Algae could help clean-up nuclear-accident sites

              The 'Mushroom-cloud' formed due to a nuclear explosion (image adapted from)

The dark images and darker consequences of several nuclear disasters like the Chernobyl (April 26, 1986), Kyshtym (September 29, 1957), Three-Mile island (March 28, 1979) and the very-ongoing Japan’s Fukushima Nuclear accident constantly remind us of the Frankensteinian possibilities of nuclear-capability. With world governments heralding nuclear energy as a panacea for all energy needs, there has been a spurt in the proliferation of nuclear technology, capability and of course, nuclear reactors. And as the world clamours for a debate on 'responsible' nuclear usage and simultaneously goes 'nuclear' at a tumbling speed we remain naked under the threatening shroud of a looming nuclear disaster. 

While we recklessly roll towards reactors, our capabilities at cleaning-up a nuclear-spillage or tidying-up post radioactive-leakage remains abysmally poor. Our technology to efficiently dispose off radioactive garbage is juvenile at best, and Nature, again, seems to show the way.

from Manhattan to Moniliferum

A recent paper [1] by Minna krejci's group, a materials scientist at Northwestern University in Evanston Illinois, claims that the common freshwater alga Closterium moniliferum might hold the key to an efficient nuclear-clean-up-act, after humans have messed up! Members of the desmid order, they are unicellular eukaryotic fresh-water alga popularly known for their distinctive crescent shapes measuring ~260 micron in length.

                                             Clostridium moniliferum (source)

These crescent-shaped C. moniliferum have an unusual ability to remove strontium from water, depositing it in crystals that form in subcellular structures known as vacuoles — an ability that could include the radioactive isotope strontium-90. The unicellular desmid green algae are ubiquitous in fresh water habitats and robust in lab in-vitro cultures, and as such are particularly suitable as a model system for Sr/Ba biomineralization [2].

Why is Strontium harmful?

Strontium lies exactly below calcium in the periodic table, and shares similar chemical properties and atomic size with calcium, so biological processes can't easily separate the two elements. That makes strontium-90, which has a half-life of about 30 years [3] a particularly dangerous radio-isotope: it can infiltrate body fluids like milk, and body tissues like bone marrow, bones, blood and others, where the harmful radiation that it emits can eventually lead to metastasis and cancer. It must be recalled that it was primarily Strontium-90 which caused havoc during the Chernobyl disaster of 1986. Unfortunately, reactor waste and accidental spills can contain up to ten billion times more of the harmless calcium than the dangerous strontium, making it extremely difficult to selectively clean up the strontium without also having to dispose of the harmless calcium. 

other methods, which are largely inefficient:

And, in the case of 90Sr, even the most advanced ion-exchange materials find it challenging to efficiently separate out Ca2+, Sr2+, and Ba2+ owing to their chemical similarity [4]. While  phytoremediation approaches utilizing the accumulation of environmental contaminants by green plants are becoming increasingly popular, the effectiveness of such approaches for Sr-90 sequestration are drastically reduced in the presence of Ca2+, due to the indiscriminate transport of Ca2+, Sr2+, and Ba2+ exhibited by most organisms [5].

Why is this alga (Clostridium moniliferum) significant?

This humble fresh-water alga has no particular interest in strontium: it mostly cares for barium. But as strontium happens to be midway between calcium and barium in atomic-size and other properties, so any of it that happens to be around gets crystallized as well. Calcium, as it turns out, even being far more abundant than either of the other two elements, is different enough to barium that it gets left behind, and doesn’t crystallize. The result is a crystal that is chiefly composed of barium, but is heavily enriched in strontium and has no calcium. 

BaSO4 crystals in C. moniliferum. a) Confocal microscopy image showing the lobes of the two chloroplasts (red); cell membrane in green. b) DIC image of BaSO4 crystals (arrow) in the terminal vacuole. c) SEM image of rhombic (arrowhead) and hexagonal (arrow) crystals that remain after cells have been ashed.

[image from paper (1)]

How do they do that?

The mechanism of barium or strontium entry into the organism is not well studied, but it is known that sulphate-rich vacuoles of the alga greatly aid formation of the crystals. Since, barium and strontium have relatively low solubility in sulphate solutions; they easily precipitate out to form crystals of BaSO4 in the sulphate rich small terminal vacuoles at the tips of the crescent-shaped cells. Do the crystals serve any physiological function? It’s not known yet.

The possibilities

> Now that it’s known that the organism actively hunts for barium, it is perhaps possible to enhance the uptake of strontium by tailoring the amount of barium in the algae's environment. 
> It would then be possible to ‘seed’ a spill of radioactive material, with barium to encourage the algae to grab the strontium of the nuclear waste. 
> It might also be possible to improve the process by tinkering with sulphate levels in the environment, thereby changing the amount of sulphate in the vacuoles, but indeed it would depend on an understanding of how cells might respond to altered conditions.

Then what?

Once isolated by the algae, the strontium could be kept in high-level nuclear waste repositories, while the rest of the waste could go to a less expensive lower-level repository, saving space and money. In any case, the hundreds of millions of litres of stored strontium containing nuclear waste in the United States alone.

questions and curiosities

> It’s not yet tested how well the algae survive in the presence of radioactivity. But, since the process begins quickly, wherein cells precipitate crystals within 30 minutes to an hour;  and of course, one can culture as much of the algae as one wants, so even if viability is compromised, they would probably live long enough to start removing strontium!
> There could be concerns regarding passing up of Sr-90 along the food-chain [which could possibly be circumvented by restricting algal-growth within a specific area devoid of its natural predators].

Overall, this definitely appears to be a good idea to start with. Additionally, organismal-sources to sink-off other radioisotopes like plutonium, cobalt, cesium, iodine, etc. needs to be found out to come up with a robust radioactive-clean up regimen.

References

[1] Selective Sequestration of Strontium in Desmid Green Algae by Biogenic Co-precipitation with Barite. Minna R. Krejci, Lydia Finney, Stefan Vogt, and Derk Joester. DOI: 10.1002/cssc.201000448

[2] A. J. Brook, The Biology of Desmids, University of California Press, Berkeley,1981.

[3] M. Eisenbud, T. F. Gesell, Environmental Radioactivity from Natural, Industrial, and Military Sources, Academic Press, San Diego, 1997.

[4] M. Manos, N. Ding, M. Kanatzidis, Proc. Natl. Acad. Sci. USA 2008, 105, 3696; A. Braun et al., Application of Ion Exchange Processes for the Treatment of Radioactive Waste and Management of Spent Ion Exchangers, International Atomic Energy Agency, Vienna, 2002. 

[5] S. Singh, S. Eapen, V. Thorat, C. P. Kaushik, K. Raj, S. F. D’Souza, Ecotoxicol. Environ. Saf. 2008, 69, 306– 311.
[6] Nature news article