PORTFOLIO

Recently certain microalgae have been discovered to form intracellular mineral inclusions called micropearls. They highly concentrate specific polluting elements (e.g. Sr and Ba) opening new possibilities to develop bioremediation techniques. This could be useful for example for waters polluted by radioactive strontium, which are produced in high quantities by accidents such as Chernobyl or Fukushima, but is also known as a major contaminant found in wastewater and sludges linked with nuclear activities. Organisms preferentially concentrating barium could be used for waters polluted by barium, linked to sludge deposits due to oil production.

These micropearls were first observed in Lake Geneva. The lake’s low concentration of pollutants uncovered these microalgaes extremely high concentration capacity. Preliminary laboratory tests further showed that these organisms are able to incorporate high concentrations of the polluting elements in only a few days. Moreover, they apparently survive in waters containing high quantities of the element they concentrate. Altogether these capacities elect these micropearl-forming organisms as very good candidates for the development of bioremediation methodologies.

Although the use of Chlorodendrophyceae for the removal of radioactive 90Sr in the environment had been suggested several times, its capacity to absorb this radionuclide and the effect of the ionizing radiation on cell viability had never been tested. We demonstrated that Tetraselmis chui cultured in seawater with concentrations of 90Sr similar to those measured in the environment after a nuclear accident shows a high 90Sr absorption by this organism. The 90Sr removal from the growth medium was correlated to the cell density of the cultures. The growth of T. chui cultures was further optimized in bubble column photobioreactors using seawater amended with natural Sr, allowing to reach cell densities higher than 1.5x 106 cells mL-1 and showing an almost complete removal of Sr in less than one week of growth. Moreover, the addition of CO2 to the culture accelerated both T. chui growth and the Sr absorption by the cells. Since this type of photobioreactors can be reproduced on a very large scale, the suitability of this culture system for the implementation of new bioremediation techniques for 90Sr decontamination is clear.

This research project addressed key questions concerning the environmental and internal conditions in which certain microorganisms form intracellular mineral inclusions and concentrate polluting elements. Micropearls are a totally new feature in biology. Although some are produced by well-studied organisms (e.g. Tetraselmis cordiformis), they had been overlooked until recently. The mineralogy of micropearls is innovative in itself, as they show internal nanoscale concentric zonations of certain elements such as Sr and Ba which have never been observed in amorphous minerals before. Furthermore, the biological process leading to the biomineralization of micropearls has no known equivalent. To address this totally new topic, we used an interdisciplinary approach that is a cross-over between geology, chemistry, biology and environmental sciences. We explored the biological processes underlying micropearl formation in order to pave the way of possible bioremediation applications that could be of great societal benefit in particular concerning radioactive Sr.

In collaboration with the Laboratory of Nuclear Physics and Chemistry of the Haute École du Paysage, D'ingénierie et D'architecture de Genève, we have demonstrated that Tetraselmis chui presents high 90Sr uptake capacities, confirming its suitability for bioremediation applications to treat environments contaminated with this radionuclide. We performed several experiments with T. chui cultures containing the radioactive isotope 90Sr and we measured its uptake rate by T. chui cells. We developed an adapted technique to measure this radionuclide in the growth medium of the cultures using liquid scintillation counting. We have successfully sequenced the complete genome of T. chui and made its respective annotation. This achievement is of great relevance as it has never been done before and will allow the identification of genes related to micropearl formation. For that purpose, we have performed a RNA differential expression study of T. chui cultures grown under different culture conditions. The results obtained so far are promising and will be published shortly.

Thanks to the collaboration established with the laboratory of Marine biotechnology of The Algarve Centre of Marine Sciences (CCMAR-Algarve, Portugal), we optimized T. chui growth in 1L-bubble column photobioreactors and obtained a Sr (natural isotope) removal efficiency from seawater of 100%. Moreover, we discovered that CO2 injections accelerate algae growth and improve Sr uptake by the cells. These results are promising since this type of photobioreactor can be reproduced on a large/industrial scale and could be used simultaneously for atmospheric CO2 mitigation. T. chui has proven to be a robust species, able to reach high cell densities in environments with different salinities and pH, which is an essential requirement for an organism to be used as a bioremediation agent. Our results also suggest the use of other species within the genus Chlorodendrophycae for bioremediation purposes regarding 90Sr pollution since it has been shown that Sr uptake is related to micropearl formation (Segovia et al, 2022).

Martignier A., Pacton M., Filella M., Jaquet J.-M., Barja F., Pollok K., Langenhorst F., Lavigne S., Guagliardo P., Kilburn M.R., Thomas C., Martini R. and Ariztegui D. (2017) Intracellular amorphous carbonates uncover a new biomineralization process in eukaryotes. Geobiology 15: 240-253.
Martignier, A., Filella, M., Pollok, K., Melkonian, M., Bensimon, M., Barja, F., Langenhorst, F., Jaquet, J.M., Ariztegui, D. (2018) Marine and freshwater micropearls: Biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the genus Tetraselmis (Chlorophyta). Biogeosciences, 15, 6591-6605.
Martignier, A., De Respinis, S., Filella, M., Segovia, I., Marin, B., Günther, G., Barja, F., Tonolla, M., Jaquet, J.-M., Melkonian, M., Ariztegui, D. (2020): Biomineralization capacities of Chlorodendrophyceae: correlation between chloroplast morphology and the distribution of micropearls in the cell. Protist 171:125760.
Martignier A., Jaquet J-M., Filella M., Pollok K., Langenhorst F., Coster M., Ariztegui D. (2022) First observation of unicellular organisms concentrating arsenic in ACC intracellular inclusions in lake waters. Geosciences.
Segovia-Campos I., Martignier A., Filella M., Jaquet J-M., Ariztegui D. (2021) Micropearls and other intracellular inclusions of amorphous calcium carbonate: An unsuspected biomineralization capacity shared by diverse microorganisms. Environmental Microbiology (doi:10.1111/1462-2920.15498);
Segovia-Campos I., Filella M., Perron K., Ariztegui D. (2022) High calcium and strontium uptake by the green microalga Tetraselmis chui isrelated to micropearl formation and cell growth, Environmental Microbiology Reports;
Segovia-Campos I. (2022) Investigating the micropearl formation process in the green microalgae Chlorodendrophyceae and their potential application in novel bioremediation techniques. Unpublished doctoral thesis, Department of Earth sciences, Faculté des sciences, University of Geneva, 191 pp.