Respiratory enzymes used for Perchlorate Reduction by Microorganisms

Funded by: National Science Foundation, September 7, 2000 - September 6, 2003
Principal Investigator: Bruce Logan, Kappe Professor of Environmental Engineering,
Contact information: Phone: 814-863-7908, Email: blogan@psu.edu

Project Abstract

Perchlorate has been detected in ground waters generally at levels of 50-200 ppb primarily as a result of its production and use as solid rocket propellent. Although there is no current drinking water standard for perchlorate, it has been included on the federal Contaminant Candidate List (CCL), 32 ppb has been proposed as a federal limit, and the current California action guideline is 18 ppb. Perchlorate endangers the drinking water supply of more than 12 million people. Surprisingly, perchlorate can be used as an electron acceptor by many newly isolated strains of bacteria. There is substantial evidence that the biochemical pathway for perchlorate reduction proceeds via ClO4--->ClO3---> ClO2---> Cl-+ O2. In the last step, a chlorite dismutase produces molecular oxygen. This is an interesting biological development because oxygen is a preferred electron acceptor. Under fully aerobic conditions, perchlorate is not reduced even by capable bacteria. Nitrate can also interfere with perchlorate reduction.

Several drinking water, wastewater, and in-situ treatment systems are being developed to biologically remove perchlorate, but there is little ongoing research directed toward the physiology of perchlorate reducing microorganisms (PRMs). Biological perchlorate treatment systems will rely upon maintenance of a functioning PRM population in these engineered reactors. Understanding the respiratory pathways used by bacteria will be important to the long term operation of such reactors. For example, is perchlorate respiration constitutive or induced among capable bacteria? If induced, what factors lead to perchlorate enzyme induction? Does nitrate stimulate perchlorate reduction, or will the presence of high concentrations of nitrate shift anaerobic respiration away from perchlorate-reducing populations toward a solely denitrifying community?

This research project is directed at analyzing the electron transport chain (ETC) used by PRMs to degrade perchlorate. Little is known about the enzymes necessary to complete the reduction of perchlorate to chloride. Many PRMs are a subset of denitrifying microorganisms but not all denitrifiers are PRMs. Denitrifiers partially develop branched respiratory pathways that shuttle electrons to reduced nitrogen forms (NO3-, NO2-, N2O and NO) under anoxic conditions. It is not known whether PRMs use aerobic respiratory enzymes, whether some of the denitrifying enzymes are necessary, or whether other, new respiratory enzymes are used for electron transport. Using respiratory inhibitors we will probe the transfer of electrons along the respiratory chain in PRMs in order to determine how perchlorate and chlorate are used for cell respiration. Through the use of other chemicals, such as chloramphenicol to inhibit protein synthesis, we will determine whether different pathways are inducible or constitutive and what conditions are necessary to maintain perchlorate reducing conditions in bacterial cultures.

Meet the researchers working on this project:

Ms. Yanguang Song (yxs144@psu.edu) is using different respiratory inhibitors to probe which respiratory enzymes are responsible for perchlorate reduction.
Lisa Steinberg M.S. Student (Since 2001) lms346@psu.edu Lisa is working on biological reduction of perchlorate.
Dr. Jianlin Xu Post-doctoral Researcher (since 2001) jxu@engr.psu.edu homepage: www.personal.psu.edu/jux1/ Jianlin is working on perchlorate respiratory pathways, particularly those associated with denitrification. He obtained his Ph.D. at the Ghent University in Belgium.