Assessing the threat of microplastics to commercial oysters, particularly those of the genus Crassostrea

Abstract

Current statistics show that China is the largest producer of plastics in the world, contributing to almost 30% of production worldwide (Plastics Europe, 2017). In particular, mega-cities in China connected with the Pearl River Estuary contribute to much of this production, and locations in the Estuary’s proximity are potential sites for plastic pollution. In this thesis, the hypothesis that Crassostrea hongkongensis oysters cultured in Deep Bay, Hong Kong would be susceptible to microplastic uptake was tested. Extensive sampling was conducted across the bay through the collection of oysters, water and sediment from five selected sites (rafts). Follow up analysis of samples revealed that an average of 15.1 ± 6.1 microplastics per individual were present within the oysters, and oysters situated in the outer part of the bay took up more microplastics compared to those situated in the inner part of the bay. The numbers and types of microplastics quantified in sediment and water samples across sites did not correspond to the number of microplastics quantified in Crassostrea hongkongensis specimens across sites, suggesting that there was an element of selection during the biological uptake of these particles, and this was potentially influenced by particles’ properties such as size, shape and polymer type. Having established that wild Crassostrea hongkongensis in Deep Bay were taking up microplastics, an experiment was set up to investigate whether a genetically similar oyster species, Crassostrea gigas, would exhibit selective uptake up microplastics according to plastics’ size, shape or polymer type, and whether microplastic exposure would induce biological responses within the oysters. Eight microplastic types of various polymers, sizes and shapes were supplied in equal concentrations to Crassostrea gigas oysters at a final concentration of 100 microplastics mL-1, where they were exposed for a 24 hour period. Findings revealed that microplastic polymer type and size did indeed influence uptake (Pr (>Chi)=0.034), and oysters readily took up 0.29% of 8-30 m polyethylene beads and 0.31% of 115-156 m polyvinyl chloride fragments supplied when exposed solely to plastics; oysters exposed to a combination of algae and plastics (same microplastic concentrations) took up 0.25% of the former plastic type, and 0.32% of the latter plastic type supplied. A post-hoc Tukey test confirmed that uptake of these two plastic types were significantly higher (p<0.05) than with other plastic types. Crassostrea gigas oysters were then exposed to polyethylene beads (8-30 m) at a concentration of 100 microplastics mL-1 for 14 days and subsequent biological assays were used to identify potential biological impacts upon exposure. Despite the tentative evidence of a small increase in phagocytosis activity and decreases in food assimilation and metabolism, no significant (p<0.05) impacts on oxidative stress, DNA damage or changes to oxygen consumption and feeding behaviour were found as a result of microplastic uptake. The research in this dissertation shows that although the size and polymer type of microplastics affects uptake, exposure to beads at low concentrations does not cause adverse biological impacts or health effects. Since oysters tend to be contaminated with microplastic fibres in the natural environment, and the microplastics used for testing exposure-induced biological impacts consisted solely of microplastic beads, they may be subject to higher levels of oxidative stress and other toxic effects beyond those reported in this study.