Key genes of electron transfer, the nitrogen cycle and tetracycline removal in bioelectrochemical systems

Abstract

Background Soil microbial fuel cells (MFCs) can remove antibiotics and antibiotic resistance genes (ARGs) simultaneously, but their removal mechanism is unclear. In this study, metagenomic analysis was employed to reveal the functional genes involved in degradation, electron transfer and the nitrogen cycle in the soil MFC. Results The results showed that the soil MFC effectively removed tetracycline in the overlapping area of the cathode and anode, which was 64% higher than that of the control. The ARGs abundance increased by 14% after tetracycline was added (54% of the amplified ARGs belonged to efflux pump genes), while the abundance decreased by 17% in the soil MFC. Five potential degraders of tetracycline were identified, especially the species Phenylobacterium zucineum, which could secrete the 4-hydroxyacetophenone monooxygenase encoded by EC 1.14.13.84 to catalyse deacylation or decarboxylation. Bacillus, Geobacter, Anaerolinea, Gemmatirosa kalamazoonesis and Steroidobacter denitrificans since ubiquinone reductase (encoded by EC 1.6.5.3), succinate dehydrogenase (EC 1.3.5.1), Coenzyme Q-cytochrome c reductase (EC 1.10.2.2), cytochrome-c oxidase (EC 1.9.3.1) and electron transfer flavoprotein-ubiquinone oxidoreductase (EC 1.5.5.1) served as complexes I, II, III, IV and ubiquinone, respectively, to accelerate electron transfer. Additionally, nitrogen metabolism-related gene abundance increased by 16% to support the microbial efficacy in the soil MFC, and especially EC 1.7.5.1, and coding the mutual conversion between nitrite and nitrate was obviously improved. Conclusions The soil MFC promoted functional bacterial growth, increased functional gene abundance (including nitrogen cycling, electron transfer, and biodegradation), and facilitated antibiotic and ARG removal. Therefore, soil MFCs have expansive prospects in the remediation of antibiotic-contaminated soil. This study provides insight into the biodegradation mechanism at the gene level in soil bioelectrochemical remediation.