Introduction
Microplastics are ingested by many marine organisms, including bivalves. Laboratory evidence, especially from the blue mussel Mytilus edulis , suggests that not all microplastics pass quickly through the digestive system and are egested but may become stuck on gills or pass into the circulatory system
. Human exposure to microplastics through bivalve consumption is likely since microplastics have been found in the soft tissue of shop-purchased bivalves
. Commercial bivalves are generally depurated prior to being marketed to reduce microbiological contamination. Depuration in a laboratory setting has shown to reduce microplastic concentrations in mussels Perna perna
. The present field study aims to assess the removal efficiency of microplastics in commercial depuration facilities.
Materials and methods
Ostrea edulis were obtained from two UK shellfish merchants in November 2017 (n = 20; five untreated and five depurated specimens per merchant). Samples were stored at -20°C and subsequent m icroplastic extractions performed based on
. Briefly, soft tissue was digested in 10% potassium hydroxide for 48 hours at 40°C and digestates filtered over 1.2 µm after neutralisation with citric acid . Contamination mitigation included working in a clean air cabinet (Bassaire 03VB, BS EN ISO14644, class 5, with additional cover), use of glass and metalware whenever possible, furnacing of filter papers at 500°C for two hours prior to use and wearing of 100% cotton clothing including a previously lint-rolled laboratory coat. Laboratory microplastic contamination was assessed using procedural blanks and dampened filter papers as airborne controls. Counts of potential microplastics in blanks were used to estimate the limit of detection (LOD) by using 3x standard deviation per particle type and colour
. Results were adjusted using the LOD. Light microscopy (magnification ≤ 160x) was used to enumerate and characterise potential microplastics and Raman spectroscopy (Renishaw inVia , 785 nm) to ascertain plastic composition. Microplastics were reported as median concentrations ± 1 standard deviation . A two-sample two-tailed t-test for unequal variances was performed.
Results
F requency of occurrence of specimens with microplastics was 20% in untreated and 40% in depurated samples. Median concentrations in specimens with microplastics were 1.0 ± 0.0 microplastics in undepurated oysters and 1.5 ± 0.96 microplastics in depurated oysters (maximum 3 microplastics) . Microplastic concentrations were not statistically different between untreated and depurated specimens (t-test, p = 0.19). By category, 55.6% of microplastics were fibres and the remainder fragments/ film. All but two microplastics were 12 - 18 μm in size (fibre diameter or largest fragment dimension ). One fragment measured 52 and other approximately 400 μm . Microplastics were identified as polyester/PET, polypropylene, polyethylene and acrylic.
Discussion
Little information about microplastic concentrations in O. edulis exist, but values are similar to concentrations in the Pacific oyster Crassostrea (Magallana) gigas at other locations
. Human exposure to microplastics through bivalve consumption is a concern. It has been suggested that bivalve depuration decreases microplastic contamination based on depuration experiments in controlled laboratory settings with mussels P. perna , M. edulis as well as C. gigas
. However, our work shows that depuration in commercial facilities does not reduce microplastic contamination in O. edulis. While concentrations are low, most microplastics are of a size that raises concern about human health. Microplastics ≤ 150 μ m have the potential to cross the human gut tissue barrier and therefore absorb into the body
. Further work is needed to evaluate microplastic removal potential at other facilities and to assess how processes could be improved to aid microplastic reduction in bivalves prior to human consumption.
Conclusion
Our trial — performed at two commercial depuration facilities — shows that depuration aimed to reduce microbiological contamination prior to human consumption does not reduce microplastic concentrations in O. edulis. While frequency of occurrence and concentrations are low, the size of microplastics found warrants further investigations aimed at reducing human microplastic exposure through bivalve consumption.
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