Microplastic Debris in Charleston Harbor: Identifying Sources and Assessing Effects on Grass Shrimp (<i>Palaemonetes pugio</i>) Immune Function

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Leads, Rachel Renee
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Microplastics are ubiquitous in the marine environment and are comprised of heterogeneous small plastic particles (<5 mm). Previous research determined that Charleston Harbor contains 414±77 microplastics/m<sup>2</sup> in intertidal sediments, with black fragments constituting >90% of the microplastics at some sites. The high concentration of black fragments in Charleston Harbor is unique to other coastal locations worldwide. The present study aimed to further explore the sources, fate, and effects of microplastics in Charleston Harbor with a focus on determining the source, identity, and toxicity of the black fragments. A survey of the Ashley, Cooper, and Wando Rivers of Charleston Harbor for microplastics (63-500 µm) showed blue fibers and black fragments to be the most abundant microplastics observed, constituting 26.2% and 17.1%, respectively, of total microplastics. Polymer analysis of the black fragments using FTIR indicated that these fragments were tire wear particles. To understand the toxicity of tire wear particles and other dominant microplastic particle types in the environment to estuarine organisms, the impact of microplastic exposure and ingestion on grass shrimp (<i>Palaemonetes pugio</i>) immune function was investigated. Grass shrimp were exposed to 63-150 µm sediment, polyethylene spheres, polypropylene fragments, tire fragments, and polyester fibers for 96 h at a concentration of 50,000 particles/L before bacterial challenge with <i>Vibrio campbellii</i>. The survival of grass shrimp challenged with <i>V. campbellii</i> did not vary significantly (p=0.43) among exposures. However, grass shrimp exposed to polyethylene spheres consistently exhibited 100% survival following bacterial challenge (a 12% increase in survival compared to controls), suggesting that polyethylene spheres may stimulate the immune response of grass shrimp. The present study provides insight into the sources, fate, and biological effects of microplastic accumulation in Charleston Harbor.