Mediterranean Sea
Microplastics (MPs) are widespread pollutants in marine ecosystems, capable of transporting contaminants and microorganisms. The Mediterranean Sea, characterized by limited water exchange and intense human activity, is particularly exposed to MP accumulation. This study investigated the distribution, composition, and microbial associations of MPs in surface seawater from 22 sites across the Mediterranean basin. MPs were found in 95.5% of samples, with an average abundance of 70.7 ± 3.2 items/L. The highest concentrations occurred near densely populated and industrialized zones such as Livorno and Istanbul. Polyethylene (PE) and cellulose-based fibers were the dominant polymers, while fragments prevailed over fibers in most samples. The majority of MPs measured between 1 and 0.1 mm. High-throughput sequencing of the 16S rRNA gene revealed that microbial communities were structured by geography and MP characteristics. Core taxa included Pseudohongiella and Salinisphaera, while hydrocarbon-degrading bacteria such as Alcanivorax and Polyciclovorans were enriched in samples with higher MP loads or specific particle sizes. Redundancy analysis indicated that MP size and concentration together explained up to 29% of microbial community variation. These results suggest that MPs provide both habitat and selective pressure for bacteria with degradation capabilities, thereby influencing marine microbial assemblages. Combining µ-FTIR polymer identification with 16S rRNA sequencing, this study offers one of the first basin-scale integrated assessments of MP composition and associated microbiota in the Mediterranean. Our findings underscore the need for continued, multidisciplinary monitoring to evaluate the ecological risks of MP pollution and its role in shaping marine microbial ecosystems.