Prevalence and Distribution of Microplastics in Oysters from the Mississippi Sound

Oysters are a foundational part of their ecosystem and research has shown they are negatively impacted by exposure to microplastics (MPs). High MP levels have been documented in waters surrounding oyster reefs, and as filter feeders, oysters can ingest MPs along with their food. Here, we determined MPs (>30 µm) in oysters (Crassostrea virginica) from ten sites across the Mississippi Gulf Coast. Further, a subset of these samples was dissected to quantify MPs within specific tissues. Average concentrations ranged from 30.7± 11.5 to 4.7 ± 0.25 putative MPs/g wet weight (ww) of whole tissue, with sites inside bays near population centers displaying higher levels of MPs than those exposed directly to the Gulf. Mantle, gill, and adductor muscle tissues had similar concentrations of putative MPs (15.9 ± 13.4, 11.5 ± 8.6 and 12.8 ± 6.7 MPs/g, respectively), whereas digestive system tissues had lower concentrations (6.8 ± 6.1 MPs/g of tissue). This suggests that most MPs in an oyster likely adhere to external tissues and are not actually ingested. Most of the MPs retained were in the smallest size fraction of 30-90 µm (80%), followed by 125-250 µm (9%), 90-125 µm (8%), and >250 µm (3%). Analysis of samples from Biloxi Bay by µ-FTIR to assess MP composition shows that polyurethane, polyethylene, and polyamide are common, but additional analyses are needed to fully characterize the MP profile across sites. Overall, this work provides much-needed empirical data on the abundances and sizes of MPs in oysters from the Mississippi Sound, as well as the tissues where they reside.