Sludge-derived hydrothermal liquid accelerates Cr(VI) bioreduction by triggering an mtrC-to-mtrF electron transfer...

The practical application of Shewanella oneidensis MR-1 in heavy metal bioremediation is greatly restricted by its low extracellular electron transfer (EET) capacity. To address this bottleneck, we developed a high-efficiency "treating waste with waste" Cr(VI) bioreduction system using sludge-derived hydrothermal liquid (HTL) as a natural electron shuttle. Our results showed that HTL prepared at 210 °C (HTL₂₁₀) increased the Cr(VI) bioreduction rate constant (k_obs) by 20.33-fold, achieving complete reduction of 20 mg/L Cr(VI) within 48 h. Furthermore, a quantitative prediction model linking k_obs to HTL spectral indices (SUVA, HIX) and electron transfer capacity (ETC) was established for non-destructive prediction of the system's reduction performance. Multi-scale analyses elucidated the underlying mechanisms of this enhancement. Extracellularly, HTL₂₁₀ accelerates EET and induces massive extracellular polymeric substance (EPS) secretion, driving rapid Cr(VI) reduction and in-situ immobilization. Intracellularly, it alleviated Cr(VI) toxicity by scavenging reactive oxygen species (ROS) and drives bioenergetic reprogramming to boost ATP/NADH supplies. Crucially, combined transcriptomic profiling and RT-qPCR validation indicated that HTL₂₁₀ induces adaptive transcriptional reprogramming of the EET pathway, suggesting a potential shift of electron flow from the energy-intensive mtrC pathway to a quinone-adapted mtrF network. This study reveals the intrinsic molecular mechanisms of HTL-enhanced EET, providing a novel paradigm for sludge HTL valorization and low-cost heavy metal-contaminated water remediation.