Comparing biotransformation of extracellular polymeric substances (EPS) under aerobic and anoxic conditions: Reactivities, components, and bacterial responses

This study aimed to better understand the transformation behaviors of extracellular polymeric substances (EPS) and their roles in regulating bacterial community in biological wastewater treatment processes. Herein, well-controlled bioassays under aerobic and anoxic conditions were performed to investigate degradation dynamics, composition variations, and bacterial response during EPS transformation. Reactivity continuum modeling showed that organic pools of EPS had continuous reactivity distributions, and most labile organic fraction with a degrading rate >0.1 h-1 was substantially higher under aerobic (20.47%) than anoxic (2.02%) condition. Rapid degradation of protein-like substances in the initial degradation stage was accompanied by the humification process, as revealed by UV absorption spectroscopy, fluorescence spectroscopy, and size exclusion chromatography with continuous organic carbon detection analysis. The 16S rRNA gene sequencing results showed that the selection effect of EPS in controlling abundant populations during their transformation, e.g., Acinetobacter was enriched, and Candidatus Competibacter was washed out relative to the source community. Furthermore, taxonomic normalized stochasticity ratio-based null model and bacterial ecological network analysis indicated higher relative importance of deterministic process in shaping the EPS-degrading communities under aerobic than anoxic condition, likely explaining the faster EPS biotransformation under aerobic condition. Intriguingly, the keystone populations driving EPS metabolism showed the environmental filtering characteristics (e.g., capable of degrading refractory and aromatic compounds or adapting to harsh environments) and cooperative interactions with the co-occurring species under both conditions. This work is expected to reveal the fates and roles of EPS in wastewater treatment plants extensively.