Microplastics in the marine environment : an ecotoxicological perspective

Oceanic plastic pollution is of major concern, with several million tons of plastic dumped in the ocean every year that are causing health threat to marine creatures. Impacts have been found at all the trophic chain levels from the zooplankton to the megafauna, but little is known on its impact on the microbial life and its crucial role in the oceanic ecosystem functioning. The objective of this thesis was to study the ecotoxicity of plastics in the marine environment. The first handled question was: how much the abundance, diversity and activity of bacterial life growing on plastic, i.e. the ‘plastisphere’ are driven by the chemical properties of the polymer and the environmental changes (Chapter 2)? Polyethylene (PE) and polylactide acid (PLA) together with glass controls in the forms of meso-debris (18mm diameter) and large-microplastics (LMP; 3mm diameter), as well as small-microplastics (SMP; of 100 m diameter with spherical and irregular shapes) were immerged during 2 months in seawater. We found that the plastic chemical composition, the successive phases of biofilm formation and the phytoplankton-bacteria interactions were more important factors driving the abundance, diversity and activity of the plastisphere as compared to material size and shape. The second handled question was: would the microplastic (polystyrene PS; 50-100 µm; three concentrations) together with their mature biofilm be toxic for the marine filter-feeder Branchiostoma lanceolatum and how much the plastisphere can influence this toxicity (Chapter 3)? We used a large set of complementary techniques to follow the microplastic ingestion (microscopy quantification) and the modification of the gut microbiota (16S rRNA Illumina Miseq sequencing), the gene expression of immune system, oxidative stress and apoptosis (Nanostring) and also histopathology (transmission electron microscopy). No obvious toxicity was observed, while microplastics could be a vector for bacteria to the gut microbiome, can induce more goblet cell differentiation and can surprisingly have a positive effect by supplying nutrients to amphioxus in the form of bacteria and diatoms from the plastisphere. The third handled question was: how much the conventional petroleum-based microbeads classically used in cosmetics can be substituted by other polymers for their biodegradability by the plastisphere in marine environment? (Chapter 4). We used complementary techniques to follow the 4 biodegradation steps including biodeterioration (granulometry, gravimetry and FTIR spectroscopy), biofragmentation (size exclusion chromatography, 1H nuclear magnetic resonance and high-resolution mass spectrometry), bioassimilation and mineralization (1H nuclear magnetic resonance and oxygen measurements). We concluded that microbeads made of polyhydroxybutyrate-co-hydroxyvalerate (PHBV) or rice and in a lesser extend polycaprolactone (PCL) and apricot were good candidates for substitution of conventional microplastics, classically made of PE or polymethyl methacrylate (PMMA) that were not biodegraded under our conditions. Interestingly, the biobased PLA was not biodegradable but the petroleum-based PCL was biodegradable under our marine conditions.