Recycling Uranium and other Rare-Earth Elements in Contaminated Waters Using Bacteria and Microalgae: Adaptation, Capture, Use and Bioremediation

Dear Colleagues,

Environments with extreme anthropogenic contamination are interesting sites for the study of evolutionary pathways, rapid adaptation, biodiversity and extremophile bioprospecting. The extreme novelty of these anthropogenic environments favours rapid adaptative radiations of facultative extremophiles. Microorganisms capable of quickly adapting to this extreme contamination also have an interesting practical application in the bioremediation of highly polluted places.

Dramatic examples of human-generated polluted environments are uranium (U) mining and milling areas, nuclear waste accumulation sites, accident zones at nuclear power plants and nuclear test sites. In addition to uranium contamination, these places often have other extreme environmental conditions that make it even more difficult for microorganisms to adapt, such as acidic or alkaline pH, other heavy metals and other radioisotopes.

At least two groups of microorganisms (i.e., bacteria and microalgae) are capable of adapting to these highly uranium-contaminated sites. Since most of these places were contaminated by uranium after 1945, it is evident that these organisms are able to adapt very quickly to this extreme contamination.

In most cases, physiological acclimatization is unable to achieve adaptation to these environments. In contrast, rare spontaneous mutations that occurred prior to the exposure to waters contaminated by uranium have allowed rapid adaptation to these extreme conditions. Adaptation to the most extreme conditions is also provided by recombination through sexual reproduction, because adaptation often requires more than one mutation. Microorganisms living in extreme environments could be the descendants of preselective mutants that had significant adaptive value to uranium contamination. These “lucky mutants” could allow for the evolutionary rescue of populations faced with rapid environmental change.

These uranium-resistant organisms can be harnessed for industrial applications. As an example, we isolated a microalgae strain (ChlSP) that inhabits extreme U tailing ponds at the Saelices mining site (Salamanca, Spain), characterized as acidic (pH 3), radioactive (around 4 μSv h⁻¹) and contaminated with different metals, mainly U (from 25 to 48 mg L⁻¹) and zinc (from 17 to 87 mg L⁻¹). After an artificial selection protocol to increase the U uptake, the artificially selected strain (ChlSG) was able to take up a total of ≈6.34 mg U g⁻¹ DB by means of two U-uptake mechanisms: the greatest proportion by biosorption onto cell walls (ca. 90%), and only a very small quantity, ~0.46 mg g⁻¹ DB, irreversibly bound by bioaccumulation.

However, the most surprising feature of these microorganisms is that they are capable of enriching uranium, preferentially capturing one of the isotopes. Relative isotope abundances are often mediated by biological processes, and biologically driven U isotopic fractionation has been identified in bacteria and microalgae. For example, freshwater Chlamydomonas sp. (ChlGS) and marine Tetraselmis mediterranea (TmmRU) took up U while inducing U isotopic fractionation with a preference for the fissile 235U isotope over 238U. The n(235U)/n(238U) isotopic fractionation magnitudes (δ235) were 23.6 ± 12.5‰ and 370.4 ± 103.9‰, respectively.

Resulting from the nuclear fuel cycle, large amounts of depleted uranium (DU) tails are piling up, waiting for possible use or final disposal. These results open up new perspectives on the re-enrichment of DU tailings.

Population growth, scientific advances and wealth have favoured the development of modern technologies, and with it, the use of very different materials (e.g., a modern microchip uses more than 60 different elements).

New approaches are needed since the modern world is dependent on these materials. These must allow the efficient and environmentally friendly recycling of non-renewable elements (i.e., rare-earth elements).

In aquatic habitats, with electronic waste that contains rare-earth elements, bacteria and microalgae can be used as sponges. Biosorption reveals itself as an emerging new biotechnology, being a more effective, cheap and environmentally friendly way to treat residues that contain non-renewable elements. Nowadays, a number of microorganisms (i.e., bacteria, microalgae, yeasts and moulds) are starting to be used as absorbent materials in aqueous solutions.

There are substantial challenges ahead that have to be addressed, and microbial biotechnology could provide great help. Frequently, “the solution is in nature”.

Dr. Victoria Lopez-Rodas
Guest Editor

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