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Calcium carbonate mineralization by bacteria

Bacteria capable of mineralizing very large quantities of calcium carbonate

Published on 7 December 2020

2019 04 02 Lac Pavin Camille, Romain préparant la bouteille Niskin.jpgAs part of its research on magnetotactic bacteria1, a team of scientists from the Bioscience and biotechnology Institute of Aix-Marseille (BIAM) has just discovered a bacterium with the particularity of performing two types of biomineralization inside its cell. This discovery is a significant step towards the development of bioremediation methods for radioelements.

It was during a scientific mission in 2015 to Lake Pavin in the Sancy Massif in Auvergne that these bacteria were observed for the first time. Located at an altitude of 1,200 m, this lake has a very special character due to its geology and geochemistry: its waters are rich in iron and it is a meromictic lake, i.e. it is chemically stratified in a permanent way, presenting zones that are totally devoid of oxygen. "These particular conditions seem favorable to the development of certain magnetotactic bacteria in the sediment zones but also in an oxygen-poor water column between 50 and 60 m deep" explains Christopher Lefèvre, coordinator of the study and CNRS researcher at BIAM. "Among the significant diversity present in this lake, bacteria adapted to the atypical conditions of this environment develop there," he continues. "It is through our sampling, but also through the improvement of our observation methods, that we have discovered bacteria that are different from what we know, not only because of their morphology, but also because of their mode of movement and the presence of a double cell compartment.

The scientists then sensed the importance of this discovery. However, in order to unravel their mystery, they had to broaden their scientific collaborations to better equip themselves.

It was by crossing different advanced microscopy techniques that the scientists were able to reveal the nature of these cellular compartments and highlight the microbial biomineralization achieved by this new type of bacteria. Belonging to the class of Alphaproteobacteria, "they are the only known unicellular organisms capable of biomineralizing two types of nanostructures surrounded by membranes: calcium carbonate inclusions and magnetic iron crystals" he continues. In animals and plants, biomineralized minerals have a structural role; in bacteria, on the other hand, the diversity of minerals and their roles are less well known. "This double biomineralization would favour the movement of bacteria in aquatic environments towards the optimal zones for their growth. On the one hand, the magnetic iron crystals would enable them to feel the Earth's magnetic field and explore their environment along the lines of this field rather than moving randomly in three dimensions, while on the other hand the calcium carbonate granules, representing up to two thirds of the cell volume, would act as ballasts facilitating their movement by gravity", comments Caroline Monteil, CEA researcher at BIAM and first author of the study.

No discovery without state-of-the-art equipment
To isolate, describe and identify these bacteria, an armada of approaches and technologies including transmission and scanning electron microscopy, synchrotron radiation, confocal microscopy, single cell sorting and isolation, and single cell genome sequencing had to be crossed, The success of these studies would not have been possible without the support of a multidisciplinary scientific network, composed of several institutes including BIAM as well as IMPMC2 and IPGP3 in Paris.

Today, this discovery raises many questions, particularly about the molecular and biochemical mechanisms responsible for this double biomineralization. According to Christopher Lefèvre, "the genes and metabolism associated with the biomineralization process are not yet elucidated. However, we know that this double mineralization allows these bacteria to sequester in large quantities certain diverse chemical elements such as iron, calcium or phosphorus". According to Dr. Caroline Monteil, "this capacity of accumulation through biomineralization makes these bacteria very promising models for the development of biotechnologies in the field of depollution". Indeed, trace amount of barium and strontium were found in biomineralized carbonates inclusions. "Imagine for a moment that we can use these bacteria to decontaminate environments polluted by radioelements such as strontium or barium radioisotopes?  Here, we have a model that not only could sequester these pollutants, but would also allow their extraction from the environment thanks to their magnetic properties. "For the past two years, the team has been observing other microorganisms in the lake, whose properties could make them valuable allies in biotechnology.

Bactéries magnéto sédiments lac Pavin_1.png©BIAM


1 Magnetotactic bacteria: group of mobile prokaryotes whose direction of mobility is guided by magnetic field lines through the biomineralisation of magnetic iron crystals.

2 IMPMC, Institute of Mineralogy, Physics of Materials and Cosmochemistry.

3 IPGP, Institute of Globe Physics in Paris.

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