The Bidirectional Electron Transfer Between Electroactive Bacteria And Solid Electrodes And Their Environmental Effects

Microbes in the natural environment exist mainly as the planktonic cells and the biofilms bunching on a solid surface. In 2004, the American scientist(Logan) found that a biofilm enriched from wastewater on a solid electrode carrier could transfer electron to the electrode, and the electron transfer efficiency was hundreds of times higher than that of the planktonic cells. Such a biofilm with a high electrochemical activity was then termed as electroactive biofilm(EAB). Compared with the traditional biofilm, direct exchange of electrons occurs between EABs and the solid carriers in addition to adsorption behaviors and material exchanges. EABs can not only donate electrons to extracellular solids such as iron oxides and electrodes, but also accept electrons and transfer them into cells. Therefore, extracellular electron transfer of EABs has attracted increasing attentions and shows great potentials of diverse applications including pollutant degradations, wastewater treatments and clean energy production. Up to now, however, the environmental effects and the electron transfer mechanisms are still not fully understood. So a deep and comprehensive investigation of EABs will be of great significance in understanding their environmental functions, the diversities of energy metabolisms as well as their applications.Base on the main line of bidirectional electron transfer between solid electrodes and EABs, this study evaluated the potential environmental effects of four different EABs from the perspectives of microbial reductive dechlorination of pentachlorophenol(PCP), denitrification of nitrate and bioelectrosynthesis(CO2 reduction), and the underlying electron transfer mechanisms were analyzed, respectively. The main research results are as follows:(1) The effects of rice straw biochars to the PCP dechloridation by Geobacter sulfurreducens were explored. The results indicated that biochars could function as a solid electron mediator to enhance electron transfer from G. sulfurreducens to PCP and thus accelerated its reductive dechloridation. Intermediate products of PCP dechloridation were identified to be 2,4,6-trichlorophenol, 2,4-dichlorophenol, 4-monochlorophenol and phenol. However, the acceleration effects varied significantly among biochars that were prepared at different temperatures. In the range of 400 ~ 900 °C, both electrical conductivities(ECs) and electron exchange capacities(EECs) of biochars increased with the increasing production temperature. Anaerobic incubation experiments showed that biochars could be used as electron acceptors for the extracellular respiration and growth of G. sulfurreducens. The higher electron-accepting capacity of biochars led to a greater biomass of G. sulfurreducens. Analysis of the mediation mechanisms of biochars indicated that the dechloridation rates correlated with both the EECs and ECs of biochars. The experiments of biochar surface modifications confirmed that biochars transferred electrons via the pathways of both the surface redox-active groups and its electrically conductive graphite regions. This study provided an effective method for in situ bioremediation of PCP contamination under anaerobic environments.(2) The abilities of direct uptake of electrode electrons for autotrophic denitrification by Alcaligenes faecalis biofilms and Thiobacillus denitrificans biofilms were evaluated in the bioelectrochemical systems(BES). The results showed that the two biofilms were capable of accepting electron from electrodes. The lower the electrode potential, the higher the rates of nitrate reduction were. A. faecalis biofilms showed a higher denitrification performance(nitrogen removal rate of 71.6%) than T. denitrificans biofilms(nitrogen removal rate of 17.8%). The former reduced nitrate via two pathways, namely denitrification and dissimilatory nitrate reduction to ammonium(DNRA), whereas the latter did not have the DNRA pathway. Electron transport inhibition experiments demonstrated that the electrode electrons were transferred to Complex I, II, III and quinone pool in the cytoplasmic membrane of T. denitrificans. These results provided an alternative electrochemical approach to the treatments of wastewater with nitrate pollution.(3) The abilities of the electrosynthesis from CO2 by A. faecalis and Clostridium thermoautotrophicum biofilms using electrodes as electron donors were investigated. The results indicated that electron uptake rates and electrosynthesis rates of the two biofilms increased with a decreasing electrode potential. The electrosynthesis products of A. faecalis included pyruvic acid and succinic acid. However, these organic matters were only the intermediate products, which could be finally transformed to the biomass or other compounds. Formate and acetate were the main electrosynthesis products of C. thermoautotrophicum biofilms, and the products accumulated faster at a higher temperature. For C. thermoautotrophicum, surface modifications of the carbon cloth electrode by Au and Neutral Red enhanced the production rates of formate by 2.9 and 1.7 times, respectively. By contrast, an electrode surface modification by graphene oxide decreased the yield of formate by 45%. This suggested that the surface groups of the electrodes had a great impact on the electron transfer reaction. Direct immobilization of C. thermoautotrophicum onto the electrode significantly accelerated electron uptake by C. thermoautotrophicum, and the yields of formate and acetate were increased by 41.3 and 9.6 times, respectively. So direct immobilization of the bacterium onto the electrode is an effective method for improving the electrosynthesis performance.(4) The bidirectional electron transfer activities of A. faecalis biofilms in BES were studied. The results indicated that the outward electron transfer(current generation) rate was much slower than the inward one. Rotenone, antimycin A, quinacrine, dicumarol and DCCD could inhibit both the outward and inward currents. This indicated that the electron transport chain on the cytoplasmic membrane, namely complex I, II, III and quinine pool were involved in both outward and inward electron transfer. Meanwhile, ATP was synthesized in these processes. There were significant differences in the protein expression levels of A. faecalis biofilms acclimated at-0.5,-0.3,-0.1 and +0.3 V vs. SHE, respectively. Cytochrome c peroxidase, electron carrier protein(Rnf B) and cytochrome c of the biofilms were significantly down-regulated in the current consumption modes(-0.5 or-0.1 V), while these proteins were significantly up-regulated in the electricity generation mode(+0.3 V). Such results indicated that the three proteins were crucial for the outward electron transfer but not for the inward one. A flagellin protein(Flagellin) of the biofilms was up-regulated(as high as 13 times) in both of the two modes. This suggested that the flagellin protein played an important role in the biofilm formation and electron transfer. In addition, CpaB(a pili protein) of A. faecalis biofilms was up-regulated in the electricity generation mode. So it might suggest that similar to the function of the nanowires of Geobacter spp., the pili of A. faecalis biofilms was involved in outward electron transfer. This work provided a new insight into the mechanisms of bidirectional electron transfer of EABs.