Mechanisms Of Extracellular Electron Transfer In Shewanella Spp. And Their Applications

Dissimilatory Metal-reducing Bacteria (DMRB) are a class of bacterial that can transfer electrons extracted from organic carbon substrates to extracellular electron acceptors such as metal oxides. DMRB with the unique capability of extracellular electron transfer (EET) can readily use insoluble metal oxides and solid electrodes as electron acceptor under anaerobic conditions, resulting in a significant role in geochemical cycle of metal elements and electricity recovery from environmental pollutants. The dissimilatory metal reduction process is also actively involved in the transformation of varies types of environmental organic pollutants in environments.DMRB showed a promising potential in remediation of various special contaminated environment. Therefore, as a type of versatility functional bacteria, DMRB are becoming a research focus in the fields of microbiology, electrochemistry and environmental engineering and science, etc. To extend the applications of DMRB in environmental remediation, the roles and mechanisms of EET in the conversion of environmental pollutants should be further understood to regulate and tune this process readily. In this work, several EET relevant processes of Shewanella spp., a model DMRB in environmental remediation were explored. The roles and mechanisms of EET in transformation of roxarsone, an organoarsenic chemicals, were investigated. Also the Cr isotope fractionation were examined to gain insights into the Cr(VI) reduction by DMRB. Furthermore, the EET performance under suppressed riboflavin-biosynthesis levels was also evaluated in electrochemical systems. These works will provide useful information on extending the applications of DMRB in environmental remediation. The main research contents and experimental results are listed below:1. Extracellular electron transfer of Shewanella oneidensis MR-1 regulated by riboflavin biosynthesis. Disruption riboflavin biosynthesis was performed through constructing a ribBA mutant. Compared with wild type, ?ribBA exhibited a 50%lower level of flavins production. While current production of ?ribBA was approximately 1-fold higher than that of the wild type in an electrochemical cell. Both the cyclic voltammetry and differential pulse voltammetry analyses further confirm that a larger catalytic current was produced by ?ribBA.The expressions of genes for the Mtr respiratory pathway in the AribBA strain were slightly changed compared to the wild type, while the expressions of cymA and omcA were up-regulated by 1.7- and 1.8-fold, respectively. Moreover, the expression of the periplasmic protein-FccA and the paralogues of mtr A and mtrB increased. These results suggest that the up-regulated expression of paralogues of the Mtr pathway and the periplasmic proteins enhanced EET in AribBA. More electrons are transferred from the cell interior to the outer membrane and finally to the electrode through the Mtr pathway and the paralogues of Mtr pathway.This work gives a new insight into the plasticity of EET and also provides a feasibility of tuning the microbial electron transfer capacity,and thus has great significance for advancing the fundamental knowledge and applications of DMRB as important environmental microbial species.2. Anaerobic transformation mehcanisms of roxarsone by Shewanella putrefaciens CN32.Bioreduction of roxarsone by S. putrefaciens CN32 was examined,and its transformation products and mechanisms underpinning this process were also investigated. The degradation pathways were clarified by using S. putrefaciens CN32 as the model. The knockout of undA/mtrC led to a 70% loss of the roxarsone bioreduction ability within the initial 48 h. Both extracellular and intracellular reductions occurred simultaneously, resulting in the production of As(?) as the main inorganic arsenic species. Adding anthraquinone 2,6-disulfonate as a mediator considerably increased the roxarsone reduction rate by 119%. The rapid anaerobic reduction of roxarsone by several EEB was also confirmed. Given the wide distribution of EEB in environments, our findings facilitate a better understanding of the transformation behaviors of arsenic compounds in natural environments and highlight the necessity of re-evaluating the environmental risks of roxarsone.3. Mechanisms of Cr isotope fractionation during Cr(?) reducing by Shewanella oneidensis MR-1 .The OmcA-MtrC protein complex has been identified as the terminal reductase during extracellular Cr(VI) reduction in S.oneidensis MR-1. The relationship between bacterial respiratory pathway and Cr isotope fractionation was studied by using Shewanella oneidensis MR-1 wild type and ?omcA/?mtrC mutant strain. The reduction rate by WT was 11.12±0.38 ?M Cr(?) h-1 (mean ± standard deviation;n = 2) at the first 4 h,while that by mutant strain was 4.91 ± 0.48 ?M Cr(?) h-1 (n = 2) (i.e., 44.2% of that of the wt). The cellular locations of reduced precipitates were determined with TEM. Extracellular matrix-associated precipitates were found on the cell surface of WT after reduction of 100 ?M Cr(?) for 30 h.While Cr element was detected in the cell interior of ?omcA/?mtrC. The results of Cr isotope fractionation analysis showed that the magnitude of isotope fractionation (?)for Cr(?) reduction by WT and ?omcA/?mtrC were -2.42 ± 0.68 ‰ and -2.70 ±0.22 ‰, respectively. The e values surprisingly showed no significant difference between two strains, suggesting isotope fractionation is independent of Mtr pathway during Cr(?) reduction by S. oneidensis MR-1. Cr isotope fractionation during microbial reduction processes is commonly recognized as a promising tool in biogeochemistry and bioremediation. A better understanding of the mechanism in Cr isotope fractionation during microbial reduction was gained.