Changes in bacterial diversity and catabolic gene abundance during the removal of dimethylphenol isomers in laboratory-scale constructed wetlands.

Constructed wetlands (CWs) are well-established wastewater treatment technologies and applied for bioremediation of contaminated water. Despite the optimal performance of CWs, the understanding of the bacterial processes in the rhizosphere, where mainly microbial degradation processes take place, is still limited. In the present study, laboratory-scale CWs planted with Juncus effusus and running under controlled conditions were studied in order to evaluate removal efficiency of dimethylphenols (DMPs), also in comparison to an unplanted bed. Next to removal rates, the bacterial community structure, diversity, and distribution, their correlation with physiochemical parameters, and abundance of the phenol hydroxylase gene were determined. As a result, better removal performance of DMP isomers (3,4-, 3,5-, and 2,6-DMP added as singles compounds or in mixtures) and ammonium loads, together with a higher diversity index, bacterial number, and phenol hydroxylase gene abundance in Juncus effusus CW in comparison with the non-planted CW, indicates a clear rhizosphere effect in the experimental CWs. An enhancement in the DMP removal and the recovery of the phenol hydroxylase gene were found during the fed with the DMP mixture. In addition, the shift of bacterial community in CWs was found to be DMP isomer dependent. Positive correlations were found between the bacteria harboring the phenol hydroxylase gene and communities present with 3,4-DMP and 3,5-DMP isomers, but not with the community developed with 2,6-DMP. These results indicate that CWs are highly dynamic ecosystems with rapid changes in bacterial communities harboring functional catabolic genes.

Delftia sp. LCW, a strain isolated from a constructed wetland shows novel properties for dimethylphenol isomers degradation.

BACKGROUND: Dimethylphenols (DMP) are toxic compounds with high environmental mobility in water and one of the main constituents of effluents from petro- and carbochemical industry. Over the last few decades, the use of constructed wetlands (CW) has been extended from domestic to industrial wastewater treatments, including petro-carbochemical effluents. In these systems, the main role during the transformation and mineralization of organic pollutants is played by microorganisms. Therefore, understanding the bacterial degradation processes of isolated strains from CWs is an important approach to further improvements of biodegradation processes in these treatment systems. RESULTS: In this study, bacterial isolation from a pilot scale constructed wetland fed with phenols led to the identification of Delftia sp. LCW as a DMP degrading strain. The strain was able to use the o-xylenols 3,4-DMP and 2,3-DMP as sole carbon and energy sources. In addition, 3,4-DMP provided as a co-substrate had an effect on the transformation of other four DMP isomers. Based on the detection of the genes, proteins, and the inferred phylogenetic relationships of the detected genes with other reported functional proteins, we found that the phenol hydroxylase of Delftia sp. LCW is induced by 3,4-DMP and it is responsible for the first oxidation of the aromatic ring of 3,4-, 2,3-, 2,4-, 2,5- and 3,5-DMP. The enzyme may also catalyze both monooxygenation reactions during the degradation of benzene. Proteome data led to the identification of catechol meta cleavage pathway enzymes during the growth on ortho DMP, and validated that cleavage of the aromatic rings of 2,5- and 3,5-DMPs does not result in mineralization. In addition, the tolerance of the strain to high concentrations of DMP, especially to 3,4-DMP was higher than that of other reported microorganisms from activated sludge treating phenols. CONCLUSIONS: LCW strain was able to degraded complex aromatics compounds. DMPs and benzene are reported for the first time to be degraded by a member of Delftia genus. In addition, LCW degraded DMPs with a first oxidation of the aromatic rings by a phenol hydroxylase, followed by a further meta cleavage pathway. The higher resistance to DMP toxicity, the ability to degrade and transform DMP isomers and the origin as a rhizosphere bacterium from wastewater systems, make LCW a suitable candidate to be used in bioremediation of complex DMP mixtures in CWs systems.