Heterogeneous microalgal extracellular polymeric substances-phenanthrene interaction mechanisms modulated by...

Microbial extracellular polymeric substances (EPS) act as critical receptors for environmental pollutants at the water-cell interface, providing binding sites, especially for tryptophan (Trp). A time variation in Trp secretion is expected during the proliferation of microalgal cells. However, how the change in Trp level regulates the micro-interfacial interactions between algal cells and pollutants remains poorly understood. We investigate the binding of a polycyclic aromatic compound (PAC) phenanthrene to EPS of Chlorella pyrenoidosa over time using a combination of in vivo and in silico analysis. We find phenanthrene primarily binds to the algal EPS at the amine portion of the amide and carboxyl group through van der Waals forces and electrostatic interactions on the second day. By the sixth day, the increased secretion of Trp-rich EPS by C. pyrenoidosa enhances its aromaticity and hydrophobicity, shifting the dominant binding mechanism to π-π stacking interactions and hydrophobic forces, with the benzene ring directly participating. Meanwhile, the presence of Trp loosens the EPS structure, promoting connections between the hydroxyl group of EPS polysaccharides and phenanthrene's electron-rich centers, indirectly facilitating hydrogen bond formation. Our study demonstrates the regulatory role of EPS substructural component Trp in the biosorption of phenanthrene by microalgae, and provides the time evolution of non-covalent interactions dictating the binding of algal EPS with phenanthrene, extending from macroscopic levels to nanoscopic levels. The findings advance the knowledge of the algal Trp mediated-behavior and bioeffects of PACs in aquatic environments. It also contributes to the theoretical foundation for refining biosorption treatment technologies for wastewater containing PACs.