Abstract
Abstract. Unicellular algae play important roles in the biogeochemical cycles of numerous elements, particularly through the biomineralization capacity of certain species (e.g., coccolithophores greatly contributing to the “organic carbon pump” of the oceans), and unidentified actors of these cycles are still being discovered. This is the case of the unicellular alga Tetraselmis cordiformis (Chlorophyta) that was recently discovered to form intracellular mineral inclusions, called micropearls, which had been previously overlooked. These intracellular inclusions of hydrated amorphous calcium carbonates (ACCs) were first described in Lake Geneva (Switzerland) and are the result of a novel biomineralization process. The genus Tetraselmis includes more than 30 species that have been widely studied since the description of the type species in 1878. The present study shows that many other 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 among 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 Sr over Ca (the Sr∕Ca ratio is more than 200 times higher in the micropearls than in the surrounding water or growth medium). This concentrating capacity varies among species and may 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 takes place in different environments (marine, brackish and freshwater) and is therefore a widespread phenomenon.
Highlights
The biogeochemical cycles of numerous elements are influenced by the biomineralization capacities of certain unicellular organisms
The present study shows that many other 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
Amorphous calcium carbonate (ACC) is an important actor in the biogenic carbonate cycle because it is a frequent precursor of calcite, as many organisms use amorphous calcium carbonates (ACCs) to build biominerals with superior properties (Albéric et al, 2018; Rodriguez-Blanco et al, 2017)
Summary
The biogeochemical cycles of numerous elements are influenced by the biomineralization capacities of certain unicellular organisms. This is the case, for example, of the coccolithophores, which play an important role in the carbon cycle through their production of biogenic calcite (Bolton et al, 2016). Amorphous calcium carbonate (ACC) is an important actor in the biogenic carbonate cycle because it is a frequent precursor of calcite, as many organisms use ACC to build biominerals with superior properties (Albéric et al, 2018; Rodriguez-Blanco et al, 2017). The precipitation of calcium carbonate in microbial mats, the Earth’s earliest ecosystem, starts with an amorphous calcite. Martignier et al.: Marine and freshwater micropearls gel (Dupraz et al, 2009), and the formation of ACC inside tissue could make coral skeletons less susceptible to ocean acidification (Mass et al, 2017)
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