|Fig. 1: Escherichia coli, an example of a common microbe capable of metal sequestration (Source: Wikimedia Commons)|
In an era of slowly-increasing dependence on nuclear power, there comes an inevitable accompanying increase in generation of radioactive waste from released byproducts of such energy generation. Waste radionuclides generated and released from these power plants (intentionally or accidentally) contribute to the already-present radioactive contamination of the environment triggered by mining and nuclear weapon testing.  Due to the long half-life of many of these hazardous radionuclides, the disposal of these radioactive contaminants presents a challenge to scientists, policymakers, and energy companies around the world. Current efforts to remove and minimize release of radioactive contaminants depend largely on the use of containment tanks or geologic depositories to isolate the toxic substances in hopes of preventing leakage into human environments. The unreasonable land demand of these containment units, however, renders this disposal strategy unfeasible in the long-term goals of expanding the nuclear energy industry. Furthermore, in the unfortunate event that containment fails, current clean-up efforts would require highly expensive and inefficient chemical and enzymatic treatments to concentrate and remove the leaked contaminants.
In the last decade, it was observed that these pools of radioactive waste were a hotbed for several unique strains of radioactive-resistant bacteria capable of performing remarkable metal chemistry on these radionuclides, including the oxidation or reduction of metallic ions analogous to current waste clean-up strategies. As such, the harnessing and development of the chemical capabilities of these low-maintenance organisms may present a feasible, cost-effective waste bioremediation process. In an effort to present an overview of the development of bacterial bioremediation, this report will briefly address: (1) the various chemistry by which microbes are capable of improving metal mobility, (2) the current research in the use of bacteria as bioremediation agents for radioactive waste, and (3) other biological systems with unique chemical faculties and their implication in radioactive waste bioremediation.
Many species of microorganisms are capable of sophisticated metallic chemistry to change the mobilization of metal species in their environment.  There are several ways in which bacteria are capable of increasing the mobility of metals, most of which result in the reduction or oxidation of environmental metals, through which properties of the metal ions - solubility, reactivity, stability, etc. - are altered. The simplest method is the direct reduction or oxidation of metal ions through enzymatic means. For example, several anaerobic strains of bacteria (ex. Geobactor metallireducens, Shewanella putrefaciens) have been cited to possesses iron-reducing enzymes, increasing the solubility of the ion as Fe(III) is reduced to Fe(II).  Some bacteria, such as those in the Thiobacillus class, are also capable of carrying out heterotrophic or autotrophic bioleaching, in which the microbes acidify their environments through the release of protons or organic acids to oxidize metal sulfides into soluble metal sulfates. [4,5] Other more specific methods of increasing metal solubility is through the use of siderophores - highly specific metal ligands capable of binding to iron, magnesium, manganese, and radionuclides such as plutonium to increase their solubility - or through the use of biomethylation, in which the microbes enzymatically transfer a methyl group to the metal ions to increase their volatility or solubility. 
On the other hand, microbes are also capable of decreasing the mobility of metals (i.e. precipitation or crystallization of environmental metals). Similar to how many microbes increase metal solubility, some, such as the uranium-reducing Desulfovibrio desulfuricans, are capable of directly reducing or oxidizing metal ions to decrease their solubility - reducing U(VI) to U(IV) in the case of D. desulfuricans or Cr(VI) to Cr(III), for example. [2,6] Many microbes are also capable of metal uptake, effectively sequestering the metals intracellularly as they localize to internal organelles.  Metal ions may also bind to extracellular components, such as polysaccharides or proteins in the cell membranes: cyanobacteria, for example, are well-known for their ability to entrap metal particles with membrane polysaccharides, while metal-binding proteins, also known as metallothioneins, can be found in common bacteria, such as Escherichia coli. [7,8] While these techniques are not all-inclusive of the types of metal chemical reactions unique to microbes, they nonetheless highlight the wide range of bacterial metal-altering capabilities that can be harnessed for potential environmental bioremediation.
|Fig. 2: Shewanella oneidensis, an organism capable of accumulating Uranium within its cell membrane. (Source: Wikimedia Commons)|
By utilizing the various chemical techniques described above amongst many others, microbes are capable of sequestering radionuclides for more effective removal from the environment. Pollmann et al. describe the capability of Bacillus sphaericus to bind to and accumulate high amounts of radioactive metals.  In this study, it was observed that B. sphaericus thrived in uranium mining waste piles - a toxic environment contaminated by various metal species, including: uranium, copper, lead, aluminum, and cadmium. B. sphaericus was reported to possess a paracrystalline proteinaceous surface, defined as the S-layer, to which these toxic metals were found to localize. Pollmann et al. disclose a filter design utilizing this bacterial S-layer to trap uranium ions as waste water or sediments are passed through the filter. In immobilizing the radioactive uranium species on the S-layer, it becomes much easier to remove large amounts of the radionuclide. Through further chemical treatment, it may even be possible to recover some of the waste uranium for reuse in power plants.
In yet another study on uranium bioremediation, Kazy et al. describe the study of a Pseudomonas strain capable of intracellularly sequestering uranium and thorium ions (a phenomenon known as biosorption), radioactive species commonly found in waste generated by nuclear energy and nuclear weapon testing.  Through transmission electron microscopy, it was observerd that uranium and thorium bind to phosphate groups on the interior face of the bacterial cell wall, resulting in visible peripheral metal deposits weighing between 43-54% of cellular dry weight. It is hypothesized that these radionuclides displace potassium and calcium from their binding sites in the bacterial cell wall. Through this bioaccumulation of radionuclides in the microbial body, it again becomes easier to remove these radionuclides and prevent radioactive leakage of nuclear wastes into ground water.
In addition to naturally-occurring radioactive-resistant bacterial strains, scientists are also looking to engineer strains of bacteria capable of accumulating even higher doses of radioactive species. Brim et al. describe the insertion of an E. coli mercury-resistant gene (merA) into Deinococcus radiodurans, the most radiation-resistant organism currently known, cited as being capable of withstanding 60 Gy/hr of radiation, a dose higher than that found in most radioactive waste sites.  It has been observed that higher radioactive resistance correlates to an ability to convert toxic, volatile metal species into a less reactive, less toxic form. By introducing merA, a gene encoding a mercuric ion reductase, into D. radiodurans, for example, it was found that the organism was, in fact, capable of converting the toxic Hg(II) ions to elemental mercury. It was further demonstrated that multiple remediation strategies can be introduced to a single strain of organism through insertion of various gene clusters; Brim et al. described conferring toluene and chlorobenzene metabolism to D. radiodurans through introduction of other genes, indicating that it is possible to use a single strain of bacteria to clean up multiple contaminants (metallic and organic) associated with nuclear waste sites.
The above-mentioned studies of bioremediation research are just representative examples of the wide range of on-going research in the field of bacterial- assisted nuclear waste bioremediation. Through studies of bacterial species that thrive in radioactively-contaminated sites, scientists are discovering even more resistant organisms capable of absorbing or detoxifying nuclear wastes.
Microbes are not the only organism capable of doing remarkable metal chemistry. In a process dubbed phytoremediation, scientists harness the capabilities of plants to accumulate radioactive species instead. Singh et al. describe the use of fast-growing Vetiver grass (Vetiveria zizanoides) to absorb Sr-90 and Cs-137 from its environment.  In vivo studies indicate that V. zizanoides shoots were capable of removing up to 91% of Sr-90 and 59% of Cs-137 from its hydroponic medium (originally supplemented with 5 × 103 kBq/L of both species) after 168 hours. In a phenomenon very similar to the biosorption of radionuclides in bacterial bodies, V. zizanoides and its capability of accumulating radionuclides in its roots and shoots present yet another feasible and potentially cost- effective nuclear waste treatment possibility. Similar radionuclide accumulation studies have also been done on water hyacinth (Eichhornia crassipes), giant milky weed (Calotropis gigantean), and even Chinese cabbage (Brassica chinensis), to demonstrate the widespread ability of plants to absorb radioactive toxins from the environment. [12-14]
Currently, the use of microbes in bioremediation of nuclear waste remains limited in scope. Even so, the concept of harnessing inherent bacterial chemistry to treat and recover radioactive species presents an incredibly promising method of sequestering and removing radionuclides from the environment. As our dependence on nuclear energy grow with the inevitable increase in energy demand, it becomes crucial that we develop feasible, cost- effective means to prevent toxic radioactive leakage into our living environment. Through further engineering of radiation-resistant bacteria and perhaps even some fast-growing plants, we are presented with a sustainable alternative to current chemical treatments of radionuclides.
© Evelyn Chang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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