Beyond metal removal: Microbial strategies for copper and zinc detoxification and implications for aquatic fish health

Copper (Cu) and zinc (Zn) contamination remains a critical threat to aquatic biodiversity, inducing systemic bioaccumulation and severe histopathological damage in teleost fish. While conventional chemical precipitation and ion-exchange technologies are established industrial standards for mass removal, their efficacy in mitigating sub-lethal biological stress in complex aquatic environments is less characterized. This integrative review provides a synthesis of the comparative roles of Escherichia coli and Aeromonas hydrophila in Cu and Zn bioremediation, shifting the focus from total chemical removal to the preservation of physiological integrity in sentinel organisms. We analyze the divergent mechanistic strategies employed by these species: E. coli provides rapid kinetic sequestration via high-affinity surface functional groups (carboxyl and phosphoryl ligands), which effectively reduces free ion activity and protects sensitive branchial tissues from acute epithelial lifting and lamellar fusion. Conversely, A. hydrophila facilitates long-term immobilization through extracellular polymeric substance (EPS)-mediated biomineralization, transforming bioavailable ions into inert mineral phases that reduce chronic hepatic necrosis. By integrating microbial efflux systems (CopA, CusCFBA, CzcCBA) and enzymatic detoxification pathways with fish health outcomes, this review establishes a decision-oriented framework for aquatic remediation. We further address the bio-validation gap and pathobiont paradox, advocating for multi-organ histopathological scoring as a standardized metric to validate the functional, rather than just chemical, success of bioremediation strategies.