| Systematic experiments were performed to examine the inhibitory effects of oxygen on perchlorate reduction and to identify the components and branches in the electron transport chains involved in perchlorate reduction and aerobic respiration by Dechlorosoma sp. KJ (ATCC strain BAA-592). To investigate the effect of dissolved oxygen (DO) on perchlorate reduction, anaerobically grown cultures of Dechlorosoma sp. KJ were exposed to near-saturated concentration of DO for various periods of time. It was determined that cells exposed to oxygen for more than 12 hours were incapable of reducing perchlorate. Cells exposed to oxygen for less than 12 hours, when re-introduced to anoxic conditions, quickly reduced the redox potential to highly negative values (-127 mV to -337 mV) and were able to reduce perchlorate or chlorite. This result suggested that aeration during backwashing of biofilm reactors, or exposure of perchlorate-degrading cell suspensions to oxygen for periods of less than 12 hours, will not be detrimental to the ability of perchlorate-degrading bacteria to use perchlorate as an electron acceptor.;Inhibitors were used to block specific respiratory enzymes, and thus to identify elements of the electron transport chain involved in the reduction of molecular oxygen and perchlorate (Chapters 3 and 4). Inhibition of dissimilative perchlorate reduction by a low concentration of cyanide was due to oxygen build up, not inhibition of enzymes used for perchlorate reduction. The threshold DO concentration to inhibit perchlorate reduction was < 0.04 mg/L. The main ETC used by cells grown on high DO concentrations likely contains cytochrome bc1, cytochrome c and cytochrome aa3. When cells were grown on perchlorate, DO uptake occurred via a shorter electron transport chain (ETC) than that used by cells grown at higher DO concentrations. The ETC to oxygen at low DO concentrations is branched off the quinone to cytochrome bd, while the ETC to perchlorate is branched off the quinone to perchlorate reductase. These results should be useful in helping to improve the design and operation of biological perchlorate treatment systems. |
