Electron Transfer Mechanisms For Pollutant Biodegradation And Electricity Generation By Electricigens

Electricigens are a class of bacteria capable of metabolizing carbon sources and transferring electrons to extracellular electron acceptor. Pseudomonas, Geobacter and Shewanella are all well-known electricigens. Especially, Shewanella, as a typical modal microorganism with relative clear gene sequence, has been widely investigated for pollutant degradation and electricity generation. In its metabolism, a wide range of substances can be ustilized as electron donors or acceptors under anaerobic conditions. Accompanied with the reduction of pollution, Shewanella could obtain energy for its growth. However, the electron transfer pathways of these processes are largely unclear. Therefore, in this work, S. oneidensis MR-1and its mutant strains were used to investigate the bioreduction mechanisms of the organic pollutants, which are commonly present in natural environments. Furthermore, the meachanisms of electron transfer in the electricity generation by electricigens were also explored.Azo-dyes, the common pollutants from textile industries, are toxic and highly persistent. They can cause water pollution and threaten human health. Therefore, treatment of azo-dyes wastewater is essential before it is discharged. Herein, the bioreduction mechanism of azo dyes was explored and a typical azo-dye with relatively simple chemical structure, methyl orange (MO), was chosen for the degradation investigation. MO was used as the sole electron acceptor for bioreduction by Shewanella oneidensis MR-1. Accompanied with MO reduction (the cleavage of azo bond occurred), S. oneidensis MR-1grew with the energy from the anaerobic respiration. A comparison of the wide type and mutant strains shows that the deletion of mtr led to80%loss in the reduction of decolorization rate. Based on the results, the MtrA-MtrB-MtrC respiratory pathway was proposed to be the main route of electron transport for S. oneidensis MR-1in its anaerobic decolorization of azo dyes. Meanwhile, knockout of cymA led to a significant loss of MO reduction ability, suggesting that CymA was the key c-type cytochrome in the electron transfer chain to MO.In the second part of this thesis, the bioreduction of another organic compound, nitroaromatic compound, by S. oneidensis Mr-1was also studied. Nitroaromatic compounds are a type of refractory pollutants released into environment almost exclusively from anthropogenic sources, such as incomplete combustion of fossil fuels and synthesis of chemicals. Effective bioreduction of nitroaromatic compounds by electrochemically active bacteria have been demonstrated, but the mechanism behind such a bioreduction is unclear yet. Nitrobenzene (NB), the simplest nitroaromatic compound, was chosen for the investigation. Different from the MO anaerobic bioreduction process, the Mtr respiratory pathway, NrfA and NapB in S. oneidensis MR-1were found to be uninvolved in the NB bioreduction. In addition, p-chloro mercuric benzoic (pCMB) was observed to pose an inhibitory effect on the NB bioreduction. Furthermore, knockout of cymA led to a67%loss in NB bioreduction efficiency compared with the wild strain. The formation of an intermediate, phenylhydroxylamine, was observed at the initial stage of NB bioreduction for cymA-deleted strain. These results demonstrate that NB bioreduction was not a pure extracellular process and that both nitrate reductase and nitrite resuctase were not involved in the NB bioreduction. More importantly, CymA was confirmed to be a key c-type cytochrome in the NB bioreduction by S. oneidensis MR-1.In the third part of this thesis, Microcystis aeruginosa IPP, another electricigen, was used for investigation. M. aeruginosa IPP is also a well known cyanobacterium that contributes to algal bloom. The mechanism for the enhanced electricity generation under illumination for a biocathode microbial fuel cell (MFC) using M. aeruginosa IPP as the cathodic culture was investigated. The experimental results demonstrate that this cyanobacterium was able to act as an effective cathodic biocatalyst under illumination. In addition, M. aeruginosa IPP was found to produce reactive oxygen species (ROS), the amount of which exhibited a positive correlation with the algal cell density. Electricity generation was severely suppressed when the ROS production was inhibited by mannitol, indicating that the ROS secreted by the cyanobacterium played an important role in the electricity generation of such a biocathode MFC. This work demonstrates that the ROS released by cyanobacteria can enhance electricity generation in MFC.