Bioavailability of microplastic in laboratory exposure studies: What are we exposing to?

To quantitatively study the uptake and effects of microplastic (MP) in planktonic species and other aquatic biota in a laboratory setup, it is paramount to monitor MP bioavailability. Many polymers are inherently hydrophobic and can elicit surface charges. Both factors can contribute to reduced bioavailability of MP in exposure media due to homoaggregation and adhesion to the surfaces of exposure vessels and analytical equipment. At present data on the stability and behavior of MP suspensions over time in laboratory exposure studies are lacking. Previous methods used to maintain MP of marginally negative buoyancy in suspension include mixing by water flow or air bubbling, ultrasound, overhead stirring and plankton wheels. Strong water flow and air bubbles are not typically used in exposures with planktonic species as they interfere with feeding behavior. In addition, air bubbling can cause heterogeneous distribution of MP in a suspension and ultrasound can damage the MP and change their properties. In the current study, the efficiency of overhead stirring to maintain spherical polystyrene (PS) MP (Ø10.4 m) in suspension was assessed using flocculators. A coulter counter was used to measure MP suspension concentrations regularly during 24 h. We studied the impacts of exposure vessel material,PS bead start concentration, surface to volume ratio, stirring speed and the dispersant PVP40 (polyvinylpyrrolidone). All parameters tested influenced the number of PS beads in suspension, but none of them ensured a stable detectable PS concentration over time. Data from studies using plankton wheels indicate a similar issue. Overall, our findings indicate that MP increasingly attach to surfaces of exposure systems, sediment and/or surface over time, likely becoming gradually less bioavailable to test organisms. Consequently, we may be underestimating the uptake rates and effects of MP in aquatic biota to a variable and undefined degree.