The Mediterranean Sea is one of the most affected areas by plastic pollution. Because of their small size, microplastics can be ingested by marine organisms that are part of the human diet. This poses the risk of transporting of chemical and microbiological contaminants. In the present study, we isolated DNA to determine the microbial communities on microplastics in seawater and shellfish on the Slovenian coast, as well as the chemical structure of the particles. We focused primarily on fragments, which are one of the types of microplastics besides filaments, beads and foams . Microplastic samples were obtained from water by filtration, and from mussels by cutting the mussels open and visual inspection with a stereomicroscope. Due to the small size of the samples, we attempted to optimize DNA isolation by adding steps to mechanically disrupt the cells to maximize DNA extraction with simulated microplastic samples. Metagenomic analysis of the 16S rRNA gene showed that the most abundant bacteria on microplastics were dominated by the strains Proteobacteria, Cyanobacteria, Firmicutes, Actinobacteria and Bacteroidetes. The bacterial communities found on microplastics belong to over 400 different species in average. Some of the bacteria isolated from shellfish are human pathogens. We cannot link all the results of the metagenomic analysis with microplastics because it is not possible to determine the polymer composition of all the particles. FTIR spectroscopy was used to determine the chemical composition of the particles. The polymer composition was confirmed for 5 of the total 23 samples. The polymers identified in the samples were polyvinyl chloride, polyethylene, acrylonitrile butadiene styrene and polypropylene.
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