Microplastics and Mercury Interactions: Microcosm Medium Toxicity and Bioconcentration and Bioaccumulation in Bivalves

The Tagus estuary is included in the group of aquatic systems particularly affected by anthropogenic activity. This coastal area is in contact with industrial activity and exposed to strong and continuous urbanisation, which means the introduction of many pollutants, such as mercury (Hg) and microplastics (MPs). The concern associated with mercury lies in its persistence. According to Directive 2000/60/EC, it is on the list of priority hazardous substances, adding the possibility of it being transformed into more toxic forms, such as its organic forms (example: Methylmercury (MeHg)). With regard to microplastics, they are also known for exerting toxic effects due to their size and to associated pollutants, namely lixiviated compounds intrinsic to microplastics or other chemical that can be adsorbed to their surface, like mercury. Additionally, it is known that microorganisms can also grow in the surface of microplastics. The focus of the present study was to investigate the interactions of two contaminants, microplastics and mercury, individually and in combination, and in the presence of microorganisms, focusing on acute toxicity and mercury bioconcentration and bioaccumulation by bivalves. Based on microcosm apparatus, several exposures with 40 mg/L of microplastics, Hg2+ concentrations ranging between 1 and 10 ppm and MeHg concentrations ranging between 0.01 and 0.05 ppm, isolated and in combination, were prepared to asses: i) the acute toxicity of contaminated water samples by the Microtox Acute Toxicity test, using the bacterium Aliivibrio fisheri and ii) the bioconcentration and bioaccumulation of mercury by Venerupis decussata (clams) and Mytilus edulis (common mussel). The clams were exposed directly to the contaminated sediment in microcosm experiments, while the mussels were exposed to the contaminants through water collected from the microcosms. The results showed that i) microplastics influence the mercury acute toxicity, ii) microplastics influence the bioavailability of mercury to bivalves and the bioaccumulation by these feeding filters organisms, playing a role as vehicle of mercury uptake and iii) the bioconcentration of mercury in the studied organisms depends on the ease of filtration/excretion of microplastic particles. It can therefore be concluded that mercury adsorbs to the surface of microplastics and that their presence is relevant to bioconcentration and bioaccumulation of mercury in organisms. Therefore, the relationship and interference between the two contaminants has led to new growing concern about the impacts on ecosystems and human health, and it is essential to commit to implementing plausible measures and planning remediation strategies.