Stress responses of sulfate-reducing bacteria sludge upon exposure to polyethylene microplastics

The stress responses of sulfate-reducing bacteria (SRB) sludge to polyethylene (PE) microplastic exposure were revealed for the first time. In this study, a lab-scale sulfate-reducing up-flow sludge bed reactor was continuously operated with different concentrations of PE microplastics in the feed (20, 100, and 500 microplastic particles (MPs)/L). Exposure to low levels of PE microplastics (i.e., 20 MPs/L) had a limited effect on SRB consortia, whereas higher levels of PE microplastics imposed apparent physiological stresses on SRB consortia. Despite this, the overall reactor performance, i.e., chemical oxygen demand removal and sulfate conversion, was less affected by prolonged exposure to PE microplastics. Moreover, as the concentration of PE microplastics increased, the SRB consortia promoted the production of extracellular polymeric substances to a greater extent, especially the secretion of proteins. As a result, protective effects against the cytotoxicity of PE microplastics were provided. Batch experiments further demonstrated that leaching additives from PE microplastics (including acetyl tri-n‑butyl citrate and bisphenol A, concentrations up to 5 μg/g sludge) exerted only a minor effect on the activity of SRB consortia. Additionally, microbial community analysis revealed active and potentially efficient sulfate reducers at different operational stages. Our results provide insight into the stress responses of SRB sludge under PE microplastic exposure and suggested that SRB consortia can gradually adapt to and resist high levels of PE microplastics. These findings may promote a better understanding of the stable operation of SRB sludge systems under specific environmental stimuli for practical applications.