Mechanism of polycyclic aromatic hydrocarbons degradation in the rhizosphere of Phragmites australis: Organic acid co-metabolism, iron-driven, and microbial response.

Microbial co-metabolism is crucial for the efficient biodegradation of polycyclic aromatic hydrocarbons (PAHs); however, their intrinsic mechanisms remain unclear. To explore the co-metabolic degradation of PAHs, root organic acids (ROAs) (phenolic ROAs: caffeic acid [CA] and ferulic acid [FA]; non-phenolic ROAs: oxalic acid [OA]) were exogenously added as co-metabolic substrates under high (HFe) and low (LFe) iron levels in this study. The results demonstrated that more than 90% of PAHs were eliminated from the rhizosphere of Phragmites australis. OA can promote the enrichment of unrelated degrading bacteria and non-specific dioxygenases. FA with a monohydroxy structure can activate hydroxylase; however, it relies on phytosiderophores released by plants (such as OA) to adapt to stress. Therefore, non-specific co-metabolism occurred in these units. The best performance for PAH removal was observed in the HFe-CA unit because: (a) HFe concentrations enriched the Fe-reducing and denitrifying bacteria and promoted the rate-limiting degradation for PAHs as the enzyme cofactor; (b) CA with a dihydroxyl structure enriched the related degrading bacteria, stimulated specific dioxygenase, and activated Fe to concentrate around the rhizosphere simultaneously to perform the specific co-metabolism. Understanding the co-metabolic degradation of PAHs will help improve the efficacy of rhizosphere-mediated remediation.

Removal pathway quantification and co-metabolic mechanism evaluation of alkylphenols from synthetic wastewater by phenolic root exudates in the rhizosphere of Phragmites australis.

Phenolic root exudates (PREs) released from wetland plants are potentially effective for accelerating the biodegradation of alkylphenols, yet the inherent behavior is still unclear. In this study, two representative root exudates (REs), namely p-coumaric acid (PREs) and oxalic acid (non-PREs) were exogenously added as specific and non-specific co-metabolic substrates, respectively, to elucidate the quantification of each removal pathway and degradation mechanism of co-metabolism for alkylphenols (i.e. p-tert-butylphenol (PTBP)) from synthetic wastewater. The results showed that soil adsorption (31-37%), microbial degradation (27-37%), and plant uptake (16-41%) are the main removal pathways of PTBP by PREs in the Phragmites australis rhizosphere. Both REs enriched anaerobic functional community (anaerobic ammonium oxidation bacteria and denitrifying bacteria) and promoted the usage of PTBP as carbon source and/or electron donor. The activity of non-specific enzyme (polyphenol oxidase) was enhanced by RE which owning a significant positive correlation with bacterial abundance, whereas only PREs strengthened the activity of specific enzyme (monophenol oxidase) catalyzing the phenolic ring hydroxylation of PTBP followed by a dehydrogenation route. Moreover, exogenous PREs significantly improved the growth of degrading-related bacteria (Sphingomonas and Gemmatimonas), especially in unplanted soils with high activity of dioxygenase catalyzing the cleavage pathway of PTBP, instead of plant presence.