Nature and Value of Freely Dissolved EPS Ecosystem Services: Insight into Molecular Coupling Mechanisms for Regulating Metal Toxicity

Extracellular polymeric substances (EPSs) dispersed in natural waters play a significant role in relieving impacts to microbial survival associated with heavy metal release, yet little is known about the association of freely dissolved EPS ecosystem services with metal transformation in natural waters. Here, we demonstrate that dispersive EPSs mitigate the metal toxicity to microbial cells through an associative coordination reaction. Microtitrimetry coupled with fluorescence spectroscopy ascribes the combination of freely dissolved EPSs from Escherichia coli (E. coli) with Cu2+/Cd2+ to a coordination reaction associated with chemical static quenching. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and computational chemistry confirm that carboxyl residues in protein-like substances of the EPSs are responsible for the coordination. Frontier molecular orbitals (MOs) of a deprotonated carboxyl integrate with the occupied d orbitals of Cu2+ and/or d, s orbitals of Cd2+ to form metal-EPS complexes. Microcosmic systems show that because the metal-EPS complexes decrease cellular absorbability of metals, E. coli survivals increase by 4.3 times for Cu2+ and 1.6 times for Cd2+, respectively. Based on bonding energies for six metals-EPS coordination, an associative toxic effect further confirms that increased bonding energies facilitate retardation of metals in the EPS matrix, protecting against E. coli apoptosis.