Evaluation of Individual and Combined Effects of Microplastics and Naphthalene on Aquatic Sediment: Disturbance of Carbon and Microbial Dynamics

Microplastics (MPs) can absorb contaminants, enhancing their distribution and toxicity in the environment. While few data are available on the co-effect of MPs and polycyclic aromatic hydrocarbons (PAHs) on aquatic sediment. In this study, the effects of different contents (0.1%, 1%, and 5%) of polylactic acid MPs (PLA MPs) on the distribution and comprehensive impact of a typical polycyclic aromatic hydrocarbon, naphthalene (NPT), were investigated analyzing the characteristics of DOM, enzymatic responses of soil, antioxidant properties of plants, and changes of microbes. Results showed that the combined pollution of 5% MPs and NPT reduced the aromaticity of DOM and significantly enhanced the humification process. Compared with single pollution, plant oxidative stress products were increased by 40.7-65.0%, and antioxidant enzymes were increased by 73.3-95.8%. The soil catalase that characterizing microbial activity was 33.9-44.1% lower than that of single pollution. According to IBR_V2-EAI model, the increase of MPs concentration enhanced the combined biological toxicity to plants and aquatic sediments. Combined pollution showed a toxic inhibitory effect on the microbial abundance of NPT degrading pollutants (Ideonella, Thauera and Clostridium, etc.), and functional genes that involved in glycolysis and TCA cycle (K00134, K00162, K00627, etc.) and NPT metabolic process (K00152, K14585, K14582, etc.). This study provides theoretical support for the joint risk of MPs and NPT in aquatic sediment environment.