A comparison between the effects of polylactic acid and polystyrene microplastics on Daphnia magna

The effects of microplastics on zooplankton are an increasing environmental concern. Both primary microplastics that are produced as constituents of cosmetic products, as well as secondary microplastics that are formed by degradation of larger plastic debris, are ubiquitous in aquatic environments. Today, primary microplastics are being phased out and replaced by plant-derived bioplastics. Whether these new materials have similar effects as oil-based microplastics on animals is currently unknown. Here, we compare the effects of secondary microplastic exposure to Daphnia magna, using polylactic acid (PLA) as a representative for bioplastics and polystyrene (PS) for oil-based plastics. To increase the ecological relevance of our tests, we also provided treatments where the particles were coated with bovine serum albumin (BSA) as a means to simulate the coating of biofilms which readily form on particles under natural conditions. Furthermore, to be able to differentiate the effects of general particles from those specific to microplastics, kaolin clay was used as a control treatments, as well as one treatment containing only algae. The objectives were to test the influence of particles on feeding rates, reproduction and growth. PS caused a higher mortality, decreased feeding rate and reproductive output, while PLA and kaolin did not produce any negative effects. BSA did not have a significant effect on reproduction or growth. However, a decrease in reproduction was observed in the plastic treatments. Degradation of PS into styrene monomers is suggested as a possible explanation for the observed toxicity and effects on life history parameters.