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.

Biogeochemical dynamics of nutrients and rare earth elements (REEs) during natural succession from biocrusts to pioneer plants in REE mine tailings in southern China.

The exploitation of ion-adsorption rare earth element (REE) deposits has resulted in large quantities of abandoned mine tailings, which pose significant risks to the surrounding environment. However, the natural evolutional patterns at early successional stages and related biogeochemical dynamics (e.g. nutrient and REE cycling) on such mine tailings remains poorly understood. To this end, a chronosequence of REE mine tailings abandoned for up to 15 years was investigated in a post-mining site in south China. Our results showed that biocrusts were the earliest colonizers on these tailings, reaching a peak of 10% of surface coverage after 10 years of abandonment. Later on, after 15 years, the biocrusts began to be replaced by pioneer plants (e.g. Miscanthus sinensis), suggesting a rather rapid succession. This ecological succession was accompanied by obvious changes in soil nutrients and microbial community structure. Compared to bulk soils, both the biocrusts and rhizospheric soils favored an accumulation of nutrients (e.g. P, S, N, C). Notably, the autotrophic bacteria (e.g. Chloroflexi and Cyanobacteria) with C and N fixation abilities were preferentially enriched in biocrusts, while heterotrophic plant growth promoting bacteria (e.g. Pseudoocardiaceae and Acidobacteriales) were mainly present in the rhizosphere. Moreover, the biocrusts showed a remarkably high concentration of REEs (up to 1820 mg kg-1), while the rhizospheric soils tended to decrease REE concentrations (~400 mg kg-1) in comparison with bulk soils, indicating that the REEs could be redistributed by biological processes. Principal component analysis and mantel tests showed that the concentrations of nutrients and REEs were the most important factors affecting the microbial communities in biocrusts, rhizospheric and bulk soils. In sum, based on the observation of nutrient accumulation and pollutant (i.e. REE) dynamics in the initial successional stages, this work provides a feasible theoretical basis for future restoration practices on REE mine tailings.

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.

Controls on rare-earth element transport in a river impacted by ion-adsorption rare-earth mining.

Rare-earth elements (REEs) are known to be a group of emerging pollutants, but the geochemistry of REEs in river waters in ion-adsorption rare-earth mining areas has attracted little attention. In this study, samples of the <0.45 µm and 0.22-0.45 µm (large colloids) water fractions and acid-soluble particles (ASPs) were collected from a river impacted by ion-adsorption rare-earth mining activities. The roles of ligand complexation, colloid binding, and particle adsorption in REE transport and distribution were also investigated. Results showed higher concentrations of REEs in the <0.45 µm fraction of all sampling sites (3.30 × 10-2-9.42 µM) compared with that in the control site (1.21 × 10-3 µM); this fraction was also characterized by middle REE enrichment at upstream sites, where REEs are mainly controlled by the <0.22 µm fraction (55%-94% of the species found in the <0.45 µm fraction) and ligand complexation (REE3+, REE(SO4)+, and REE(CO3)+). At downstream sites, heavy REE enrichment was observed, which was largely determined by binding to large colloids (68%-83% of the species found in the <0.45 µm fraction) and adsorption to particles (>90% of the acidified bulk water). Furthermore, REE patterns indicated that the REE-associated large colloids were mineral or mixed mineral-organic matter (OM) at upstream sites and OM-dominated or functionalized at downstream sites. The particles were mainly coated by inorganic matter substances (e.g., Fe/Al oxyhydroxides). In summary, our results reveal that REE patterns provide a useful tool to study the fate of REEs in ion-adsorption rare-earth mining catchments.

[Tolerance and enzyme response of ectomycorrhizal fungi Xerocomus chrysenteron to DDT stress].

The growth effect, tolerance and oxidative enzymes activities of Xerocomus chrysenteron under different concentrations of DDT were studied at the condition of pure culture. The changes of biomass accumulation and laccase activity were also examined along with the liquid medium period under the DDT concentration of 80.0 mg L(-1). The results show that various DDT concentrations don't change the growth mode of the studied ectomycorrhizal fungi, which are all in the mode of classic Logistic growth. Xerocomus chrysenteron has a good tolerant ability to the DDT stress, whose hemi-inhibit concentration reaches 139.75 mg L(-1). Under the liquid medium of 80.0 mg L(-1) of DDT, Xerocomus chrysenteron grows normally and after 36 days and the residue of DDT in the liquid medium is only 3.5% of the original concentration. Under the high concentration of DDT, the laccase and peroxidase activities significantly increase. The laccase is detected since the 16th day. After 36 days' culture, the laccase activity and specific activity in liquid medium reach 107.24 U L(-1) and 61.77 U g(-1) respectively. The ectomycorrhizal fungi Xerocomus chrysenteron responses to the DDT stress in various ways, which suggests large potential of biodegradation or mineralization of DDT.