| In recent years, microbial electrochemical system (MES) is becoming a hot research topic in the field of energy and environment. The anode exoelectrogen, one of the key components in the MES, acts as a biological catalyst which receiving electrons from the oxidation of organic substrates and transfering them to the anode through their electron transport system. Existing researches showed that, the interfacial electron transfer between the exoelectrogen and anode (also named extracellular electron transfer) was the key step in the MES. It can be performed by the cytochromes near the interface and characterized by the electroanalysesa. Some related researches showed that anode exoelectrogens’electrochemical signals could change with the different working potential. Some researchers speculated that the anode exoelectrogen, in contact with the electrode, could sense the electrode potential and change its own electron transfer system accordingly, so as to maximize the energy gain. However it is still unclear whether the anode exoelectrogen changes its electron transfer system in the physiological level. In addition, those anode exoelectrogens worked under low anode potential can improve the performance of the MES. Related literatures generally regard the oxidation potential of anodic substrate as the anodic thermodynamic limit, and argue that the known model electric bacteria are near that limit when respiring extracellularly. Nevertheless, the former discover in our lab is different with that concept.Therefore, electrochemical signals of Geobacter sulfurreducens, a model anode exoelectrogen under four different working potential of-0.25V,-0.20V,0.00V and +0.20V, were analyzed in this article. And the gene expression levels of Geobacter sulfurreducens on the anode with obvious different electrochemical signals were compared so as to provide a proof of the direct relevance between the change of electrochemical signals of anode exoelectrogens and the physiological change of anode exoelectrogens’electron transport system in molecular level and also supply some theoretical basis for explaining the mechanism of some important processes in natural. It also helps to further the understandings on the anode thermodynamic limit in MES.(Standard hydrogen electrode acts as the referenced standard in this paper.)1 Anode exoelectrogen of G. sulfurreducens could produce electricity under all poised potentials including the lowest potential level of -0.25 V.Chronoamperometry results showed that anode exoelectrogens started to produce catalytic current after 3 h both under the working potential level of+0.20 V and 0.00 V, but gained current densities of 90±6 uA/cm2 and 85±2 uA/cm2 in about 70± 5 h and 72±5h respectively when the anode reached a first activity maximum. However, under the working potential of-0.20 V, the time for the reactor to start up extended to about 7.5 h, and the anode reached a first activity maximum at about 111 ±4 h resulting in a current density of 14±5 uA/cm2. For the anode exoelectrogens working under the potential level of-0.25 V, the catalytic current was firstly observed at about 164.00 h, and the anode reached a first activity maximum at about 368±5 h when the current density was 3 ±1uA/cm2. In addition, under this working potential, the maximum current density in the subsequent feeding cycles was 14±4 uA/cm2.2 Anode exoelectrogens of G. sulfurreducens could attach to the anode under all the four setted working potential. But their adhesion amounts were different. The adhesion amount of G. sulfurreducens under the potential level of-0.25 V was less.The carbon cloth acted as the anode in this thesis. Scan results gained by the scanning electron microscope (sem) showed that carbon fibers were covered by some very dense and even biofilm with an average thickness of 1.0 um under the working potential of 0.20 V level, but with less dense biofilm under the level of 0.00 V. The thickness of the biofilm existed on the widest carbon fiber which working under the potential of 0.00V was 1.0um. However, under the-0.20 V level, there was only thin monolayer biofilm on the carbon fibers with an average thickness of 0.4 um; Then, under the-0.25 V level, only some anode exoelectrogens sparsely attached on carbon fibers. As a result, under a certain range, the higher the electrode potential was, the more energy the anode exoelectrogen obtained, and the more easier the biofilm formed.3 According to the comparison of CV and DPV signals originated from the terminals of electron transfer system of anode exoelectrogens of G. sulfurreducens, which worked under those four different poised potential, there were many differences between those electrochemical signals gained in the catalytic phase. The poised potential of the main pair of redox peaks in G. sulfurreducens worked under the potential of-0.25V was-0.238±0.004V, which suggested two-electron electron transfer.DPV results in the catalytic phase showed that under the potential level of -0.25 V, the electrochemical signal for G. sulfurreducens consists a main pair of redox peaks with the potential in a range of-0.25～-0.20 V, and indicated a transfer process of two electrons. But under the potential level of-0.20 V, the electrochemical signal was a mixed bimodal with a potential for the major peak of-0.187±0.006 V. The main redox pair also possibly represented a transfer process of two electrons. However, under the potential level of 0.00 V and+0.20 V, electrochemical signals with a unimodal,came from the fusion of bimodal, were appeared. Peak potentials were both close to 0.1580.006 V meaning a transfer process of single electron. According to CV results gai±ned in catalytic Phase, under potential levels of-0.20 V and-0.25V, the potentials of the active center were-0.182 ± 0.003V and-0.237±0.003V respectively. And under the potential levels of 0.00 V and+0.20 V, the active center potential of anode exoelectrogens both were about-0.158±0.002V, which were consistent with the results of DPV.DPV results in the non-catalytic phase show that electrochemical signals of G. Sulfurreducens working under different levels were basically the same, mainly including two pairs of peaks with the mid-point potential of-0.180±0.005 V and-0.090 ± 0.004 V respectively. The ratio of peak height was almost the same. CV results were consistent with those phenomena. The potentials of two activity centers in the CV results,which made a major contribution to catalytic current, were basically the same with the DPV results. Therefore, in a word, under different levels of anode potential, electrochemical signals for anode exoelectrogens of G. sulfurreducens in the catalytic and noncatalytic phase were different. The anode potential had a significant influence on the electrochemical signal of G. sulfurreducens in catalytic phase.4 Analysis on the differences in genes expressionThe difference of electrochemical signals from G. sulfurreducens between the level of+0.20 V and-0.25V in catalytic phase was evident. Hence, the gene transcription level for anode biofilms grown under these two potential levels were analyzed. As a result,373 genes with significant differences between the two samples were identified including 366 common genes to those two samples,3 unique genes to the sample gained from the potential level of+0.20 V,4 unique genes to the sample gained from the potential level of-0.25 V. Compared to the+0.20 V sample,282 genes with significant differences were up-regulated in the sample gained from the potential level of-0.25 V, including 24 genes involved in cytochrome C. Besides,91 genes with significant differences were down-regulated. And there were 114 genes in transcriptome with significant difference in transcription, and their multiple of difference were more than four times.Acorrding to those 282 up-regulated genes, GO enrichment analysis results showed that 19 GO pathways were involved, including 8 terms of Biological Process (BP),2 terms of Cell Components (CC) and 9 terms of Molecular Function (MF). Among those BP terms, there was only one significantly enrichmented term, which was related to "electronic transmission". But among those MF terms, there were five significantly enrichmented terms, which were associated with "electron carrier activity", "NADH dehydrogenase activity", "NADH and NADPH related oxidoreductase activity", "molybdenum ion binding" and "NADH dehydrogenase (quinone) activity" respectively. For those CC terms, there was also only one significantly enrichmented term, which was related to "membrane"; Among the 91 down-regulated genes,23 GO items were involved including 12 BP items and 11 MF items.5 MF items, which were significantly enriched, were all related to the transmembrane transport of matters.Then, analysis results of the enrichmented KEGG pathway showed that 52 KEGG pathways were involved based on all those significantly different genes. There were 45 KEGG pathways,those up-regulated genes took part in, including one significantly enriched pathway, which was related to energy metabolism of oxidative phosphorylation (p_fdr<0.001); As for down-regulated genes,21 KEGG pathways were involved. In addition, there were 3 pathways significantly enriched which were related to "sulfur related metabolism" or "genetic information processing".Therefore, based on the analysis of differences in genes expression, among a certain range, the different anode potential can cause the change of the electron transport system for anode-respiring bacteria in catalytic phase. |
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