Transcriptome Analysis Reveals the Inhibitory Effect of Cu²⁺ on Polyferric Sulfate Floc Reduction by Shewanella...

Polyferric sulfate (PFS), an economical coagulant widely used for removing heavy metal contaminants from water, is susceptible to reduction and transformation by iron-reducing bacteria prevalent in sediments. However, the effect of heavy metal ions adsorbed in PFS flocs on this biological process remains unclear. According to our results, compared with other heavy metal cations (e.g., Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺, Pb²⁺, and Co²⁺), Cu²⁺ had a stronger inhibitory effect on PFS floc reduction by Shewanella putrefaciens CN32, a typical dissimilatory iron-reducing bacterium. The presence of Cu²⁺ remarkably influenced the global transcription of CN32, resulting in 782 upregulated genes and 713 downregulated genes that are mainly annotated in energy production, amino acid metabolism, protein biosynthesis, and oxidation‒reduction processes. The anaerobic TCA cycle for energy (electron) production was significantly activated in the presence of Cu²⁺, while the transcription of many genes related to the extracellular electron transfer pathway was downregulated, which is responsible for the decreased Fe³⁺ reduction. Moreover, the pathways of assimilatory sulfate reduction and subsequent cysteine biosynthesis were significantly enriched, which is hypothesized to result in the consumption of abundant energy produced from the enhanced anaerobic TCA cycle, revealing a strategy to address the oxidative stress caused by Cu²⁺. This work elucidates the unusual suppressive effects of Cu²⁺ on the microbial reduction of PFS flocs, which reveals the high resistance of PFS flocs to microbial destruction when used to treat Cu²⁺ pollution in water, thus demonstrating their tremendous practical prospects.