Microbial Solutions in Bioremediation of Radioactive Waste

We read about radioactivity and its possible hazards in the last article titled Radioactive pollution: a never ending hazard. We also learnt about a bacterium Deinococcus radiodurans and its ability to survive high intensity radiation, which is otherwise lethal to all life forms. This article features the applications of the strain in bioremediation of radioactive waste as well as sites. In 1956 a can of meat which had been sterilized by high doses of X-rays was found to be spoilt. On culturing, it was discovered that the contaminant was a red-pigmented bacterium and was named Deinococcus radiodurans. This is currently the most radioactive-resistant life form known on Earth. It has tremendous ability to withstand high doses of radiation and hence it is being used for radioactive waste clean-up initiative funded by the US Department of Energy (DOE). Read: Bioremediation – The Easy Way Out Of Chaos Since D. radiodurans does not possesses the ability of degradation of toxic compounds that are leached from radioactive sites, genetically engineered variants of the strains can perform the function. Degradative genes from Pseudomonas could be easily introduced in this strain which will make it more robust in bioremediation. Below are the types of mechanisms through which microbes can be used in remediation of such radioactive wastes:
  1. Indirect enzymatic reduction of radionuclides: In this process, bacteria reduce the metals available in the surrounding and the by-products formed help in chemical reduction of the radionuclides.
  2. Direct enzymatic reduction of radionuclides: Soluble forms of radionuclides are converted to insoluble precipitates by the action of reductases and hydrogenases (two different class of enzymes). This helps in preventing contamination of water and other such sources.
  3. Biomineralization of radionuclides: Chelating agents which are used to precipitate nuclear wastes are naturally formed by some bacteria. Citrobacter is known to produce citric acid which acts as a chelating agent in decontamination of nuclear waste.
  4. Genetically modified micro-organisms: Recombinant strains having supreme capabilities of degradation of environmental pollutants are being created by gene modifications. Such strains are capable of sustaining in hazardous conditions and bioremediate the sites.
  5. Biosorption and bioaccumulation: Positively charged metals and radionuclides are attracted towards negatively charged surface receptors and cell organelles of bacteria. Further the sequestered radionuclides are precipitated by slime, capsule and EPS which are produced by the micro-organisms.
  6. Biostimulation:In areas which are highly contaminated with heavy metals and radionuclides, introduction of microbes having synergistic action speeds up the process of remediation. Similarly introduction of diverse micro-flora in the system enhances capacity of microbes by natural transformation, thus helping in acclimatisation of the remediation site.
As the above mentioned techniques are useful in remediation, new technologies can be developed by conjugation of many techniques together. The U.S Department of Energy has already applied recombinant strains of D.radiodurans in nuclear contaminated sites and have witnessed positive results. Lastly, the use of these microscopic tiny creatures will be a boon for reclamation of the environment. Sources: http://large.stanford.edu/courses/2016/ph241/chang2/ K. Pollmann et al., “Metal Binding by Bacteria From Uranium Mining Waste Piles and Its Technological Applications,” Biotechnol. Adv. 24, 58 (2006). S. K. Kazy, S. F. D.Souza, and P. Sar, “Uranium and Thorium Sequestration by a Pseudomonas sp.: Mechanism and Chemical Characterization,” J. Hazard. Mater. 163, 65 (2009). E. J. P. Phillips, E. R. Landa, and D. R. Lovley, “Remediation of Uranium Contaminated Soils with Bicarbonate Extraction and Eicrobial U(VI) Reduction,” J. Ind. Microbiol. 14, 203 (1995). H. Brim et al., “Engineering Deinococcus radiodurans For Metal Remediation in Radioactive Mixed Waste Environments,” Nat. Biotechnol. 18, 85 (2000).