Understanding the transfer of different size microplastics and their fate in organisms of different marine environmental compartments.
Supervisors: Nerea García-Velasco, Manu Soto (UPV/EHU
Microplastics (MPs) have been documented in nearly all marine environmental compartments and have been identified as a growing hazard to marine biota due to their broad dispersion and potential interactions with marine animals. The current study sought to determine the accumulation, localization, and depuration dynamics of fluorescent polystyrene (PS) MPs in the target tissues of the mussel Mytilus galloprovincialis, the polychaete Hediste diversicolor, and the shrimp Palaemon serratus, as well as the transfer of MPs between sediment and water compartments concerning particle size. The organisms were exposed to an experimental microcosm containing a sediment layer and seawater column with two different set ups. One treatment with 1 μm sized MPs spiked in seawater and 5 μm MP spiked in sediment, while another with 5 μm and 1 μm MP spiked in seawater and sediment, respectively; both compartments spiked with 1.5×107 total MPs. Organisms were maintained in experimental conditions during the course of a 5-day exposure period and a 14-day depuration and were then subjected to procedures for both quantitative (alkaline digestion followed by filtration) and qualitative (cryosectioning) examination under fluorescence microscopy. All species accumulated MPs spiked in the water column and sediment irrespective of their habitat, and MPs appeared to transfer vertically by the influence of biota in relation to particle size. Mussels showed a higher accumulation of MPs spiked in the seawater column with a trend of increased uptake for bigger particles. Particles were found in distinct parts of the digestive tract (lumen of the intestine, stomach) and gills of mussels. Depuration effectively eliminated accumulated MPs, with a significant reduction for smaller particles whereas the same was not observed for the larger particles. Polychaetes accumulated a comparable amount of particles that spiked in the water column and sediment regardless of particle size, being these MPs identified predominantly in the hindgut. Unlike mussels, after depuration, a considerable increase of all MP sizes except 1 μm when spiked in sediment was noted for polychaetes. Shrimps have accumulated both size MPs with a size specificity where they preferred to ingest bigger particles than the smaller ones via seawater as a route of entry. MPs were mainly localized in the digestive tract and the tegument and pereopods. No discernible reduction in MP elimination was seen after depuration, and both size MPs were found in the hepatopancreas. This work has enabled to understand the transfer of MPs between different environmental compartments and their fate in target tissues of organisms with different feeding behavior in relation to particle size.

Keywords: Microplastics, Mytilus galloprovincialis, Hediste diversicolor, Palaemon serratus, Polystyrene, Particle size, Depuration, Sediment.