Microplastics are a ubiquitous emerging contaminant across marine systems. Because of their small size and widespread distribution, microplastics occupy the same size fraction as the food source of many suspension feeders, such as marine bivalves, and ingestion of plastic particles could pose a threat to the health of these organisms. For example, microplastics may act as a vector for anthropogenic chemicals and foreign or pathogenic microbial species. Ingestion of plastic particles with adsorbed compounds or microbial communities could potentially affect the gut microbiome of the host through mechanical blockages or abrasions, leaching of plasticizers or adsorbed chemicals, or direct influence on microbial communities in the gut. In many species, the microbial community of the gut aids in digestion, mediates abiotic stressors, and affects host immunity. Similar functions may be performed in bivalves. Thus, changes in the gut microbiome may have indirect effects on bivalve health.
To test whether microplastics affect the gut microbiome or tissues of the blue mussel, Mytilus edulis, animals were exposed to nylon microfibers (length = 500 µm, diameter = 30 µm), Spartina spp. particles, or no particle, for 21 days. Mussels were fed a microalgal diet of Tetraselmis spp. (concentration in microcosm of 10,000-15,000 cells/mL) and Shellfish DietÒ (concentration in microcosm of 5,000 cells/mL). All particles were aged for three days to develop a biofilm. Two experiments were conducted, one with an exposure concentration of approximately 50 particles/L/hr /mussel/day, and one using a concentration of approximately 100 particles/L/hr /mussel/day. Spartina spp. particles were of comparable size and aspect ratio to nylon fibers and were used to control for the presence of indigestible particles. Genomic DNA was extracted from gut tissue , nylon, Spartina spp., and stock water samples and sequenced using 16S high throughput techniques to determine community taxonomic composition. Data indicate that intrinsic control over gut microbial communities outweighed exposure to the microbial community on nylon fibers (Figure 1). The microbial community on nylon microfibers differed from the community in seawater and on Spartina spp. particles, however, gut microbial communities of mussels exposed to microfibers were similar to both mussels exposed to Spartina spp. particles and control mussels.