Bioelectrochemical and transcriptional mechanisms underlying Cl-/SO42--induced microbial synergistic corrosion of X65 carbon steel.

This study simulated four ionic environments and combined electrochemistry, surface characterizations and metatranscriptomics to explore the individual and coupled effects of Cl- and SO42- on microbiologically influenced corrosion (MIC) of X65 carbon steel. The four environments were: control group (W1), high-chloride (W2), high-sulphate (W3), and composite ionic (W4). After 21 days of immersion, corrosion rates followed W4 > W2 > W3 > W1. Cl- destroyed the passive film and caused severe pitting corrosion, while SO42- enhanced SRB metabolism and promoted a 50 μm-thick dense biofilm. Under combined Cl-/SO42- stress, chemical corrosion and MIC acted synergistically, forming loose porous products with a maximum pitting depth of 8.77 μm. Metatranscriptomics demonstrated that composite ions reshaped microbial gene networks and upregulated genes for sulfur metabolism, energy production, transmembrane transport and biofilm synthesis. Key ribosome and stress-response genes (cspA, rmf) showed the highest expression in W4. SRB and halotolerant bacteria were co-enriched to drive interfacial corrosion. These findings reveal that ion-regulated microbial metabolic transcription enhances extracellular electron transfer (EET) and interfacial electrochemical activity, which constitutes the core bioelectrochemical mechanism for synergistic corrosion. This study reveals the bioelectrochemical and transcriptional mechanism by which ions regulate microbial metabolism to accelerate synergistic corrosion.