Groundwater is a critical resource - providing water for drinking, industry and agriculture, as well as sustaining surface waters, wetlands, and the ecosystems they support. Accumulation of toxic metals in water supplies is an extremely serious environmental problem and a costly one to remediate.
As part of their natural activity, some bacteria can detoxify or immobilize heavy metals. To enable environmental engineers to design more suitable clean-up technologies for contaminated sites, researchers at the University of California, San Diego (UCSD) are studying the mechanisms used by bacteria to detoxify environmental contaminants. Dr. Brad Tebo leads a team of scientists studying two different biological strategies to reduce the toxicity of heavy metals:
Direct biological transformation of toxic metals into a less toxic form. Specifically, the UCSD researchers are investigating the reduction of the mutagenic and carcinogenic form of chromium, hexavalent chromium (Cr(VI)), to the less toxic Cr(III).
Scavenging and immobilization of toxic metals on the surface of highly reactive, enzymatically produced, metal oxides. Specifically, Tebo's group is investigating the mechanisms involved in bacterial transformation of soluble manganese (Mn(II)) to the insoluble oxides Mn(III) and Mn(IV). Mn oxides are strong oxidants, and have negatively charged surfaces and high surface areas. As a result, heavy metals such as arsenic, cobalt, copper, lead, nickel, and zinc can either be adsorbed onto the surface of the oxides or incorporated into their crystal lattice - effectively locking them in the solid matrix of soils and sediments and removing the metals from the water.
The UCSD research is focused to increase our understanding of the processes by which bacteria are able to carry out these reduction/oxidation reactions and to identify specific genes and proteins responsible for the biochemical activity.
The UCSD Cr(VI) reduction studies are concentrated on isolating and characterizing Cr(VI) reducing bacteria from contaminated waters, soils and sediments, and examining the biochemical basis for Cr(VI) reduction. Not only will identification of these mechanisms provide valuable information for the development of bioremediation processes, but Dr. Tebo believes that identification of the genes and proteins expressed in response to Cr(VI) could lead to the development of biomarkers of environmental Cr(VI) exposure. Recently, the UCSD researchers:
Identified two proteins that are specifically expressed in response to Cr(VI) by aerobically grown cells of the Cr(VI) reducing bacterium, Shewanella.
Identified conditions under which Shewanella is able to reduce Cr(VI) at chromium concentrations that are usually toxic to bacteria. They determined that when Cr(III) is bound into solution as it being produced, the ability of the bacterium to withstand toxic levels of chromium is greatly increased. They are continuing to study this trend and are trying to determine if the compound that binds to Cr(III) is of biological origin.
Manganese cycling between its solid and soluble forms is largely driven by microbial activities, and the UCSD researchers are working to discover the mechanism of Mn(II) oxidation. They have demonstrated that the protein involved in Mn(II) oxidation is one of a family of multi-copper oxidase proteins. In addition, they have:
Isolated a number of spore-forming bacteria in coastal marine sediments that produce dormantspores which enzymatically oxidize soluble Mn(II) to insolubleMn(IV) oxides. These are the first active Mn(II)-oxidizingenzymes identified in spores or gram-positive bacteria and this is also the first enzyme of its kind identified in a marine bacterium. This work suggests that the commonly held view that bacterial sporesare merely inactive structures in the environment should berevised.
Determined that the Mn(II)-oxidizing enzyme (Mn(II) oxidase) is also capable of oxidizing certain phenolic compounds. This suggests that the enzyme could play a role in strategies for dual remediation of organics and metals. The researchers are currently devising a means to produce sufficient quantities of the Mn(II) oxidase to study the biochemical details of Mn(II) oxidation.
Heavy metal contamination is widespread in the U.S., threatening water supply sources as well as pristine watersheds. Harnessing naturally occurring processes for the purposes of remediation is a far more cost-effective proposition than physically removing non-degradable contaminants such as heavy metals. Stabilizing and transforming toxic metals to environmentally benign forms is currently being explored at a variety of Superfund sites with some success. A detailed scientific study of the mechanisms involved at the molecular level will help refine the current strategies and render them more effective.
For More Information Contact:
Bradley M. Tebo
Department of Environmental and Biomolecular Systems OGI School of Science and Engineering Beaverton, OR 97006 Tel: 503-748-1992 Email:
To learn more about this research, please refer to the following sources:
Francis, Chris M. and Bradley M. Tebo. 2002. Enzymatic manganese(II) oxidation by metabolically dormant spores of diverse Bacillus species. Applied and Environmental Microbiology. (http://aem.asm.org/) 68(2):874-880.
Obraztsova, Anna Y., Chris M. Francis, and Bradley M. Tebo. 2002. Sulfur disproportionation by the facultative anaerobe Pantoea agglomerans SP1 as a mechanism for chromium(VI) reduction. Geomicrobiology Journal. (http://www.informaworld.com/smpp/title~conten t=t713722957~db=all) 19(1):121-132.
Francis, Chris M., Edgie-Mark Co, and Bradley M. Tebo. 2001. Enzymatic manganese(II) oxidation by a marine ±-proteobacterium. Applied and Environmental Microbiology. (http://aem.asm.org/) 67(9):4024-4029.
68(2):874-880. 
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