Manuel Rodriguez-Gallego

Conservation of Ornamental Stone by Myxococcus xanthus-Induced Carbonate Biomineralization

ABSTRACT Increasing environmental pollution in urban areas has been endangering the survival of carbonate stones in monuments and statuary for many decades. Numerous conservation treatments have been applied for the protection and consolidation of these works of art. Most of them, however, either release dangerous gases during curing or show very little efficacy. Bacterially induced carbonate mineralization has been proposed as a novel and environmentally friendly strategy for the conservation of deteriorated ornamental stone. However, the method appeared to display insufficient consolidation and plugging of pores. Here we report that Myxococcus xanthus-induced calcium carbonate precipitation efficiently protects and consolidates porous ornamental limestone. The newly formed carbonate cements calcite grains by depositing on the walls of the pores without plugging them. Sonication tests demonstrate that these new carbonate crystals are strongly attached to the substratum, mostly due to epitaxial growth on preexisting calcite grains. The new crystals are more stress resistant than the calcite grains of the original stone because they are organic-inorganic composites. Variations in the phosphate concentrations of the culture medium lead to changes in local pH and bacterial productivity. These affect the structure of the new cement and the type of precipitated CaCO3 polymorph (vaterite or calcite). The manipulation of culture medium composition creates new ways of controlling bacterial biomineralization that in the future could be applied to the conservation of ornamental stone.

Precipitation and Growth Morphology of Calcium Carbonate Induced by Myxococcus Xanthus: Implications for Recognition of Bacterial Carbonates

Abstract It is thought that morphologies of bacterial carbonates can be used to identify microbial fossils and/or precipitates in sediments and rocks. This study shows that calcite and vaterite formed in a gel medium in the presence of Myxococcus xanthus display a range of morphologies that depend on whether the bacteria are live or dead. Metabolic activity of the bacteria induced: (1) aggregates of calcified bacteria formed at maximum supersaturation; (2) vaterite spheres (final growth stage of dumbbell fibrous-radiated aggregates); and (3) dipyramid- and disphenoid-like calcite crystals (combination of {011} and {0001} forms). Morphologies (2) and (3) developed at a lower supersaturation and are typically found in gel-like media. Dipyrimidal-like calcite crystals were also obtained abiotically in gel medium. Dead M. xanthus cells induced heterogeneous precipitation of calcite with rhombohedral morphologies at low supersaturation. A growth mechanism resulting from self-assembly of calcium carbonate nanocrystals may account for the observed morphologies, crystal microstructure, and crystallite size measurements. All of the above-mentioned morphologies of bacterial carbonate have been observed in other laboratory experiments and in continental and marine environments. However, all of them have also been produced abiotically, with the exception of calcified bacterial cells. This may make it more difficult to identify bacterial activity in the rock record. Nonetheless, bacterially induced alkalinization appears to be a prerequisite for the development of spherulitic and dipyramid- or disphenoid-like forms in natural mucilaginous biofilms and microbial mats. The morphologies reported here may facilitate the recognition of early and recent marine and continental microcrystalline bacterial carbonates and cements.

Precipitation of Barite by Myxococcus xanthus: Possible Implications for the Biogeochemical Cycle of Barium

ABSTRACT Bacterial precipitation of barite (BaSO4) under laboratory conditions is reported for the first time. The bacterium Myxococcus xanthus was cultivated in a solid medium with a diluted solution of barium chloride. Crystallization occurred as a result of the presence of live bacteria and the bacterial metabolic activity. A phosphorous-rich amorphous phase preceded the more crystalline barite formation. These experiments may indicate the involvement of bacteria in the barium biogeochemical cycle, which is closely related to the carbon cycle.

Bacterially Induced Mg-Calcite Formation: Role of Mg2+ in Development of Crystal Morphology

ABSTRACT The production of several morphologies of magnesian calcite crystals by the soil bacterium Myxococcus xanthus is reported for the first time. Mg-calcite crystallization occurred in agar-agar gel culture media in the presence of the live bacteria. The agar-agar used to solidify the nutritive microbial solution acted as a porous system that allowed slow counterdiffusion of cations and anions and of the bacterial metabolites produced. Under these conditions, crystal nucleation and growth occurs, apparently as a consequence of the localized ion supersaturation produced by the microbial metabolites and by microbial supply of heterogeneous nuclei for crystallization. Several morphologies of Mg-calcite typical of those formed under biotic and abiotic conditions developed simultaneously. The crystals produced were not compositionally zoned and showed no significant variation in Mg content, probably as a consequence of the sponge-like character of the precipitates.

Bacterial biomineralization: new insights from Myxococcus-induced mineral precipitation

Abstract Bacteria have contributed to the formation of minerals since the advent of life on Earth. Bacterial biomineralization plays a critical role on biogeochemical cycles and has important technological and environmental applications. Despite the numerous efforts to better understand how bacteria induce/mediate or control mineralization, our current knowledge is far from complete. Considering that the number of recent publications on bacterial biomineralization has been overwhelming, here we attempt to show the importance of bacteria–mineral interactions by focusing in a single bacterial genus, Myxococcus, which displays an unusual capacity of producing minerals of varying compositions and morphologies. First, an overview of the recent history of bacterial mineralization, the most common bacteriogenic minerals and current models on bacterial biomineralization is presented. Afterwards a description of myxobacteria is presented, followed by a section where Myxococcus-induced precipitation of a number of phosphates, carbonates, sulphates, chlorides, oxalates and silicates is described and discussed in lieu of the information presented in the first part. As concluding remarks, implications of bacterial mineralization and perspectives for future research are outlined. This review strives to show that the mechanisms which control bacterial biomineralization are not mineral- or bacterial-specific. On the contrary, they appear to be universal and depend on the environment in which bacteria dwell.

Struvite and calcite crystallization induced by cellular membranes of Myxococcus xanthus

In this work we have proved that struvite and calcite crystals can be obtained in the presence of the cellular membrane fraction of Myxococcus xanthus, when appropriate supersaturated solutions are used. Probably, the negative charged points of the external side of the cellular structures could reduce the metastability field of struvite and calcite, acting as heterogeneous nuclei of crystallization.

An urban model for dolomite precipitation: authigenic dolomite on weathered building stones

Abstract Formation of authigenic dolomite within alteration crusts developing on limestone surfaces of a historical building is confirmed by XRD, polarising microscopy, and SEM with EDX data. Two different types of authigenic dolomite occur: limpid stoichiometric dolomite and Ca-rich ‘protodolomite’. The presence of gypsum, calcite, pollution-derived particles, clay minerals, and organic material within the crusts confirms their recent formation, together with that of dolomite, in a micro-environment of high evaporation and ionic concentrations. Such environments are typically found in many urban centres. On the basis of these findings an ‘Urban Model’ for dolomite precipitation is proposed. There are two sources of Mg: (1) dissolution of nearby dolostones overlying the limestones, as a result of leaching by atmospheric SO2, oxidised and hydrolysed as H2SO4; and (2) Mg-rich metallic atmospheric particles. Elevated Mg/Ca values are achieved by precipitation of gypsum, which removes Ca from the solution. ‘Protodolomite’ then precipitates and later recrystallises as limpid dolomite due to changes in the pore fluids, related with the environmental conditions. CO32− and CO3H− are provided by leaching of both the limestone support and the nearby dolostones. Additional sources of CO2 are provided both by the polluted urban atmosphere, rain water, and the degradation of organic material within the crust. Organic matter (fungi and bacteria), pollution-derived particulate matter and clays seem to play a key role in promoting dolomite precipitation in this micro-environment by altering kinetic barriers and providing active sites for dolomite nucleation. Finally, the formation of urban dolomite is compared with Recent dolomites forming in natural environments with alternating evaporative/fresh-water conditions.

Struvite production by Myxococcus coralloides D

Abstract Myxococcus coralloides D has been shown to produce extracellular crystals, which have been identified via scanning electron microscopy and X-ray diffraction to be struvite (MgHN4PO4·6H2O). This is the first occasion that the production of this mineral by a myxobacterium has been described.

Morphological diversity of Struvite crystals produced by Myxococcus Coralloides and Myxococcus Xanthus

Abstract We studied the morphological diversity of struvite crystals produced by Myxococcus coralloides and Myxococcus xanthus in different culture conditions. We discussed the similarities of these crystals with the struvite morphology studied previously following the theory of the periodic bond chains.

Myxococcus xanthus Colony Calcification: An Study to Better Understand the Processes Involved in the Formation of this Stromatolite-Like Structure

Calcium carbonate precipitation is a common phenomenon in nature and has been observed to be mediated by a number of microorganisms (for a review, see Castanier et al. 2000; Wright and Oren 2005). Bacterially induced carbonate mineralization is important in a wide range of processes including atmospheric CO2 budgeting (Braissant et al. 2002; Ehrlich 2002), carbonate sediment and rock formation (Riding 2000; Ben Chekroun et al.