Mechanisms Of Syntrophic Interaction Of Microbial Populations And Metabolism Regulation Of Extracellular Electron Transfer In Microbial Electrochemical Systems

Microbial electrochemical system?MES?offers the versatile technology to achieve wastewater treatment,bioremediation and sustainable bioenergy generation,which has been widely studied in recent years.MES has many configurations,such as microbial fuel cell?MFC?and microbial electrolysis cell?MEC?.The performance of MES depends on microbial community composition of electrode biofilm.Microbial community structure and extracellular electron transfer?EET?metabolism of MES are affected by reactor configuration,electrode materials and ecological factors.The interaction of electrode biofilms and regulation mechanism of extracellular electron transfer metabolism under different ecological conditions remains a scientific problem to improve the performance of MES.This study was aimed to 1)investigate the succession of microbial community and analyze the genes related to EET and gene expressions in metabolic process.2)explore the response of MFC performance and electrode microbial community structures to Fe²⁺and Fe³⁺.3)reveal the mechanism of microbial population interactions in process of hydrogen and methane production in MEC systems,which provides the theoretical basis and technical support for the application of MES and the optimization of the reaction system.To understand the regulation mechanism of microbial metabolism in MFC anode under the condition of substrates restriction,metatranscriptomic analysis was used to investigate the functional gene?especially EET gene?expression in multi-anode MFC.In the process of substrate utilize,9867 down-regulated and2744 up-regulated genes were observed in 12h vs.34h.Compare with 12h,the number of down-regulated genes was 3525 and the up-regulated genes was up to13706 at 44h.With substrates consumption,the dominant Proteobacteria populations based on mRNA species annotation transferred from Geobacter to Pesudomonas and finally Acinetobacter.The relative abundance of fermentative bacteria-Proteiniphilum acetatigenes decreased with acetic acid decreasing.However,other exoelectrogenic bacteria,such as Desulfomonile tiedjei,Desulfovibrio magneticus and Desulfobulbus propionicus,and archaea Methanosarcina mazei expressed better competence at substrates lacking condition?44 h?.The functional genes analysis indicated that gene GSU1445?iron transport?,Gmet₃344,Gura₄233,Gura₄239,Despr₂052,Despr₂060?NADH?,met C and DaAHT2₁471?sulfur metabolism?were highly expressed when substrates were abundant.However,some cytochrome c genes?for example,cox2,Astex₂616,petA and petB?were significantly up-regulate when substrates decreasing,which makes MFC a better EET capability at 34h.The results revealed the transition of dominant exoelectrogenic bacteria and the difference of EET-related genes expression under substrates consuming.To study the capability of EET response to ecological factors,Fe²⁺and Fe³⁺with different concentrations were added into MFCs to investigate the MFC power output and microbial community.Voltage production showed that Fe²⁺of 100?mol/L facilitated MFC start-up compared to 150?mol/L,200?mol/L and without supplement of Fe²⁺.However,higher concentration of Fe²⁺?200?mol/L?had an inhibitive influence on current generation after 30 days of operation.Illumina Hiseq sequencing indicated that Fe²⁺changed obviously microbial community structures of both anode and cathode biofilms.Fe³⁺could also affect the current generation of MFC.Insoluble Fe³⁺?200?mol/L?improved electrochemical activity of the MFCs microbial biofilms during start-up and resulted in a higher maximum power density of 0.95 W/m².Illumina Hiseq sequencing indicated that the predominant populations in the anode biofilms of the MFCs belonged to Geobacter.Microbial cathode biofilm communities were more susceptible to Fe³⁺,as an obvious shift in the cathode biofilm community structures occurred as Fe³⁺concentration was increased.The most predominant populations in the MFC cathode biofilms without Fe³⁺and with 200?mol/L Fe³⁺were affiliated with Thauera?46%and 35%?,whereas no absolutely predominant populations were present in the MFC cathode biofilm with 1000?mol/L Fe³⁺.The above results indicated that lower concentration of Fe²⁺and Fe³⁺facilitated exoelectrogenic Geobacter enrichment.However,higher concentration of Fe²⁺and Fe³⁺shifted the community structures of the anode and cathode biofilms,especially inhibited exoelectrogenic activity.To verify the microbial population interactions and control biogas production of MEC in complex substrates treatment,we discussed the feasibility of using biocathode MEC for biohydrogen and biomethane production.Fe?0?was used to improve waste sludge degradation and biohydrogen production.Meanwhile,the alkali-pretreated sludge fed MECs?AS-MEC?showed the highest biohythane?hydrogen<20%?production rate of 0.148 L·L^-1-reactor·d^-1,which is 40%and 80%higher than raw sludge fed MECs?RS-MEC?and anaerobic digestion?open circuit MEC,RS-OCMEC?.The result of current density,metabolites utilization and the ratio of biogas confirmed that alkaline pretreatment and MEC system greatly improved waste sludge degradation and biohythane production.Illumina Miseq sequencing of 16S rRNA gene amplicons indicated that anode biofilm of AS-MEC was dominated by exoelectrogenic Geobacter?22%,relative abundance?and fermentative Alistipes?10%?.The biocathode biofilm was dominated by fermentative/exoelectrogenic Clostridium?15%?.The archaeal hydrogenotrophic Methanobacterium?98%?were the dominant population in biocathode biofilm of AS-MEC,while in RS-MEC they were Methanocorpusculum?77%?.The research demonstrated that multiple pathways of biogas production were existed in AS-MEC system,including fermentative and electrolytic H2 production,as well as hydrogenotrophic methanogenesis and electromethanogenesis.Real-time quantitative PCR analyses showed that higher amount of methanogens were enriched in AS-MEC than that in RS-MEC and RS-OCMEC,suggesting that alkali-pretreated sludge and MEC facilitated hydrogenotrophic methanogen enrichment.