When you imagine a pool of water filled with high concentrations of toxic metal debris and uranium from a nuclear power plant, what do you think of? Do you think lethal, deadly or completely inhospitable?
If so, you would be, for the most part, correct. Such conditions are fatal to nearly every living cell on the planet. However, a recently-isolated algae found in a nuclear material storage site named Coccomyxa actinabiotis is able to not only survive in such conditions, but can even thrive. Such an organism can prove a useful tool in slowing the increasing levels of nuclear waste due to the development of nuclear power.
In our current situation of constantly increasing demand for energy, nuclear power presents a valuable source of electricity: nuclear reactors currently supply the world with 13% of its total electricity, and nuclear energy processes produce virtually no greenhouse gases. With carbon emissions becoming an increasing threat to global temperatures by the year, nuclear power is likely to become a popular alternative to more air-polluting energy production tactics such as coal-burning plants. However, with this
revolution in energy will come a different pollutant in the form of particles known as radionuclides. Radionuclides are high-energy, unstable atoms that are an inevitable product of nuclear fission that takes place in energy-producing nuclear plants1.
Many nuclear power plants utilize a water-based system in which water is run by a reactor to harness the heat energy of the nuclear reactions. In this way, radionuclides leak into the water, which must be removed before the water is reused1. The methods by which this is done (such as precipitation, evaporation, ion-exchange, etc.) are energy-intensive and expensive2. Additionally, any leaks of radioactive material, such as the explosion of the Fukushima power plant in 2011, are dealt with via blunt and inefficient methods, such as manual
washing and concrete isolation of contaminated surfaces3. Therefore, a radionuclide-cleaning technique such as bioremediation presents an appealing alternative to current detoxification processes.
Bioremediation is the use of microorganisms to render hazardous materials harmless. Such a
strategy is cost-effective, allows decontamination of large areas such as bodies of water, and is generally gentle on the environment. This technique is already used in the nuclear industry using organisms such as genetically-modified yeasts, though these microbes require a significant amount of nutrients and are not as efficient at concentrating radionuclides as certain cells that are naturally resistant to radioactivity. Examples of these promising specimens with high natural radiation resistance are mostly cells without nuclei, such as Pyrococcus furiosus or Deinococcus radiodurans. However, the nucleated algae Coccomyxa actinabiotis presents one of the most convenient specimens for bioremediation deployment for a multitude of
factors: it has demonstrated extreme resistance to both radionuclides and heavy
metals, it has natural mechanisms to turn radionuclides into inert substances,
and it is photosynthetic2.
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cells. Note the bright green coloration
indicative of the photosynthetic
chloroplasts in the algae.
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First, what makes environments filled with radionuclides so hostile? In high levels of radiation, very small particles such as gamma or beta rays are emitted from decaying radionuclides. These rays are able to penetrate cells and nuclei where they can fragment and mutate the cell's DNA. Excessive levels of genetic destruction are fatal for the cell. Amazingly, C. actinabiotis is capable of surviving and reproducing in radioactivity of up to 20,000 Gy - a concentration of radionuclides that can shatter glass and plastics2. As a more relatable point of comparison, a full-body dose of radiation that is internally measured at only 5 Gy is lethal to 50% of humans after one month of the initial exposure4.
The methods by which C. actinabiotis and other cells survive this harsh environment are varied, but all depend on maintaining the integrity of the organism's DNA. It is known that C. actinabiotis actively accumulates deadly radionuclides and uses them in certain biochemical pathways, partially detoxifying them and concentrating them within the cell membrane. Currently, however, there is little thorough research on the specific mechanisms by which this resistant algae lives in such intense radiation. Studies on the non-nucleated, radiation-resistant organism D. radiodurans indicate that microbes can use techniques such as enhanced gene repair mechanisms and maintaining multiple genome copies to combat radioactive gene damage. Additionally, supplementary experiments have suggested that C. actinabiotis is
capable of restoring its entire genome in the event of severe radioactive mutation, allowing continued growth and multiplication2.
In addition to radionuclides, C. actinabiotis can also withstand extremely high concentrations of toxic metals such as cobalt and silver. Using a high-resolution imaging technique, it was found that cobalt ions become highly concentrated outside of C. actinabiotis chloroplasts, suggesting that cobalt is possibly used within the chloroplasts to produce nutrients. The imaging also revealed that silver ions are spread evenly throughout the inside of the cell. Silver may or may not be used in C. actinabiotis biochemical reactions, but it is, at the very least, highly isolated from compartments in which it can interfere. Alternatively, silver ions may be bound to other molecules that render it harmless to vital biochemical pathways5.
These properties, when uniquely combined in a single organism, make for a relatively superior tool for cleaning hazardous environments. While species such as D. radiodurans are capable of surviving in high concentrations of radionuclides, they are heterotrophic microbes. This means that they require other microorganisms to feed upon, which is difficult for these cells to accomplish in bioremediation environments when nuclear material kills most, if not all, D. radiodurans prey species6. C. actinabiotis, on the other hand, is a photosynthetic algae. Thus, all it requires to clean toxic substances is light and a carbon source, making it far more applicable in
situations in which bioremediation is necessary. Additionally, most microbes that are resistant to either radioactivity or heavy metals are not resistant to the other: D. radiodurans, while highly radionuclide-resistant, cannot tolerate high toxic metal concentrations. C. actinabiotis is capable of handling high concentrations of both types of
substances simultaneously, giving the organism unparalleled versatility in the detoxification of dangerous materials7. Lastly, most currently used bioremediation species require intensive genetic manipulation in order to possess traits such
as detoxification and resistance6. C. actinabiotis is capable of performing these actions naturally, requiring less lengthy and expensive manipulations of DNA2.
So, finally, how would C. actinabiotis actually be used as a bioremediation tool? Upon being placed on a contaminated surface or in a contaminated body of water, C. actinabiotis will begin absorbing radionuclides and heavy metals at a rapid pace, concentrating and purifying them to a certain extent. Eventually, most of the contaminants will be contained within cell bodies, which can then be gathered and disposed of by incineration or other such disposal methods. Alternatively, trapped heavy metals can be recycled from these algae
cells to be reused in various industries2.
In conclusion, the process of using C. actinabiotis as a nuclear decontamination tool can occur without
genetic manipulation and with only a source of light energy and carbon
atoms. Thus, the end result of such a
treatment is an area cleansed of both dangerous radionuclides and toxic metals
with relatively little energy investment and minimal environmental harm2. No other microorganisms currently known,
nucleated or not, can equal the versatility, resilience and simplicity by which
this novel algae absorbs and concentrates deadly atoms. Acquiring such an efficient technique of
resolving radionuclide pollution can allow nuclear power to completely meet
world energy demands without suffering the consequences of radioactive
contamination.
References:
1. Zinkle, S.J. (2013). Materials
challenges in nuclear energy. Acta Materialia. 61(3). Pgs. 735-758.
Retrieved from
http://www.sciencedirect.com/science/article/pii/S1359645412007987
2. Rivasseau, Corinne, et. al. (2012). An extremely radioresistant green eukaryote for radionuclide
bio-decontamination in the nuclear industry. Energy and Environmental
Science. 6. Pgs. 1230-1239. Retrieved from
https://hal-agrocampus-ouest.archives-ouvertes.fr/hal-00796855/document
3. Brumfiel, Jeff. (2011). Fukushima set for epic clean-up. Nature. 472. Pgs. 146-147.
Retrieved from http://www.nature.com/news/2011/110411/full/472146a.html
4. Caso, C. et. al. (1999). Section 5.4 Radioactivity and Radiation Protection. Review of
Particle Physics. Retrieved from http://xdb.lbl.gov/Section5/Sec_5-4.pdf
5. Leonardo, T. et. al. (2014). Determination of elemental distribution in green micro-algae using
synchrotron radiation nano X-ray fluorescence (SR-nXRF) and electron microscopy
techniques – subcellular localization and quantitative imaging of silver and
cobalt uptake by Coccomyxa actinabiotis. Metallomics. 6. Pgs. 316-329.
Retrieved from
http://pubs.rsc.org.ezp1.lib.umn.edu/en/content/articlepdf/2014/mt/c3mt00281k
6. Misra, Chitra Seetharam et. al. (2012). Recombinant D. radiodurans cells for bioremediation of heavy metals
from acidic/neutral aqueous wastes. Bioengineered. 3(1). Pgs. 2165-5979.
Retrieved from http://www.tandfonline.com/doi/full/10.4161/bbug.3.1.18878
7. Rivasseau, Corinne et. al. (2011). U.S. Patent No. 20130078707.
Retrieved from http://www.google.com/patents/US20130078707
8.
Associated Press. (Mar. 10, 2013). Japan
struggles to clean up after tsunami, nuclear disaster. The Oregonian. Retrieved from http://www.oregonlive.com/today/index.ssf/2013/03/japan_struggles_to_clean_up_af.html
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