| Energy crisis and environmental pollution were two issues which have been particularly focused by current society. Considering the energy-climate problems caused by the use of fossil fuels and that those fuels cannot provide enough energy to meet the future society development, researchers over the world have made great efforts into exploiting new, sustainable and environmental-friendly energy resources. Microbial fuel cells (MFCs), a technology combining traditional biodegradation and electrochemical technology, has gained increasing attentions and developed rapidly in the past decade. MFCs employ microorganisms to directly transform chemical energy stored in biodegradable organic/inorganic matters to electricity, performing many merits including environment-harmless, high efficiency in simultaneous biodegradation and electricity generation etc. In current, MFCs is making a challenging step from laboratory fundamental research to field application and in which a promising future has been indicated. More fundamental researches are needed to support the rapid development of MFCs application. The anodic microorganisms play a dominant role in organic compounds degradation and current generation. However, less is known about the microbial electron transfer mechanism to date, which has been considered one of the most significant limitations for MFCs development.Due to the wide distribution in various natural and polluted environments and having the most diverse respiratory strategies, the Shewanella species have been frequently used as the model bacteria in MFCs studies. This study constructed dual-chamber MFCs using S. decolorationis S12 as anodic inoculum to research the bacterial electron transfer mechanism in MFCs. The S12 inoculated MFCs generated a maximum power density upto 5.3 W/m3 and a columbic efficiency of 7.8% when LB was used as medium in anode chamber. During current generation, the anode surface formed significant biofilms which was demonstrated the major strategy of S12 in current generation. Since seldom pilus-like baterial nanowire was observed in the anode biofilms, the C-type cytochromes was supposed to be the major electron transfer conduit by S12. The ccmA mutant of S12 subsequently confirmed this predication. The mutant could not biosynthesis mature C-type cytochromes and showed no current generating capacity, eventhough the flavin-secretion capacity was sustained. Biofilms developed on anode surface was evidenced the predominant role in anodic electron transfer while the planktonic cells seemed likely a minor role. Self-secreted flavins including riboflavin and Flavin mononucleotide were also detected which has been shown essential in electron transfer from Shewanella c-type cytochromes to electrode. S12 anode biofilm formed in closed-circuit (current-generating) MFCs sustained 98% viability within 96 h while that in the open-circuit MFCs decreased to 72% within 96 h. Live/Dead staining and confocal laser scanning microscope analysis showed the biofilm viability lost in open-circuit MFCs most occurred at the inside layer of the biofilm, indicating that current generation in MFCs was an efficient strategy to sustain or even enhance biofilm viability. Those findings offered a biological insight to support the facts that MFCs performs higher biodegradation efficiency than traditional anaerobic biological technology.Azo dyes and Polybrominateddiphenylethers (PBDEs) were chose as targeted pollutants to test their degradation efficiency in MFCs. In S12-catalazed biocathode MFCs, azo dye amaranth could be reduced by S12 using cathode as the sole electrondonor. In the S12 cathode chamber, 1 mM amaranth was completely reduced within 50 h. However, lower reduction efficiency was observed in MFCs biocathode chamber when compared to a normal anaerobic bacterial reduction. It could be understood by that the less cell growth in cathode chamber and a large part of the electrons generated from anode chamber was consumed by the overpotential and the bacteria metabolism. This is the first demonstration that the anodic-generated electrons are available for the cathodic azo respiration by Shewanella, without the need for external power resource. Further researches are needed to detail the underlying mechanism of the biocathode process.In the case of PBDEs degradation, a mix sediment microorganism consotia was used as inoculum in anode chamber. MFCs displayed higher deda-brominated diphenylether (BDE-209) biodegradation efficiency. 46.5% of the Br- ions from BDE-209 were substituded while that of the normal anaerobic culture was 12%. GC-MS showed 81.4% and 64% degradation efficient of the BDE-209 in MFCs and normal anaerobic culture, respectively. However, no detectable low-bromianted PBDEs as reduction products was observed. This might be resulted by the absorption of PBDEs on electrodes cation exchange membrane, as well as the possibility that BDE-209 was degraded via the cleavage of phenyl or ether bond. Microorganism community composition in the debromination MFCs were analysis by both PCR-DGGE and 454 high throughput sequencing. Different microorganism community compositions were showed between the MFCs culture and normal anaerobic culture, as well as between the anode biofilms community and the planktonic community. Dissimilatory metal reduction bacterial, Geobacter spp. was dominant in the anode biofilms community. The previously reported environment functional bacteria Alcaligenes spp. in MFC was dominant in the planktonic community. In addition to the dominant members, other enriched bacteria in MFCs have been previously reported to be able of reductive dechloration were observed, indicating that those bacteria might be responsible for the current generation or the enhanced BDE-209 degradation efficiency in MFCs.In conclusion, this study used strain S12 as a model bacterium to study the electron transfer menchinsm in both anodic and cathodic chamber of MFCs and achieved a serial of new findings. For the first time to our knowledge, this study researched the electron transfer mechanism of S. decolorationis in MFCs anode chamber. The finding that a nonpoised electrode could serve as sole electron donor for S. decolorationis azo reduction in a biocathode MFCs provide a foundation for the research of electron transfer mechanism from electrode to bacteria cells. In MFCs-based biodegradation, current generation could enhance many biological capacities (e.g. biofilm viability, upregulated functional gene expression) which could further promoted the biodegradation in MFCs, providing fundamental direction and assist to the wide application of MFCs in the near future.
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