Investigating the potential of intracellular mineral inclusions in microalgae as a novel bioremediation method for radioactive 90Sr water pollution Segovia-Campos, I. 1, Martignier, A. 1, Jaquet, J-M. 1, Barja, F. 2, Filella, M. 1 & Ariztegui, D. 1

1 Section of Earth and Environmental Sciences, University of Geneva, Switzerland 2 Microbiological Unit, University of Geneva, Switzerland

Several unicellular within the order 2. The concentration of dissolved Ca in the growth medium Chlorodendrales have recently been discovered to form decreases at a similar rate as cell concentration increases in intracellular carbonates called Micropearls. These mineral cultures. In addition, once the growing stationary phase is inclusions are composed of possibly hydrated amorphous reached, Ca concentration stagnates. calcium carbonate (ACC) in which other elements such as 2.5 0.4 Sr can also accumulate (Martignier et al., 2018). A B

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1.5 O previously observed in some cyanobacteria (Benzerara et al., 0.1 Control 2014). Very little is known, however, about the molecular contracta 1.0 0.0 0 96 144 192 264 312 360 408 0 96 144 192 264 312 360 408 mechanisms involved in this biomineralization process and T i m e (h o u r s ) Time (hours) the biological function of the inclusions. A) Variation of the concentration of Ca in the culture medium measured with a Ca ion-selective electrode. Blue diamonds: culture medium without inoculum, red squares: culture of T. contracta. B) Growth curve of T. contracta. Because Chlorodendrales are widespread in the aquatic environment and show a strong capacity to concentrate Sr, they may have a larger impact on the geochemical cycle of 3. The addition of up to 5 mM of Sr in the culture medium this alkaline-earth metal than previously thought. These does not affect growth. observations suggest that Chlorodendrales should be considered as potential candidates for developing new bioremediation methods against radioactive 90Sr water pollution.

1. Micropearls are present in the cells throughout the growth cycle and the distribution pattern can change. This suggest that new micropearls are generated continuously.

A B Growth curve of T. chui at different Sr concentrations.

4. The algae show a strong capacity to concentrate Sr when is added at a low concentration in the culture medium. This ability is still present at higher concentrations with a lower efficiency.

0.45 2.5 C D 0.4 A B 0.35 2 0.3 1.5 0.25 0.2 Sr/Ca ratio in the 1 Micropearls Sr/Ca ratio Sr/Ca 0.15 0.1 Sr/Ca ratio in the 0.5 culture medium 0.05 0 0 50 µM 100 µM 1 mM 5 mM SEM images of the evolution of Tetraselmis contracta at different growing phases. Added Sr in the culture medium A) T. contracta immediately after inoculation (0 hours of culture). B) T. contracta after 6 days (144 hours) of culture. C) T. contracta after 11 days (264 hours) of culture. D) T. EDX analyses of T. chui micropearls. Comparison of the Sr/Ca ratio of the inclusions contracta after 17 days (408 hours) of culture. (green) and the initial Sr/Ca ratio in the growth medium (blue).

Martignier, A., Filella, M., Pollok, K., Melkonian, M., Bensimon, M., Barja, F., Langenhorst, F., Jaquet, J.M. and Ariztegui, D. (2018). Marine and freshwater micropearls: Biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the This project is supported by the MICROBIALS initiative of the genus Tetraselmis (). Biogeosciences, 1-22. Gebert Rüf Stiftung Foundation, Basel, Switzerland.

Benzerara, K., Skouri-Panet, F., Li, J., Férard, C., Gugger, M., Laurent, T., ... & Margaret-Oliver, I. (2014). Intracellular Ca-carbonate biomineralization is widespread in cyanobacteria. Proceedings of the National Academy of Sciences, 111(30), 10933-10938.