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The Plants, Stress & Metals team

Characterization of the micro-algae Coccomyxa actinabiotis

Published on 24 November 2016



Coccomyxa actinabiotis, a Microalga Extremely Resistant to Ionizing Radiation and to Metals: Fundamental and Applied Aspects.
We characterized the properties of a new microalga discovered in a high ionizing radiation nuclear environment. This alga was isolated from a pool used to store spent fuel elements in a nuclear reactor. The new species, identified at the morphological, biochemical and genomic level, belongs to the Coccomyxa genus and was named Coccomyxa actinabiotis, from the characteristics of the place it lives in (Rivasseau et al. 2010; 2013). Its radioresistance evaluated using physiological and metabolic analyses is outstanding for a eukaryote. C. actinabiotis withstands huge ionizing radiation doses, up to 20 000 Gy. Half the population survives 10 000 Gy, which is comparable to the hyper-radioresistant well-known prokaryote Deinococcus radiodurans. The cell metabolic profile investigated by nuclear magnetic resonance was hardly affected by radiation doses of up to 10 000 Gy. Its ability to fix radionuclides and toxic metals has also been examined. This outstanding microalga strongly accumulates radionuclides. As an example C. actinabiotis is able to concentrate 110mAg and 60Co by a factor 450 000 and 40 000, respectively. Therefore, this microalga combines both properties of extreme radioresistance and radionuclide accumulation, being able to fix radionuclide via metabolically inactive and active processes even in a highly radioactive environment, which is particularly interesting for 14C decontamination. It is therefore an excellent candidate for new remediation solutions in a highly radioactive environment. Its use for the bio-decontamination of radionuclides on a real-scale was validated in the storage pool of a nuclear facility (Rivasseau et al. 2013). Indeed, in one hour, the microalga revealed as effective as the conventional physico-chemical ion exchangers to purify nuclear effluents with 85% of decontamination in 24 h. Using this organism, an efficient real-scale radionuclide biodecontamination process was performed in a nuclear fuel storage pool with an important reduction of waste volume compared to the usual physico-chemical process. The feasibility of new decontamination solutions for the nuclear industry and for environmental clean-up operations was demonstrated.
We have also undertaken the study of the mechanisms involved in the accumulation and detoxification of two toxic metals, silver and cobalt (Leonardo et al. 2014). Elemental distribution inside C. actinabiotis cells was determined using synchrotron radiation nano X-ray fluorescence spectroscopy (SR-nXRF) at the ID22 nano fluorescence imaging beamline of the European Synchrotron Radiation Facility. The high resolution and high sensitivity of this technique enabled the assessment of elemental associations and exclusions in subcellular micro-algae compartments. A quantitative treatment of the scans was implemented to yield absolute concentrations of each endogenous and exogenous element with a spatial resolution of 100 nm and compared to the macroscopic content in cobalt and silver determined using ICP-MS. The nXRF imaging was complemented by TEM-EDS, yielding differential silver distribution in the cell wall, cytosol, nucleus, chloroplast and mitochondria with unique resolution. The analysis of endogenous elements in control cells revealed that iron had a unique distribution; zinc, potassium, manganese, molybdenum, and phosphate had their maxima co-localized in the same area; and sulphur, copper and chlorine were almost homogeneously distributed among the whole cell. The subcellular distribution and quantification of cobalt and silver in micro-alga, assessed after controlled exposure to various concentrations, revealed that exogenous metals were mainly sequestered inside the cell rather than on mucilage or cell wall, with preferential compartmentalization. Cobalt was homogeneously distributed outside of the chloroplast. Silver was localized in the cytosol at low concentration and in the whole cell excluding the nucleus at high concentration. Exposure to low concentrations of cobalt or silver did not alter the localization nor the concentration of endogenous elements within the cells. To our knowledge, this is the first report on element co-localization and segregation at the sub-cellular level in micro-algae by means of synchrotron nXRF.

Conception and development of a bioprocess pilot for the treatment of nuclear waste
The nuclear industry generates radioactive toxics and requires processes for their decontamination inside the facilities themselves and of the effluents released into the environment. Radionuclide decontamination is currently performed using physico-chemical methods. Despite their robustness and efficiency, these methods are expensive, do not remove completely certain elements, especially 14C, one of the main radionuclides released in effluents, and generate large volumes of secondary wastes when applied to environmental contaminations. Biological methods have proved to be efficient and competitive in various industrial applications. However, no viable method is presently available for the bio-decontamination of highly radioactive media. To address this issue we developed a treatment unit based on the use of the photosynthetic micro-alga, Coccomyxa actinabiotis. The technical specification was draft to design the process and construct the pilot unit taking into account the constraints linked to the use of a biological matrix in a nuclear environment. The pilot-scale treatment unit, based on this micro-alga, includes different tasks to ensure the objectives of the process: algae have first to be produced in a growth medium and harvested before ensuring the treatment of the contaminated effluent. The feasibility of these operations is studied at laboratory scale. Operating conditions and monitoring and optimization tools for each step, (i) biomass production, (ii) biomass separation and concentration by microfiltration (Gouvion Saint Cyr et al. 2014) (iii) effluent decontamination of silver-110m, cobalt-60, carbon-14, are sought. Based on the results obtained at laboratory scale, the feasibility of bio-decontamination of radionuclides by the micro-alga at pilot-scale was studied and demonstrated. Through this work, the development of an innovative process has to be considered for the decontamination of liquid effluents from the nuclear industry. This work has confirmed the high potential of algae to ensure the pollutants elimination.