Extracellular polymeric substances (EPS) are a complex mixture of high molecular weight chemicals excreted by microorganisms (bacteria, fungi, virus, etc.), comprising mainly of polysaccharides and proteins. Despite the potentially high importance, the study on the significance of EPS in mediating the fate and toxic effects of heavy metal and semi-metal ions is still very limited. We recently found that several environmentally concerned heavy metal and semi-metal ions (Ag+, Au3+ and AsO43-) can be readily reduced by dissolved aqueous EPS extracted from a variety of microbes. Ag+ and Au3+ are reduced to silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), respectively, while AsO43- is reduced to AsO33-. Collective spectroscopic investigation suggests that the reduction is induced by the hemiacetal groups of reducing saccharides of EPS on bacterial surfaces. Manipulation of EPS in E. coli suspensions (e.g., removal of EPS attached to cells by sonication/centrifugation or addition of EPS) demonstrated its critical role in hindering intracellular silver penetration and enhancing cell growth in the presence of Ag+. Additionally, the reduction of Au3+ and AsO43- was shown to obey the second order reaction kinetics with respect to the concentration of the reduced ion and the equivalent concentration of the hemiacetal groups of EPS, regardless of the microbial source. The results highlight that EPS produced by microbes may play an important and previously unrecognized role in the transformation and the mineralization, as well as the toxic effects of heavy metal and semi-metal species with environmental concern.

Abiotic reduction of Ag+, Au3+ and AsO43- by extracellular polymeric substances (EPS) excreted by microorganisms: Mechanisms and environmental implication

251st National Meeting (2016)

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