A study on microbial mechanism in response to different nano-plastics concentrations in constructed wetland and its carbon footprints analysis

The increasing accumulation of environmental nano-plastics (NPs) poses significant environmental risk. While constructed wetlands (CWs) have proven effective in treating NPs, and most research efforts primarily emphasize on denitrification capacities. However, there are still knowledge gaps concerning the carbon footprints characteristics within CWs as well as the response mechanisms of microbial communities to perturbations caused by NPs. To evaluate these effects, a 360-day CWs microcosm experiment was conducted with nano-plastic treatments of 0 μg/L, 1 μg/L, 10 μg/L, and 100 μg/L. Results showed that COD removal efficiencies were 28.42 % for the CK, decreasing significantly to 11.92 % at 100 μg/L concentrations. TN removal efficiencies for the 1 μg/L, 10 μg/L, and 100 μg/L treatments showed average reductions of 8.65 %, 11.05 % and 13.47 %, respectively, in comparison to the control, which had a mean TN removal efficiency of 22.82 %. Simultaneously, the concentration of NPs had a direct impact on greenhouse gas (GHG) emissions from CWs, which increased from 5326.60 CO₂-eq/m³·y to 30109.47 g CO₂-eq/m³·y as NP concentration increased, and the contribution of microorganisms to CWs decreased from 23.28 % to 19.56 %. These phenomena could be attributed to the following reasons: as NPs concentration increased, the abundance of genes associated with nitrogen and methanogenesis metabolism (amo and cdh) decreased, resulting in a decline in COD and TN removal rates. Concurrently, the genes abundance related to methane formation (mtr and mcr) increased, leading to higher emissions of methane gas as a result of the consumption of accumulated CH-H₄MPT and CH₂-H₄MPT intracellularly. Through this research, we gain fresh insights into sustained structural and functional adaptations of indigenous microbial communities in response to NPs. These findings serve as a theoretical foundation for future engineering enhancement of CWs geared towards more effective NPs treatment.