Interaction of Silica Nanoparticles with Microalgal Extracellular Polymers

The properties of engineered nanoparticles (NPs) in the marine environment are influenced not only by the high ionic strength of seawater but also by the interaction of NPs with naturally occurring components of seawater, especially natural organic matter. The aim of this study was to investigate the interaction of engineered silica nanoparticles (SiO₂ NPs, diameter of 12 nm) with microalgal extracellular polymers (EPS) released by the marine diatom Cylindrotheca closterium. Dissolved organic carbon (DOC) content of the prepared EPS suspension (200 μg mL⁻¹) used throughout the study was 3.44 mg C L⁻¹. The incorporation of individual SiO₂ NPs (height range 10–15 nm) and their nanoscale aggregates (height up to 25 nm, length up to 600 nm) into the EPS network was visualized by atomic force microscopy (AFM), whereas their molecular-level interaction was unraveled by the change in the signal of the Si-O group in their FTIR spectra. AFM imaging of C. closterium cells taken directly from the culture spiked with SiO₂ NPs (10 μg mL⁻¹) revealed that the latter are bound to the EPS released around the cells, predominantly as single NPs (height range 10–15 nm). Since AFM and dynamic and electrophoretic light scattering results demonstrated that SiO₂ NPs dispersed in seawater without EPS showed enhanced aggregation (aggregate diameter of 990 ± 170 nm) and a 2.7-fold lower absolute zeta potential value compared to that measured in ultrapure water, our findings suggest that the presence of EPS biopolymers alters the aggregation affinity of SiO₂ NPs in the marine environment. This might be of outmost importance during microalgal blooms when increased EPS production is expected because EPS, by scavenging and stabilizing SiO₂ NPs, could prolong the presence of NPs in the water column and pose a threat to marine biota.