Removal of PAHs in paddy fields: How natural periphytic biofilm enhances the removal process and the mechanisms involved

Previous studies on the removal of polycyclic aromatic hydrocarbons (PAHs) from farmland have mostly relied on anthropogenic biotic or abiotic amendments. Here, an indigenous paddy periphytic biofilm (PB) composed of complex symbolic communities was enriched, and its essential role, as well as the potential mechanisms of PAHs dissipation, was investigated. A pot experiment revealed that PB achieved 92.87% and 92.76% removal of the USEPA priority PAHs from the overlying water and soil of a heavily PAHs-contaminated paddy field, respectively. This subsequently decreased sigma PAHs accumulation in rice shoots to 4.12 +/- 1.08 ng g-1, under the thresholds based on food safety standards. The physiology of PB in an aquatic culture with PAHs was assessed to understand the toxicity and removal mechanism. Extracellular polymeric substances (EPS) adsorption occurred in the early stage, followed by late biodegradation, which was considered the primary mechanism for removing PAHs. In this process, the PB contributed more to the high molecular weight PAHs compared to the low molecular weight. Furthermore, 16SrRNA sequencing demonstrated specific functional phyla (Proteobacteria, Ascomycota, and Chloroplastida) with significantly higher relative abundances in periphytic communities with PAHs, which facilitated PAHs degradation. The predicted synergetic metabolic pathway involved 33 PAHs-degrading genes as inferred by PICRUSt. These findings suggest that PB could provide a promising strategy for target PAHs remediation in situ and food safety in contaminated paddy fields.