Modélisation expérimentale de la précipitation des minéraux carbonatés lors de l'activité bactérienne

La minéralisation induite par l'activité microbienne joue un rôle majeur dans le fonctionnement des écosystèmes passés et présents. Cette étude présente les données expérimentales de précipitation de CaCO3 à partir de cultures pures de deux types de bactéries anoxygéniques phototrophiques (APB) : Rhodovulum steppens A-20s haloalcaphilique et Rhodovulum sp. S-17-65 neutrophilique halophilique, ainsi que de cyanobactéries Gloeocapsa sp.. Ces bactéries représentent deux groupes importants d'organismes photosynthétiques depuis les temps les plus anciens jusqu'à nos jours. Dans ce contexte, cette thèse a pour objectif principal de caractériser les processus biologiques de précipitation de CaCO3 et d'évaluer l'existence d'un processus métabolique protégeant les bactéries étudiées contre la minéralisation de carbonates à leur surface. Pour cela, des expériences cinétiques, des mesures de potentiel zêta et d'adsorption de Ca à la surface bactérienne, couplées à des observations par Microscopie Electronique à Balayage (MEB), en Transmission (MET) et des analyses chimiques par émission et diffraction de rayons X (EDX et XRD) ont été menées. Les résultats de cette étude démontrent que les bactéries étudiées prennent une part active dans la précipitation de CaCO3 . Les mesures de potentiel zêta suggèrent l'existence d'un mécanisme de protection de la cellule pour les APB étudiées, basé sur le maintien métabolique d'une charge de surface plus positive à pH alcalin, préservant les bactéries actives de l'adsorption de Ca2+ et de la précipitation subséquente de carbonates à leur surface. L'existence d'un tel mécanisme n'est pas confirmée pour Gloeocapsa sp. Ainsi, deux mécanismes différents de nucléation de CaCO3 peuvent être mis en évidence : un premier mettant en jeu une sursaturation non-spécifique pour les bactéries anoxygéniques phototrophiques (APB), et un deuxième par nucléation spécifique au niveau de la membrane cellulaire pour les cyanobactéries Gloeocapsa sp..Microbially-induced mineralization is considered as one of the main natural processes controlling CO2 levels in the atmosphere and a major structural and ecological player, in the modern and in the past ecosystems. In this study are presented the data of laboratory experimental work on CaCO3 precipitation with pure cultures of two anoxygenic phototrophs bacteria (APB): haloalcaliphilic Rhodovulum steppens A-20s and neutrophilic halophilic Rhodovulum sp. S-17-65; and cyanobacteria Gloeocapsa sp.. These bacteria represent two important groups of photosynthetic organisms in the past and at present time. APB is the oldest microorganism which could be dominant during the anoxygenic period of Earth's life (approximately 4 billon years ago) whereas the origin of oxygen evolving microorganisms (cyanobacteria) is placed at about 3.5 billion years ago as based on oxidation records of the Earth's crust. In modern ecosystems, cyanobacteria are the dominant primary producers. Nonetheless, the potential of APB are abundant in the modern microbial mats and stromatolites and thus may represent a considerable fraction of the standing biomass. However, biomineralization induce by these bacteria has not been thoroughly studied up to now. In this context, the aim of this thesis is to characterize the process of biological CaCO3 precipitation and to assess the existence of metabolic processes protecting studied bacteria against carbonate mineralization on their surfaces. For this, kinetic experiments, SEM and TEM imaging, EDX and XRD analyses, zeta-potential measurements and Ca adsorption into bacterial surface were carried out. The result of this study demonstrates the participation of studied bacteria in CaCO3 precipitation. Zeta-potential measurements suggest the existence of a cells protection mechanism for studied APB, based on the metabolic maintenance of a positive surface charge at alkaline pH, preserving active bacteria against Ca2+ adsorption and subsequent carbonate precipitation on their surfaces. The existence of the same mechanism for Gloeocapsa sp. was not confirmed. Overall, the results of this study show two different mechanisms of CaCO3-nucleation: an unspecific supersaturation by APB and a specific nucleation at the cell wall by cyanobacteria Gloeocapsa sp.

Can Mg isotopes be used to trace cyanobacteria-mediated magnesium carbonate precipitation in alkaline lakes?

The fractionation of Mg isotopes was determined during the cyanobacterial mediated precipitation of hydrous magnesium carbonate precipitation in both natural environments and in the laboratory. Natural samples were obtained from Lake Salda (SE Turkey), one of the few modern environments on the Earth's surface where hydrous Mg-carbonates are the dominant precipitating minerals. This precipitation was associated with cyanobacterial stromatolites which were abundant in this aquatic ecosystem. Mg isotope analyses were performed on samples of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments. Laboratory Mg carbonate precipitation experiments were conducted in the presence of purified Synechococcus sp cyanobacteria that were isolated from the lake water and stromatolites. The hydrous magnesium carbonates nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4(OH)25(H2O)) were precipitated in these batch reactor experiments from aqueous solutions containing either synthetic NaHCO3/MgCl2 mixtures or natural Lake Salda water, in the presence and absence of live photosynthesizing Synechococcus sp. Bulk precipitation rates were not to affected by the presence of bacteria when air was bubbled through the system. In the stirred non-bubbled reactors, conditions similar to natural settings, bacterial photosynthesis provoked nesquehonite precipitation, whilst no precipitation occurred in bacteria-free systems in the absence of air bubbling, despite the fluids achieving a similar or higher degree of supersaturation. The extent of Mg isotope fractionation (?26Mgsolid-solution) between the mineral and solution in the abiotic experiments was found to be identical, within uncertainty, to that measured in cyanobacteria-bearing experiments, and ranges from ?1.4 to ?0.7 ‰. This similarity refutes the use of Mg isotopes to validate microbial mediated precipitation of hydrous Mg carbonate

Biomineralisations en carbonate de calcium chez les métazoaires : tendances macro-évolutives - Défis pour la décennie à venir.

16 pagesInternational audienceCalcium carbonate-based biominerals, also referred as biocalcifications, are the most abundant biogenic mineralized products at the surface of the Earth. In this paper, we summarize general concepts on biocalcifications and we sketch macro-evolutionary trends throughout the history of the Earth, from Archean to Phanerozoic times. Then, we expose five fundamental issues that represent key-challenges in biocalcification researches for the coming decade: the first one concerns the comprehension of the micro- and nano-structure of calcium carbonate biominerals from a mineral viewpoint, while the second one deals with the understanding of the dynamic process of their fabrication. The third one treats the subtle interplay between organics and the mineral phase. The fourth issue focuses on an environmental challenge related to ocean acidification (OA); at last, the diagenetic processes that affect biogenic calcium carbonate mineral constitute the fifth issue.Les biocalcifications, ou biominéraux en carbonate de calcium, sont les minéralisations biogéniques les plusabondantes à la surface du globe. Le présent article montre comment les biocalcifications sont à l’origine de certainsconcepts scientifiques d’importance, et comment elles ont évolué au cours des temps géologiques, de l’Archéen au Phanérozoïque.Cinq défis majeurs y ayant trait sont ensuite identifiés pour les années à venir : le premier vise à comprendrela structure des biocalcifications aux échelles micro- et nanométriques, tandis que le second s’interroge sur leprocessus dynamique de leur formation. Le troisième défi traite des interactions complexes entre constituants organiqueset phase minérale. Le quatrième se focalise sur des questions environnementales cruciales, notamment l’acidificationocéanique. Le dernier défi consiste à comprendre comment les phénomènes diagénétiques et la fossilisationaffectent les biocalcifications dans leur globalité

Microbial and diagenetic steps leading to the mineralisation of Great Salt Lake microbialites.

12 pagesInternational audienceMicrobialites are widespread in modern and fossil hypersaline environments, where they provide a unique sedimentary archive. Authigenic mineral precipitation in modern microbialites results from a complex interplay between microbial metabolisms, organic matrices and environmental parameters. Here, we combined mineralogical and microscopic analyses with measurements of metabolic activity in order to characterise the mineralisation of microbial mats forming microbialites in the Great Salt Lake (Utah, USA). Our results show that the mineralisation process takes place in three steps progressing along geochemical gradients produced through microbial activity. First, a poorly crystallized Mg-Si phase precipitates on alveolar extracellular organic matrix due to a rise of the pH in the zone of active oxygenic photosynthesis. Second, aragonite patches nucleate in close proximity to sulfate reduction hotspots, as a result of the degradation of cyanobacteria and extracellular organic matrix mediated by, among others, sulfate reducing bacteria. A final step consists of partial replacement of aragonite by dolomite, possibly in neutral to slightly acidic porewater. This might occur due to dissolution-precipitation reactions when the most recalcitrant part of the organic matrix is degraded. The mineralisation pathways proposed here provide pivotal insight for the interpretation of microbial processes in past hypersaline environments

Slicing the Pie: How Big Could Carbon Dioxide Removal Be?

The current global dependence on using fossil fuels to meet energy needs continues to increase. If 2°C warming by 2050 is to be prevented, it will become important to adopt strategies that not only avoid CO2 emissions, but also allow for the direct removal of CO2 from the atmosphere, enabling the intervention of climate change. The primary direct removal methods discussed in this contribution include land management, mineral carbonation and bioenergy and direct air capture with carbon capture and reliable storage. These methods are discussed in detail and their potential for CO2 removal assessed. The global upper bound for annual CO2 removal was estimated to be 12, 10, 6, and 5 GtCO2/yr for BECCS, DACS, land management, and mineral carbonation, respectively – resulting in a cumulative value of about 33 GtCO2/yr. However, in the case of DACS, global data on the overlap of low-emission energy sources and reliable CO2 storage opportunities – set as a qualification for DAC viability – was unavailable and the potential upper bound estimate is thus considered conservative. While direct CO2 removal at the upper bounds identified in this review is insufficient to completely mitigate the projected 1,800 GtCO2 emissions projected by 2050, the cumulative impact of these methods could counteract up to ~60% of these emissions. The upper bounds on the costs associated with the direct CO2 removal methods varied from approximately $100/tCO2 (land management, BECCS, and mineral carbonation) to in excess of$1000/tCO2 (again, these are the upper bounds for costs). In this analysis these direct CO2 removal technologies are found to be technically viable and potentially important options in preventing 2°C warming by 2050. However, caution is warranted in moving forward with implementation of CO2 removal, especially in the case of attempting to rapidly decrease atmospheric concentrations; it is recommended that the risks of scaling up too quickly be weighed against the existing risks associated with global warming. Please click Additional Files below to see the full abstract

Modélisation expérimentale de la précipitation des minéraux carbonatés lors de l'activité bactérienne

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Organic matrices in metazoan calcium carbonate skeletons: composition, functions, evolution.

9 pagesInternational audienceCalcium carbonate skeletal tissues in metazoans comprise a small quantity of occluded organic macromolecules, mostly proteins and polysaccharides that constitute the skeletal matrix. Because its functions in modulating the biomineralization process are well known, the skeletal matrix has been extensively studied, successively via classical biochemical approaches, via molecular biology and, in recent years, via transcriptomics and proteomics. The optimistic view that the deposition of calcium carbonate minerals requires a limited number of macromolecules has been challenged, in the last decade, by high-throughput approaches. Such approaches have made possible the rapid identification of large sets of mineral-associated proteins, i.e., ‘skeletal repertoires’ or ‘skeletomes’, in several calcifying animal models, ranging from sponges to echinoderms. One of the consequences of this expanding set of data is that a simple definition of the skeletal matrix is no longer possible. This increase in available data, however, makes it easier to compare skeletal repertoires, shedding light on the fundamental evolutionary mechanisms affecting matrix components

Effect of cyanobacteria Synechococcus PCC 7942 on carbonation kinetics of olivine at 20°C

International audienceBy accelerating the naturally-occurring carbonation of magnesian silicates, it would be possible to sequester some of the anthropogenic excess of CO2 in more geologically-stable solid magnesium carbonates. Reaction rates can be accelerated by decreasing the particle size, raising the reaction temperature, increasing the pressure, using a catalyst, and hypothetically, by bacterial addition. We aimed here at assessing quantitatively the added value of photosynthetic microbial activity on the efficiency of Mgsilicates carbonation processes. Synechococcus PCC 7942 (freshwater cyanobacteria) was selected for this study. Two magnesian silicate minerals (substrates) were chosen: a synthetic forsterite with nanometersized grains and an industrial ultramafic slag (scoria). All tests were performed at 20 ± 1 C in closed and sterile 1L Schott glass bottle reactors. With the aim to elucidate the interaction between mineral phases and bacteria, we used pH and concentration measurements, scanning and transmission electron microscopy along with Raman spectroscopy. The results show that, at ambient temperature, cyanobacteria Synechococcus can accelerate silicate dissolution (i.e. Mg2+ release) and then magnesium carbonate nucleation and precipitation by adsorption on the produced exopolymeric substances and local pH increase during photosynthesis, respectively

The Shell of the Invasive Bivalve Species Dreissena polymorpha: Biochemical, Elemental and Textural Investigations.

The zebra mussel Dreissena polymorpha is a well-established invasive model organism. Although extensively used in environmental sciences, virtually nothing is known of the molecular process of its shell calcification. By describing the microstructure, geochemistry and biochemistry/proteomics of the shell, the present study aims at promoting this species as a model organism in biomineralization studies, in order to establish a bridge with ecotoxicology, while sketching evolutionary conclusions. The shell of D. polymorpha exhibits the classical crossed-lamellar/complex crossed lamellar combination found in several heterodont bivalves, in addition to an external thin layer, the characteristics of which differ from what was described in earlier publication. We show that the shell selectively concentrates some heavy metals, in particular uranium, which predisposes D. polymorpha to local bioremediation of this pollutant. We establish the biochemical signature of the shell matrix, demonstrating that it interacts with the in vitro precipitation of calcium carbonate and inhibits calcium carbonate crystal formation, but these two properties are not strongly expressed. This matrix, although overall weakly glycosylated, contains a set of putatively calcium-binding proteins and a set of acidic sulphated proteins. 2D-gels reveal more than fifty proteins, twenty of which we identify by MS-MS analysis. We tentatively link the shell protein profile of D. polymorpha and the peculiar recent evolution of this invasive species of Ponto-Caspian origin, which has spread all across Europe in the last three centuries

Zeta potential of anoxygenic phototrophic bacteria and Ca adsorption at the cell surface: possible implications for cell protection from CaCO3 precipitation in alkaline solutions.

10 pagesInternational audienceElectrophoretic mobility measurements and surface adsorption of Ca on living, inactivated, and heat-killed haloalkaliphilic Rhodovulum steppense, A-20s, and halophilic Rhodovulum sp., S-17-65 anoxygenic phototrophic bacteria (APB) cell surfaces were performed to determine the degree to which these bacteria metabolically control their surface potential equilibria. Zeta potential of both species was measured as a function of pH and ionic strength, calcium and bicarbonate concentrations. For both live APB in 0.1M NaCl, the zeta potential is close to zero at pH from 2.5 to 3 and decreases to -30 to -40 mV at pH of 5-8. In alkaline solutions, there is an unusual increase of zeta potential with a maximum value of -10 to -20 mV at a pH of 9-10.5. This increase of zeta potential in alkaline solutions is reduced by the presence of NaHCO(3) (up to 10 mM) and only slightly affected by the addition of equivalent amount of Ca. At the same time, for inactivated (exposure to NaN(3), a metabolic inhibitor) and heat-killed bacteria cells, the zeta potential was found to be stable (-30 to -60 mV, depending upon the ionic strength) between pH 5 and 11 without any increase in alkaline solutions. Adsorption of Ca ions on A-20s cells surface was more significant than that on S-17-65 cells and started at more acidic pHs, consistent with zeta potential measurements in the presence of 0.001-0.01 mol/L CaCl(2). Overall, these results indicate that APB can metabolically control their surface potential to electrostatically attract nutrients at alkaline pH, while rejecting/avoiding Ca ions to prevent CaCO(3) precipitation in the vicinity of cell surface and thus, cell incrustation