Mechanistic insights into the response of oxygen reducing biocathode to Ni^2 +: Microbial activity and electron transfer behavior

Heavy metals affect the electrochemical activity of microorganisms in bioelectrochemical systems. However, effects of heavy metals on microbial viability and extracellular electron transfer of oxygen reducing biocathodes remains elusive. This study indicated that the impact of Ni²⁺ shock was concentration-dependent. At 1 mg L^–1, current density and electrochemical performance improved, with the highest live cell proportion (73.2 %). Conversely, higher concentrations (10 and 100 mg L^–1) showed negative effects, with increasing cell death and biocathode damage. Functional groups (i.e. carbonyl C[dbnd]O, N-H, C-H, and C-O-C), and proteins, humic acid of LB-EPS were confirmed to bind with Ni²⁺, preventing the penetration into cells. Increased Ni²⁺ levels intensified interactions with biofilm functional groups, reducing direct electron transfer between oxygen reducing biofilm and cathode. Ni²⁺ shock altered the structure and microbial richness of the cathode biofilms, decreasing electron transfer gene abundance (e.g., nikA, sodN) and increasing heavy metal resistance genes (e.g., czcB, TC.HME) at higher Ni²⁺ concentrations. Molecular docking showed Ni²⁺ interacted variably with electron transfer enzymes, strongly binding to succinate dehydrogenase and cytochrome bc1 complex, affecting enzyme activity and metabolism. This research enhanced understanding of biofilm response to Ni²⁺ shock and provided insights for improving performance in heavy metal wastewater.