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

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

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

Calcium carbonate precipitation by anoxygenic phototrophic bacteria.

16 pagesInternational audienceCarbonate biomineralization is considered as one of the main natural processes controlling CO2 levels in the atmosphere both in the past and at present time. In contrast to extensive studies of cyanobacterial calcification, biomineralization of anoxygenic phototrophic bacteria (APB) remained largely underestimated, despite their potentially important role on CaCO3 precipitation in the biomats, notably in the past. Haloalcaliphilic Rhodovulum steppense A-20s and halophilic neutrophilic Rhodovulum sp. S-17-65 were examined with respect to their ability to precipitate CaCO3 under controlled laboratory conditions. To characterize the link between the rate of bacterial growth (biomass production) and the rate of CaCO3 precipitation, batch kinetic experiments with live, dead and inactivated bacteria both in nutrient solution and in inert electrolyte were performed and produced precipitates were examined by SEM, TEM and XRD techniques. Active strains A-20s and S-17-65 precipitated calcite from initially supersaturated solutions (Ωcalcite = 10 to 40) via increasing Ωcalcite to 80–120 before the precipitation. The amount of precipitated CaCO3 (mole) was directly correlated with the amount of organic C in bacterial biomass produced with a slope of dependence ranging from 0.3 to 0.6 and from 0.1 to 0.3 for A-20s and S-17-65, respectively, depending on the initial solution composition. For both bacterial strains, only live actively photosynthetizing bacteria were capable of effectively decreasing Ca concentration and form CaCO3 with apparent bulk precipitation rates ranging from 0.001 to 0.0150 mmol/h at 10–20 gwet/L of biomass, similar to rates reported for other bacteria. SEM and XRD analyses of precipitates reveal the dominance of calcite with some amount of vaterite and monohydrocalcite forming spheres up to 100 μm diameter. The TEM analysis of bacterial suspension at the end of precipitation experiments did not demonstrate the presence of CaCO3 at the surface or in the vicinity of live cells. This suggests the existence of certain cell protection mechanism against carbonate precipitation at the cell surface. Given the lower efficiency of photoheterotrophic APB, compared to photoautotrophic cyanobacteria, to precipitate CaCO3 in natural conditions, it is possible that the overall potential of phototrophic community to form massive carbonate deposits was strongly limited before the appearance of oxygenic phototrophs

Experimental modeling of calcium carbonate precipitation by cyanobacterium Gloeocapsa sp.

17 pagesInternational audienceThe impact of cyanobacteria Gloeocapsa sp. on calcium carbonate precipitation has been examined by combining physico-chemical macroscopic and in-situ microscopic techniques. For this, Ca adsorption and assimilation and kinetic experiments were used to assess the existence of the metabolic process responsible for CaCO3 mineralization by Gloeocapsa sp. Experimental products were characterized by Scanning and Transmission Electron Microscopy (SEM and TEM) imaging, XRD analyses, coupled with Confocal Laser Scanning Microscopy (CLSM) and Raman micro-spectroscopy. Ca carbonate precipitation experiments were performed at an initial pH of 7.8 to 9.4 and 25 °C in supersaturated solutions (Ωcalcite = 1.5 to 150) in the presence of active cyanobacterial cells. During cyanobacterial photosynthesis, the solution pH increased up to 9.5-10.8 after the first 5-10 days of growth, the Ca concentration decreased and the supersaturation index attained a maximum followed by a gradual decrease due to progressive CaCO3 precipitation. Ca adsorption at the surface of live and inactivated Gloeocapsa sp. cells and Ca intracellular assimilation during cell growth were measured as a function of pH and Ca concentration in solution. The contribution of surface adsorption and intracellular uptake to total Ca removal from solution due to biocalcification does not exceed 10%. The presence of calcium carbonate, identified as calcite using Raman spectroscopy, on active Gloeocapsa sp. surfaces and in the vicinity of bacterial cell surfaces was evidenced using SEM. TEM and CLSM demonstrated cyanobacterial cell encrustation by CaCO3 precipitated in the form of nano-spheres adjacent to the cell surface. In contrast to other previously investigated calcifying bacteria, no cellular protection mechanism against Ca2 + adsorption and subsequent carbonate precipitation has been demonstrated for Gloeocapsa sp. This is most likely linked to the specific cellular organization of this species, which involves several cells in one single capsule. As such, planktonic cultures of Gloeocapsa sp. exhibit significant calcifying potential, making them important CO2-fixing microorganisms for both paleo-environmental reconstructions and technological applications

Carbonate Precipitation in Mixed Cyanobacterial Biofilms Forming Freshwater Microbial Tufa

22 pagesInternational audienceMixed cyanobacteria-dominated biofilms, enriched from a tributary of the Mérantaise (France) were used to conduct laboratory experiments in order to understand the relationship between the morphology of carbonate precipitates and the biological activity (e.g., cyanobacterial exopolymeric substances (EPS) production, photosynthetic pH increases). DNA sequencing data showed that the enriched biofilm was composed predominantly of two types of filamentous cyanobacteria that belonged to the Oscillatoriaceae and Phormidiaceae families, respectively. Microscopic analysis also indicated the presence of some coccoid cyanobacteria resembling Gloeocapsa. Analysis of carbonate precipitates in experimental biofilms showed three main morphologies: micro-peloids with different shapes of mesocrystals associated with Oscillatoriaceae filaments and theirs EPS, lamellae of carbonate formed directly on Phormidiaceae filaments, and rhombic sparite crystals wrapped in EPS. All crystals were identified by FT-IR spectroscopy as calcite. Similar structures as those that formed in laboratory conditions were observed in the microbial-tufa deposits collected in the stream. Microscopic and spectroscopic analysis of laboratory and natural samples indicated a close proximity of the cyanobacterial EPS and precipitated carbonates in both. Based on the laboratory experiments, we conclude that the microbial tufa in the stream is in an early stage of formation

Experimental modeling of bacterially-induced Ca carbonate precipitation: new insights on possible mechanisms.

19 pagesInternational audienc

Magnesium isotope fractionation during hydrous magnesium carbonate precipitation with and without cyanobacteria

The hydrous magnesium carbonates, nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4(OH)2·5(H2O)), were precipitated at 25 °C in batch reactors from aqueous solutions containing 0.05 M NaHCO3 and 0.025 M MgCl2 and in the presence and absence of live photosynthesizing Gloeocapsa sp. cyanobacteria. Experiments were performed under a variety of conditions; the reactive fluid/bacteria/mineral suspensions were continuously stirred, and/or air bubbled in most experiments, and exposed to various durations of light exposure. Bulk precipitation rates are not affected by the presence of bacteria although the solution pH and the degree of fluid supersaturation with respect to magnesium carbonates increase due to photosynthesis. Lighter Mg isotopes are preferentially incorporated into the precipitated solids in all experiments. Mg isotope fractionation between the mineral and fluid in the abiotic experiments is identical, within uncertainty, to that measured in cyanobacteria-bearing experiments; measured ?26Mg ranges from ?1.54‰ to ?1.16‰ in all experiments. Mg isotope fractionation is also found to be independent of reactive solution pH and Mg, CO32?, and biomass concentrations. Taken together, these observations suggest that Gloeocapsa sp. cyanobacterium does not appreciably affect magnesium isotope fractionation between aqueous fluid and hydrous magnesium carbonate