Optimization of experimental conditions for exposure of larval mussels (Mytilus californianus) to microplastic particles

Inputs of plastic pollution in marine environments continue to increase, making it essential to understand potential impacts on commercially important marine and estuarine species. Bivalve larvae have been shown to take up micro- and nano-sized plastics; however, exposures detailed in the literature often do not account for particle dynamics, such as eco-corona formation, agglomeration, or settlement. There is clearly a need to define and maintain suspended particle concentrations during exposure periods, while using culture systems that support the growth and survival of the test organism. Therefore, experiments were designed to optimize the components of an exposure system for both particle suspension, as well as larval mussel growth and survival. The suspension and clumping of 2 μm ultra-pure latex beads were assessed to determine the influence of flask shape, agitation, and dispersant type. These results were paired with an assessment of the effects of antibiotics, flask shape, flask rotation, and dispersant type on mussel larvae growth and survival. The combination of these experiments revealed that the optimum system for exposure of bivalve larvae to micro-sized plastics was in rotating, dimple-bottom flasks with additions of 2.5 mg.L−1 methyl cellulose and 2 mg.L−1 chloramphenicol in natural, 1 μm filtered seawater. This optimized system was used to assess the impacts of 2 μm ultra-pure latex beads on the survival and growth of Mytilus californianus larvae. First, a dose-response experiment was carried out to assess the impacts of a two-week exposure of larval mussels to microplastics concentrations that were 0%, 20%, 33.3%, 42.9%, and 50% of total cell volume of the algal ration. For these experiments the algal rations were all kept at 100%. Subsequently, a second experiment was carried out to assess impacts of a two-week exposure of larval mussels to microplastics where both the percent by volume of the algal (0–100%) ration and beads (0–100%) varied so that the total particle volume in the exposures remained constant (100% total). When algal food rations were held constant, a dose-dependent decrease in growth was observed with increasing concentrations of beads. When algal rations decreased, growth and survival decreased to a greater extent when microplastic beads were present. These results indicate that both concentrations of microplastic particles and volumetric ratios of algae rations to microplastic particles determine responses of mussel larvae, with microplastics having a greater effect on larval survival when added to low algal rations.