Agathe Martignier, Montserrat Filella, Kilian Pollok, Michael Melkonian, Michael Bensimon, François Barja, Falko Langenhorst, Jean-Michel Jaquet, Daniel Ariztegui 

The genus Tetraselmis (Chlorophyta) includes more than 30 species of unicellular micro-algae that have been widely studied since the description of the first species in 1878. Tetraselmis cordiformis (presumably the only freshwater species of the genus) was discovered recently to form intracellular mineral inclusions, called micropearls, which had been previously overlooked. These non-skeletal intracellular inclusions of hydrated amorphous calcium carbonates (ACC) were first described in Lake Geneva (Switzerland) and are the result of a novel biomineralization process.
The present study shows that many Tetraselmis species share this biomineralization capacity: 10 species out of the 12 tested contained micropearls, including T. chui, T.convolutae, T.levis, T. subcordiformis, T. suecica and T. tetrathele. Our results indicate that micropearls are not randomly distributed inside the Tetraselmis cells, but are located preferentially under the plasma membrane and seem to form a definite pattern, which differs between species. In Tetraselmis cells, the biomineralization process seems to systematically start with a rod-shaped nucleus and results in an enrichment of the micropearls in strontium over calcium (the Sr/Ca ratio is up to 219 times higher in the micropearls than in the surrounding water or growth medium). This concentrating capacity varies from one species to the other, which might be of interest for possible bioremediation techniques regarding radioactive 90Sr water pollution.
The Tetraselmis species forming micropearls live in various habitats, indicating that this novel biomineralization process can take place in different environments (marine, brackish and freshwater) and is therefore a widespread phenomenon.