Microbial Mechanism Of Magnetite-mediated Syntrophic Propionate Degardation To Methane

In the process of anaerobic fermentation of organic matter to methane,direct interspecies electron transport?DIET?is a new alternative to traditional interspecies hydrogen/formate transfer.DIET,in which electron transfer does not depend on the diffusive chemical carriers,is more efficient than interspecies hydrogen/formate transfer.Studies have demonstrated that conductive magnetite nanoparticle can facilitate DIET between electron-donating microorganisms and electron-accepting microorganisms,its function may be related to the conductive pili or cytochrome C,thus promote the efficience of DIET and accelerate methane production.These findings are of great significance to the biogeochemical cycles of carbon and iron,the generation of biomass energy,and environmental remediation.This thesis outlines the research progress on DIET by now,the previous researches mainly use ethanol as the electron donor to study the mechanism of DIET.It has been reported that conductive magnetite can stimulate DIET process and thus accelerate the syntrophic oxidation of acetate,propionate,or butyrate and CH₄production.However,the related microbial community and its function involved in DIET are still poorly studied,especially under the condition that propionate serves as the sole electron donor.In this study,the effects of synthetic magnetite and biogenic magnetite on propionate degradation to CH₄ were investigated by anaerobic digestive sludge and rice paddy soil.The microbial community structures and functional microorganisms in the magnetite-amended system were analyzed by metagenome analysis,stable isotope probing?DNA-SIP?technology and 16S rRNA high-throughput sequencing technology.The main conclusions from this study are as follows:1.The addition of synthetic magnetite can promote propionate degradation and CH₄ production.The higher concentration of magnetite,the more apparent stimulatory effect on propionate conversion to CH₄.Metagenomic analysis showed that there was no significant difference in the key genes involved in propionate metabolism pathway between the magnetite-amended treatments and the magnetite-free controls.Methanoregula and Methanosaeta were the main methanogens in both treatments.It was found that Methanosaeta contained the genes of CO₂ reduction in both treatments,demonstrated the presence of DIET-mediated methanogenesis driven by Methanosaeta in both treatments.The presence of magnetite affected the microbial community structure.The higher relative abundances of Candidatus Cloacimons acidaminovorans were observed in the control treatments,while the relative abundances of Geobacter metallireducens in the magnetite-amended treatments group were much higher than those in the controls.Metagenomic analysis showed that the level of OmcS gene in the magnetite-amended treatment was down-regulated,suggesting that magnetite might functions as a substitute for OmcS and promotes direct electron transfer between syntrophic microbes,thus accelerating methanogenesis process in the magnetite-amended treatments.2.Biogenic magnetite also has stimulatory effect on propionate degradation and CH₄ production.Propionate degradation in the presence of biogenic magnetite was insignificantly influenced by the change of hydrogen partial pressure,while the change of hydrogen partial pressure suppress the degradation of propionate/acetate in the presence of biogenic vivianite,thus demonstrating the existence of DIET-mediated methanogenesis in the biogenic magnetite treatments and interspecies hydrogen transfer-dependent methanogenic process in the biogenic vivianite treatments.DNA-SIP coupled with 16S rRNA high-throughput sequencing analysis showed that the formation of biogenic magnetite and biogenic vivianite affected the microbial community structures.Magnetospirillum was the functional microorganisms in the biogenic magnetite treatments,while Geobacter was functional microorganisms in the biogenic vivianite treatments.No methanogens were significantly enriched in the heavy layer area.We speculate that in the treatments with biogenic magnetite,Magnetospirillum is capable of DIET function that involved in propionate degradation and directly transferred electrons to the dominant methanogenic microorganism,Methanosaeta,then reduce CO₂ to CH₄.Our findings not only expand the understanding of the DIET-mediated syntrophic propionate oxidation and methane production,but also provide a new perspective for us to further understand the mechanism of the anaerobic degradation of organic matter.