Time-course distribution of fluorescent microplastics in target tissues of mussels and polychaetes

The majority of the plastic produced in the last century is accumulated in the environment, leading to an exacerbated contamination of marine environments due to transport from land to the ocean. In the ocean, mechanical abrasion, oxidation, and photodegradation degrade large plastics into microplastics (MPs) - 0.1 μm to 5 mm (EFSA, 2016) which are transported through water currents reaching the water surface, water column, and sediments. Further, they can be accumulated by aquatic and benthic species, entering the trophic chain and becoming a potential threat to humans. In the present research, we aimed to decipher the accumulation and distribution time-courses between different organs or target tissues of organisms inhabiting coastal areas such as mussels Mytilus galloprovincialis and polychaetes Hediste diversicolor. Both were exposed in microcosm experiments to fluorescent polystyrene MPs (1 μm) which were spiked at two doses (10 and 10 particles/mL) for 1, 4, 24, and 72 h. Mussels and polychaetes were digested with 10% KOH and filtered to quantify the number of MPs incorporated. Different anatomical parts of the body were selected and processed for cryosectioning and posterior microscopic localisation of MPs. Both species accumulate MPs spiked in water column, mainly after exposure to the highest dose. In mussels, particles were found in distinct parts of the digestive tract (stomach, digestive diverticula, ducts) and gills. Even if the majority of MPs were localised in the lumen of the digestive tract, in some cases, were inside the digestive epithelium. The identification of MPs and their internalization in the digestive system was studied using Raman spectroscopy. A decreasing trend with time regarding MPs number in the digestive tract (stomach) of mussels was observed while the opposite was recorded for polychaetes and sediments. The combination of microscopical observations of frozen sections and Raman, appeared to be accurate methodologies to address MPs abundances and to reveal their localisation in different organs. This work has enabled to understand the distribution and fate of MPs in different environmental compartments and it could contribute to gain knowledge about their impact after ingestion by coastal organisms.