Magnetite Promote Anaerobic Degradation Of Organic Matters

The degradation of organic matter under anaerobic environment always involves ?syntrophy‘, in which two or more microorganism combine their metabolic capabilities for metabolic process. Classical mechanisms of syntrophy are hydrogen interspecies electron transfer or/and formate interspecies electron transfer, in which hydrogen or formate are the electron carriers for syntrophic partners. Direct interspecies electron transfer(DIET), a novel alternative in which electrons are transferred from the electron-donating microorganism to the electron-accepting microorganism via biological electrical connections or conductive materials. Growing evidences demonstrate that DIET mediated by(semi)conductive iron-oxide minerals plays a significant role in biogeochemical cycle of C, bioenergy process and soil bioremediation.Two experiments were conducted in this paper to explore the application of conductive mineral mediated DIET in bioenergy production and soil bioremediation. The first experiment was to investigate the effect of magnetite on the conversion of acetate to methane by livestock wastewater sludge under high concentration of ammonium nitrogen. Chemical analysis including the concentrations of methane, acetate and Fe(II) were performed. The second experiment was to investigate the effect of conductive iron minerals on anaerobic degradation of benzoate under sulfate-reducing conditions. Chemical analysis including the concentration of benzoate, acetate and sulfate were conducted. Microbial analysis in the two experiments were performed by 16 S r RNA gene sequencing on the Illumina Miseq platform. The main conclusions are listed as follows: 1 Acetoclastic methanogenesis is the main methanogenic pathway for the conversion of acetate to methane under low concentration of ammonium nitrogen(0.5g/L NH4+-N), whereas syntrophic acetate oxidation is the primary methanogenic pathway under high concentration of ammonium nitrogen(5g/L NH4+-N). The supplementation of conductive magnetite has no significant effect on methanogensis under low concentration of ammonium nitrogen, however, magnetite increased the rate of methane production by 35% under high ammonium nitrogen stress. This stimulatory effect probably resulted from DIET-mediated methanogenesis in which electrons were transferred between syntrophic acetate-oxidizing microbes and methanogens via conductive magnetite. Molecular biology analysis showed that Methanosaetaceae and Methanosarcinacea are the primary methanogens under low concentration of ammonium nitrogen, whereas Methanobacteriaceae and Methanosarcinaceaare the major methanogens under high ammonium stress. Geobacter and Methanobacteriaceae/Methanosarcinaceae are evidently enriched after the addition of magnetite, and they are the important syntrophic microorganisms involved in magnetite-mediated DIET. 2. The addition of conductive iron-oxides(magnetite and hematite) accelerated the rate of benzoate degradation under sulfate-reducing conditions, and the stimulatory effect was increased by the increasing concentration of magnetite. The facilitating effect of conductive iron oxides on benzoate degradation could be ascribed to the role of conductive minerals serving as electric conduct for the direct interspecies electron transfer between syntrophic benzoate-oxidizing microbes and sulfate reducers. Molecular biology analysis demonstrated that Bacteroidales, Syntrophaceae, Geobacteraceae and Desulfobulbaceae are highly involved in the syntrophic benzoate degradation in the presence of magnetite.In summary, conductive iron minerals could accelerate syntrophic degradation of organic matter under anaerobic conditions, the stimulatory effect was related to the direct interspecies electron transfer between the syntrophic partners. Our results provide an effective strategy for enhancing the stability for energy recovery from anaerobic digestion. Also, considering the ubiquitous presence of iron minerals within soils and sediments, the findings of this study will increase the current understanding of the natural biological attenuation of aromatic hydrocarbons in anaerobic environments.