Extracellular Electron Transfer Mechanisms Of Hydrogen-Producing Clostridium Mediated By Iron Oxide Minerals

Microorganism extracellular electron transfer(EET)is a new anaerobic energy embolism manner and has a promising application in sewage treatment,bioremediation and clean energy production.Iron oxide minerals(IOMs),especially magnetite,play a stimulative role in direct interspecies electron transfer(DIET)process.However,the reports on the effect of IOMs on EET of hydrogen-producing Clostridium and its application in clean energy were rare.Here,we researched the effect of IOMs on extracellular electron transfer of hydrogen producing Clostridium for enhancing hydrogen production.In the first part,an electrochemical activity hydrogen producing Clostridium was isolated.The strain C.bifermentans EZ-1 was isolated from the Yellow River Delta.The morphology and biology characters were tested by scanning electron micrography(SEM),gas chromatography(GC),high performance liquid chromatography(HPLC),microbial fuel cell(MFC)and cyclic voltammetry(CV)technologies or methods.The results showed that:a.the strain was rod,3-5?m and Gram-positive;b.The strain could use glucose accompanied with hydrogen,carbon oxide,acetate and butyrate production in the anaerobic condition;c.The strain was capable of reducing Fe(?)in the soluble ferric citrate,poor crystal ferrihydrite and crystal magnetite;d.Furthermore,the strain was electrochemically active and held redox compound,the current density peaked at6.5 mA/m~2.From the above,the stain was versatile in the electron output pathway.In the second part,we detected the effect of IOMs on EZ-1,especially on electron transfer of EZ-1.The results of metabolic products testing and stoichiometric calculation suggested that:a.The different kinds and concentrations of IOMs exerted a significant effect on the carbon and electron flows of EZ-1.The effect was related to concentration of iron oxide minerals,when the concentration was low,the dominant electron output pathway of EZ-1 was producing hydrogen,however,the dominant electron output pathway converted to Fe(?)reduction as the concentration increased;b.Fe(?)reduction shunted the electrons of hydrogen evolution,resulting in the decreased hydrogen production.Out of expectation,the decreased hydrogen could not meet the need of Fe(?)reduction actually.In other words,the total electron output efficient was up-regulated.The results showed that the total electron output efficient of EZ-1 with magnetite and ferrihydrite addition increased by 31%and 61%,respectively;c.Moreover,IOMs could not only regulate the electron flow but the carbon flow.IOMs redirected the carbon flow to low reducing production(acetate)rather than high reducing production(butyrate).Given that more electrons could attain through the acetate production pathway,IOMs could extract more electrons by regulating EZ-1's carbon metabolic pathways.In the third part,a typical hydrogen production bacterium,Clostridium pasteurianum,was used to deeply exploit the effect of IOMs on hydrogen producing Clostridium with different capability of hydrogen evolution and Fe(?)reduction.Given the hydrogen yield of C.pasteurianum is higher than that of EZ-1 while EZ-1 is superior in Fe(?)reduction.We aim to enhance hydrogen production of C.pasteurianum with IOMs that was capable of extracting electron from hydrogen producing Clostridium.The results showed that:a.IOMs elevated hydrogen production of C.pasteurianum which was not accordant in that of EZ-1;b.Glucose consumption was accelerated accompanied with acetate and butyrate accumulation.Otherwise,IOMs promoted the growth of C.pasteurianum as the biomass elevated;c.Although ferrous also could promote hydrogen production,the promoting effect was lower than that of ferrihydrite,indicating that the mechanism of ferrous and ferrihydrite was different;d.The electron extracting effect of ferrihydrite was better than that of magnetite/hematite and this result was consistent with that of the second part,the maximum hydrogen production was elevated 1.8 time with 100 mg/L ferrihydrite addition.In conclusion,the hydrogen production of C.pasteurianum was elevated by 1.8time basing on the electron extracting effect of IOMs.Ferrihydrite was the best extractor among tested IMOs and the optimal concentration was 100 mg/L.This study showed that extracellular electron transfer of hydrogen-producing C.pasteurianum was regulated by iron oxide minerals for enhancing hydrogen production.This work played an important role in enhaceing hydrogen production and expanded the microorganism extracellular electron transfer.