Contribution of pristine and reduced microbial extracellular polymeric substances of different sources to Cu(II) reduction

Microbial extracellular polymeric substances (EPS) significantly influence metal behavior in the environment, but the electron transfer reaction between EPS and copper that determine the speciation and fate of copper is lacking. Here, we investigated the role of EPS from Shewanella oneidensis MR-1, Bacillus subtilis, and Saccharomyces cerevisiae and its redox state in the Cu(II) reduction under anoxic conditions. Both pristine and reduced EPS mediated copper transformation from Cu(II) to Cu(I) within 10 min. The Cu(II) reduction efficiency by the reduced EPS was ten times higher than that by the pristine EPS, which could be ascribed to the varied electron transfer ability of EPS. Multiple spectroscopic results indicated that c-type cytochromes and O-/N-containing groups were effective redox moieties responsible for copper transformation. The c-type cytochromes contributed for about 80% to the overall electron flux in S. oneidensis MR-1 EPS, which was significantly higher than in B. subtilis (27%) and S. cerevisiae EPS (22%). In contrast, functional groups such as phenolic and amide, dominated Cu(II) reduction for the B. subtilis and S. cerevisiae EPS. This study emphasizes the significant contribution of microbial EPS that serve as reducing agents and electron transfer mediators for cupric reduction and cuprous formation in the natural environments.