Seventy million cubic meters of ground and three trillion liters of groundwater have been contaminated by leaking radioactive waste generated in the United States during the Cold War. A cleanup technology is being developed based on the radiation resistant bacterium Deinococcus radiodurans that is being engineered to express bioremediating functions.
In 1992, the DOE surveyed 91 out of 3,000 contaminated sites at 18 US research facilities. The most common contaminants from DOE wastes that have been found in ground and groundwaters include the radionuclides 235Uranium (gamma, alpha)E, 238Plutonium (alpha)E, 99Technetium (beta-)E, 90Strontium (beta-)E, and 137Cesium (gamma, beta-)E; and the metals, Chromium, Lead and Mercury; and myriad toxic organic compounds (e.g., toluene and trichloroethylene (TCE)). One third of the 91 characterized sites are radioactive with some reported radiation levels as high as 10 mCi/L, within or close to the contaminating sources. These high radiation levels in combination with the chemical hazards are extremely damaging to living organisms over extended periods, often resulting in cell death.
Of the 3,000 waste sites disclosed by the DOE, the total cleanup cost, by methods that utilize costly pump and treat technologies and/or soil excavation and incineration, was estimated recently between $189 and $265 billion (Internet: http://www.em.doe.gov/bmr96; The 1996 baseline management report). DOE budget projections for cleanup activities for the next ten years exceed $60 billion. These vast waste sites are, thus, potential targets for less expensive in situ bioremediation technologies utilizing specialized microorganisms that can detoxify both metallic and organic contaminants. However, the utility of microbiological methods for the primary treatment of highly radioactive environmental wastes will largely be determined by 1) the ability of microorganisms catalyzing the desired function(s) to survive and function under radiation stress; and 2) the ability of basic research to produce bioremediation systems that do not cause undesired secondary effects that threaten the general public or further damage the environment.
Numerous bacteria (including Shewanella and Pseudomonas spp.) have been described and studied in detail for their ability to transform, detoxify, or immobilize a variety of metallic and organic pollutants. Like most organisms, however, these bacteria are sensitive to the damaging effects of radiation, and their use in bioremediation likely will be limited to environments where radiation levels are very low. Therefore, radiation resistant microorganisms that can be used for environmental cleanup need to be found in nature or engineered in the laboratory to address this problem.
The bacterium D. radiodurans shows remarkable resistance to a range of damage caused by ionizing radiation, desiccation, ultraviolet radiation, oxidizing agents, and electrophilic mutagens. It is an aerobic, tetrad-forming soil bacterium that is most famous for its extreme resistance to ionizing radiation; it not only can survive acute exposures to gamma radiation that exceed 15,000 Gy without lethality or induced mutation, but it can also grow continuously in the presence of chronic radiation (60 Gy/hour) without any effect on its growth rate or ability to express cloned genes. For comparison, an acute exposure of just 5-10 Gy is lethal to the average human. Adding to the growing resource of genetic technologies available for D. radiodurans is the recent whole-scale sequencing, annotation and analysis of its genome. This combination of factors has positioned D. radiodurans as the most promising candidate for the development of microbiological treatments of radioactive environments. D. radiodurans shows remarkable genome plasticity. It is able to maintain, replicate and express extremely large segments of foreign DNA inserted into its genome by tandem duplication. This capability has been exploited recently to show that it can accommodate and functionally express highly amplified DNA duplication insertions encoding bioremediation functions. This strain supports >2,000,000 bp of foreign DNA and it can metabolize toluene or chlorobenzene while at the same time resisting and reducing toxic ionic Hg(II) to volatile elemental Hg(0). Therefore, there is good prospect for introducing into a single D. radiodurans host the many different bioremediating gene systems that will be necessary for cleanup of heterogenous radioactive waste environments.