Roles of polystyrene micro/nano-plastics as carriers on the toxicity of Pb2+ to Chlamydomonas reinhardtii

Little information could be consulted on the impacts of micro-plastics as carriers on toxicity of heavy metals, especially for micro-plastics of different sizes. Therefore, this study investigated the adsorption and desorption of Pb on polystyrene plastics with nano- and micro-size (NPs and MPs), and further evaluated the roles of NPs and MPs as carriers on the toxicity of Pb to Chlamydomonas reinhardtii (C. reinhardtii). The results showed that NPs showed higher adsorption capacities and a lower desorption rate for Pb than MPs. The growth inhibitory rates (IR) of mixed and loaded Pb with MPs to C. reinhardtii were 18.29% and 15.76%, respectively, which were lower than that of Pb (22.28%). The presence of MPs decreased the bioavailability of Pb to C. reinhardtii by a competitive adsorption for Pb between MPs and algal cells, and suppressed membrane damage and oxidative stress caused by Pb. Maximum IR was observed for the mixture of NPs with Pb (35.64%), followed by Pb loaded on NPs (30.13%), single NPs (26.71%) and Pb (21.01%). The internalization of NPs with absorbed Pb intensified lipid peroxidation. The mixed and loaded microplastics with Pb had more negative effects on C. reinhardtii than the single microplastics. The size-dependent effect was observed in the capacity of heavy metal ions carried by microplastics and the roles of microplastics as carriers on the toxicity of Pb. The results showed that the indirect risk of microplastics as 'carriers' could not be ignored, especially for NPs.