| Antibiotics entering into the environments can result in evolution of novel antibiotic resistant genes and bacteria, and then has enormous ramifications for human health. Thus antibiotic resistance problem has attracted global attentions. For humans, chlorinated nitroaromatic antibiotic, chloramphenicol (CAP) can lead to aplastic anemia and have potential genotoxicity and carcinogenicity. Trace CAP and the CAP-resistant genes were frequently detected in diverse environments, thus elimination of the antibacterial activity of CAP is very important during wastewater treatment. The bioelectrochemical system (BES) with biocathode is an emerging technology that could reductively degrade various pollutants. Importantly, microbes worked as the cathode catalyst has great potential as the characteristic of self-renewal and environmental friendliness. The nitro group of CAP is the essential functional group determining its antibiotic properties. In this study BES biocathode was employed for the reductive degradation of CAP to the corresponding dechlorinated aromatic amine product. Corresponding nitro group reduction and dechlorination reactions eliminated the antibacterial activity of CAP. The key microbial community structure and functional genes composition for the accelerating the cathodic electrons transfer associated with the CAP reduction was revealed. It discovers that the low temperature performed biocathode had stronger stress response to environment temperature changes and keeping the stable catalytic activity. These results provides the important basis for the development of efficient degradation biotechnology for the antibiotics wastewater treatment.The CAP reduction speed constant of biocathode was10and1.6times higher than an abiotic cathode and opened biocathode (pure anaerobic microbial reduction) experiment, respectively. Biocathode enhanced reduction of CAP to the corresponding aromatic amine product AMCl2, and the AMCl2was further dechlorinated to AMCl with dehalogenase catalysis. Toxic intermediates, hydroxylamino (HOAM), and nitroso (NO), from CAP reduction were obviously accumulated only in the abiotic cathode, and electrochemical hydrodechlorination was responsible for the dechlorination of AMCl2to AMCl in abiotic BES. Acetylation of one hydroxyl of CAP (CAP-acetyl) was briefly accumulated exclusive in the biocatalyzed process. Cathodic CAP reduction lost the antibacterial activity. The electrochemical analysis indicated the obvious decrease of overpotentials (positive shift of400mV) for the CAP reduction at the biocathode compared with abiotic cathode, suggesting that the cathodophilic microbes maybe improve the cathodic electrons transfer. Some dominant genera from biocathode had the nitroaromatics reduction ability (Salmonella, Enterobacter, Stenotrophomonas, Clavibacter and Pseudomonas) and cathodic electrochemical activity (Enterobacter, Pseudomonas and Dechloromonas), indicating that these functional microbes involved in the enhanced reduction of CAP.The cathode biofilm showed significant higher CAP reduction efficiency than that of the pure anaerobic biofilm. The cathode providing electrons stimulation had the selectivity, which significantly altered the community structure and functional genes composition of cathode biofilm, and obviously lowered the microbial diversity based on the highthroughput functional gene array (GeoChip) and Illumina16S rRNA gene sequencing analysis. The electrochemically active Gram-positive Lactococcus (P=0.004;51.50±24.60%) was significantly enriched in the biocathode community. Lactococcus could self-excrete electrons transfer mediator was consistent with that the cathode biofilm detected significantly lower cytochrome c gene abundances (P=0.023). Importantly, Lactococcus was capable of reducing nitroaromatics to the corresponding aromatic amines. While anaerobic glucose fermentative bacteria Escherichia (P=0.002;10.64±4.49%) and Dysgonomonas (P=0.015;25.25±13.17%) were significantly enriched in the anaerobic biofilm, among them Escherichia was capable of reducing CAP. Some other dominant genera such as Desulfovibrio, Geobacter, Pseudomonas and Klebsiella all were capable of reducing nitroaromatics to corresponding aromatic amines. The significant higher CAP reduction efficiency in the biocathode community, which significantly positively (r=0.78; P=0.003) correlated with the dominant Lactococcus, while significantly negatively correlated with the significantly lower cytochrome c genes (r=-0.5802; P=0.048), suggested that electrons transfer mediator was mainly employed to capture cathodic electrons for accelerated reduction of CAP within biocathode communities.Environment temperature changes greatly in northern cold regions, it is crucial for the enrichment of cathode biofilm that have stress responses ability to stably reduce CAP upon temperature changes. Comparing with the room temperature25°C, low temperature10°C performed biocathode and the corresponding15°C decrement and increment for the performed biocathode BES reactors respectively, the results showed that the10°C performed biocathode had more stress response ability to stably reduce CAP upon temperature changes, and the formation efficiency of reduced product AMCl under room temperature was not significant different from that of the low temperature performed biocathode. Based on the GeoChip and Illumina16S rRNA gene sequencing analysis, the results indicated that the15°C increment significantly altered the microbial phylogenetic community structure and functional genes composition. Before and after of temperature changes, the cathode biofilm was not obviously differed at the phylum level (enriched>60%Proteobacteria and>20%Firmicutes) but at the genus level showed the significant differences. The10°C and25°C performed biocathode enriched1067and2113unique functional genes respectively, thereinto both enriched9unique genes that related to electrons transfer. Further analysis revealed that the key functional genes and dominant genera were employed for the maintaining the biocathode catalytic function. Cold-adapting Aeromonas and Vagococcus dominated in the10°C performed biocathode, while the25°C performed biocathode had higher abundance of nitroaromatics-reducing Raoultella and nitroreductase genes. Genes related to heat shock protein were significantly enriched in the25°C performed biocathode, however the abundances of important electrons transfer genes such as cytochrome c and hydrogenase was not significantly differed. These important functional gene categories and dominant genera in the cathode biofilms involved in the stress response to environment temperature changes, which played the key role for the maintaining the stability of CAP reduction. |
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