Marine and freshwater mussels as biomonitors for microplastic concentrations: A comparative laboratory study

Microplastics are pervasive in both freshwater and marine waters, posing potential hazards to a wide range of species. Evaluating environmental microplastic pollution is crucial for ecological risk assessment. However, current monitoring methods, such as sampling nets, are inefficient for quantifying small microplastics (< 25 µm) and non-buoyant particles, leading to potential underestimation of pollution levels. Biomonitors, like suspension-feeding organisms, can serve as complementary tools. This study takes a first step in evaluating bivalves as biomonitors by assessing whether microplastic uptake in their tissues correlates with environmental concentrations in both marine and freshwater conditions. Two mussel models, the marine Mytilus galloprovincialis and the freshwater Dreissena spp., were exposed to varying concentrations of microplastics (from 0 to 2000 MPs.L^-1) over 48 h. Both marine and freshwater mussels followed a linear model for MP uptake. However, an exponential model appeared more suitable for M. galloprovincialis whereas a Gaussian model better described the uptake pattern in Dreissena spp., suggesting the presence of a threshold in MP capture for the latter species. Microplastics primarily accumulated in the digestive gland compared to other tissues (i.e., byssus, gills, mantle, and others). After 48 h of depuration, marine mussels exhibited a high microplastic depuration rate (from 88 to 97%), while freshwater mussels showed moderate depuration ability (from 0 to 71%). These results support M. galloprovincialis as an effective biomonitor for marine microplastic pollution. In freshwaters, the non-linear accumulation of microplastics by Dreissena spp. limits their suitability for precise pollution assessment, but may help set pollution alert levels based on MP content and sub-lethal effects. This study contributes to addressing the challenge of accurate MP quantification in aquatic environments by highlighting the potential of bioindicators in complementing traditional methods.