Microbial extracellular polymeric substances as biogeobatteries: Mechanisms and geochemical controls of...

Accumulating evidence demonstrates that microbial extracellular polymeric substances (EPS) are a complex mixture of macromolecules at the soil-water-microbe interface, playing an essential role in determining the environmental fate of redox-sensitive elements (RSEs) such as arsenic, chromium, uranium, antimony, and selenium. Microbial EPS include redox-active components like reducing saccharides, redox-active proteins, quinone-like moieties, flavins, and tryptophan that function as "biogeobatteries" (i.e., biological systems capable of storing and transferring electrons), enhancing extracellular electron transfer in both aquatic and terrestrial environments. This review critically synthesizes recent advances in understanding how EPS mediate direct, indirect and mineral-associated electron transfer pathways for the reduction of RSEs in microbial extracellular environments. We present a comparative analysis of the detailed mechanisms across different elements and explore how interactions between EPS and minerals influence the oxidation state and mobility of RSEs in subsurface settings. Particular attention is given to the role of EPS composition, redox state, and functional groups in either promoting the immobilization or facilitating the release of specific RSEs. Additionally, the dual role of EPS in reducing RSEs and altering iron mineral phases is discussed, highlighting the implications for remediation and the potential risks of contaminant mobilization. Finally, this review outlines key mechanistic differences in RSEs reduction among intact bacterial cells, purified EPS, and soil-based EPS systems, while identifying critical gaps in knowledge and proposing future research directions to improve EPS-based strategies for immobilizing RSEs in aquatic and terrestrial environments.