Sulfate-Reducing Bacteria Buffer Aged Microplastic Leachate Stress during Sludge Acidogenesis
Aged polyethylene microplastic (PE-MP) leachates can impair sludge carbon recovery by inducing cellular toxicity and disrupting microbial carbon conversion. Here, we demonstrate that sulfate-reducing bacteria (SRB) bioaugmentation can provide biological detoxification against aged PE-MP leachate during short-chain fatty acid (SCFA) production from waste activated sludge. Increasing leachate exposure progressively intensified oxidative stress, impaired cellular integrity, restricted substrate solubilization and hydrolysis, and suppressed acidogenesis, resulting in a 71.9% decrease in peak SCFA production at 100 mL leachate. SRB markedly alleviated these toxic effects within a defined exposure window. At 25 mL leachate, SRB increased peak SCFA production by 25.4% and shifted the IC50 from 38.5 to 44.2 mL, indicating an increased tolerance threshold. This detoxification response was accompanied by 4.7-14.9% lower reactive oxygen species levels, 8.2-18.1% higher ATP availability, and partial recovery of hydrolytic activity and substrate-to-SCFA conversion. Meanwhile, aqueous SO42--S decreased by 50.9-52.4% in SRB-amended systems, linking enhanced sulfur transformation with cellular energy maintenance and mitigation of oxidative stress. Metagenomic and taxon-to-function analyses revealed that SRB did not replace indigenous carbon-converting microorganisms; instead, sulfur metabolism supported a protective division of labor that enabled hydrolytic, fermentative, and stress-adapted taxa to retain key functions in substrate depolymerization and carbon conversion. However, detoxification weakened at 50-100 mL leachate because persistent oxidative stress, incomplete energy recovery, and an unfavorable balance between SCFA formation and consumption constrained process recovery. These findings establish SRB-mediated sulfur metabolism as a biological detoxification strategy for enhancing the resilience of sludge carbon valorization to aged microplastic leachates.