Spontaneous assembly of microbial extracellular polymeric substances into microcapsules involved in trapping and immobilizing degradation-resistant oxoanions.

Despite the ubiquity of microbial extracellular polymeric substances (EPS) in soils and aquatic environments, the roles played by EPS in the nonreductive transformation of toxic and degradation-resistant oxoanions are poorly understood. Here, we used perchlorate, which is ubiquitous in surface environments, as an initiator to study the spontaneous assembly of EPS into microcapsules involved in trapping and immobilizing oxoanions. The results confirmed that ClO oxoanions could be rapidly trapped in 20 min by EPS extracted from a common Bacillus subtilis, whereas no chemical reduction of ClO occurred in 48 h. Integrated spectroscopic analyses with florescence quenching microtitration and theoretical models showed that amino functionalities of EPS are responsible for sequestering ClO, with lower pH values being more favorable to formation of EPS-ClO micelles. Combined molecular dynamics scheme with wave function analyses showed that besides amino residues, the protonated side-chain amino groups in the basic proteins have a greater capacity for sequestering ClO through a noncovalent H-bonding mechanism in which dissociable protons serve as the nodes to bridge ClO. A quantitative association between the number of hydrogen bonds and bioavailability revealed that immobilization by EPS mitigates the uptake of toxic oxoanions by forage ryegrass, reducing their risk exposure to edible produce. MAIN FINDING OF THE WORK: Micelles formed by freely dissolved EPS mitigate the uptake of toxic oxoanions by forage ryegrass.