| Methanogen is the only microbe which is capable of methane synthesis.It is strictly anaerobic and can only use acetic acid,hydrogen,formic acid and other simple organic to produce methane.Methanosarcina barkeri is one of the few methanogens with both hydrogen and acetate metabolism capacity,and it is a model strain for studying the methanogenesis mechanisms.Recent studies have found that M.barkeri can directly take in extracellular electron to produce methane.Methanogen can accept extracellular electrons via cytochrome proteins,pili of the partner bacteria or conductive solids.Magnetite is widely used in studying the mechanisms of methane production because of its good conductivity.Deep understanding of methanogenisis mechanisms can provide a solid theoretical basis for realizing the reduction of methane emission and rational use of methane as an energy source.Therefore,in this paper we took M.barkeri as the research object and established its pure culture,bioelectrochemical system(BES)and coculture system,respectively.Through comprehensive analysis of changes in the composition of gases,morphology and structure of nanoFe3O4,electrochemical performance and distribution of microbes,we studied the interaction between nanoFe304 and M.barkeri,and clarified the effects of nanoFe304 on the methanogenisis of M.barkeri.The main conclusions are as follows:(1)NanoFe304 could promote M.barkeri to produce methane from acetate decomposition.The maximum methane production rate of pure culture of M.barkeri was 2.91 mmol/L d,and the maximum methane production rate of M.barkeri with nanoFe304 was 6.15 mmol/L d.The addition of nanoFe3O4 increased methane production by 1.11 times.The growth of M.barkeri did not affect the morphology and structure of nanoFe3O4,but nanoFe3O4 could promote the aggregation of microbes with itself.The reasons for nanoFe304 to promote methane production were probably that the good conductivity of nanoFe3O4 decreased the impedance in the reaction system,enhanced the redox aciticity,and improved the ability of M.barkeri to metabolize acetate.(2)M.barkeri could accept cathodic electrons to reduce CO2 to methane.Under a cathodic potential of-0.64 V(vs.SCE),M.barkeri’s maximum methane production was 22.88 μmol/L.Adding nanoFe3O4 could accelerate M.barkeri to produce methane,increasing the maximum methane concentration(34.51 μmol/L)by 0.51 times.Immobilizing M.barkeri to cathodes with carbon nanoparticles(CNP)further promoted cathodic electron uptake.Compared to the methane concentration(62.24μmol/L)in BES with natural M.barkeri biofilms,immobilization of M.barkeri with CNP increased the maximum methane concentration(90.55μmol/L)by 0.45 times.M.barkeri-immobilized BES with the presence of nanoFe3O4 further increased the maximum methane concentration to 135.15 μmol/L,which was 1.17 times higher than that for BES of natural M.barkeri biofilms.Electrochemical experiments showed that nanoFe3O4 improved the redox activity of the BES.In situ electrochemical Fourier infrared spectrum revealed that M.barkeri contained a large number of cytochrome c,which might be used to accept cathodic electrons.(3)NanoFe3O4 could promote GSS01 and M.barkeri to produce methane via direct interspecies electron transfer(DIET).The maximum methane production rate of GSS01 and M.barkeri was 1.99 mmol/L·d,while adding nanoFe3O4 enhanced the maximum methane production rate(3.06 mmol/L·d)by 0.54 times.This two kinds of microbes shared electrons through hydrogen and DIET.NanoFe3O4 could promote microbial aggregation,shorten the physical distance in the DIET process,reduce the impedance of the syntrophic system,and thus improve the efficiency of electron transfer.In summary,nanoFe3O4 could improve the ability of M.barkeri to decompose acetate and accept extracellular electrons.Our research not only proved that M.barkeri could accept electrode electrons,but also provided a new research perspective to understand the microscopic mechanism of methanogenisis. |
