Molecular composition determines the adsorption behaviors of loosely- and tightly-bound extracellular polymeric substances (EPS) from Shewanella oneidensis MR-1 on hematite nanoparticles

Abstract

Extracellular polymeric substances (EPS) on the surface of dissimilatory iron-reducing bacteria (DIRB) play a crucial role in bacterial colonization and iron bioreduction. Surface EPS has a double-layer structure, consisting of loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS). Although different molecular compositions of TB-EPS and LB-EPS have been reported, how they interact with iron oxide minerals remains elusive. This study investigated the adsorptions of LB-EPS and TB-EPS from a model DIRB, Shewanella oneidensis MR-1, on hematite nanoparticles (NPs). The adsorption dynamics were monitored via a quartz crystal microbalance with dissipation (QCM-D). LB-EPS formed a soft and thick layer on the hematite surface via a rapid initial deposition and gradually transitioned into a rigid and compact structure, while TB-EPS exhibited less adsorption which initiated with a compact structure. EDLVO analysis suggested that the rapid adsorption rate of LB-EPS was attributed to long-range electrostatic attraction. The adsorption mechanisms were further revealed via two-dimensional Fourier-transform infrared correlation spectroscopy (2D-FTIR-COS). Carbon moieties of polysaccharides in LB-EPS dominated the initial interfacial interactions and phosphate groups and protein components stabilized the EPS layer in subsequent longer-term adsorption. Proteins in TB-EPS contributed significantly to the initial adsorption and resulted in a compact structure. Our findings reveal the mechanisms underlying the different adsorption behaviors of LB-EPS and TB-EPS on hematite and provide new insights into their distinct roles in DIRB–iron oxide interaction.