José M. Arias

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.

Spectroscopic and Microscopic Characterization of Uranium Biomineralization in Myxococcus xanthus

In this work, synchrotron-based X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) studies were carried out to elucidate at molecular scale the interaction mechanisms of Myxococcus xanthus with uranium at different pH values. Extended X-ray absorption fine structure (EXAFS) spectroscopic measurements showed that there are significant differences in the structural parameters of the U complexes formed by this bacterium at pH 2 and 4.5. At very low acidic pH of 2, the cells accumulated U(VI) as organic phosphate-metal complexes. At pH 4.5, however, the cells of this bacterium precipitated U(VI) as meta-autunite-like phase. TEM analyses demonstrated that at pH 2 the uranium accumulates were located mainly at the cell surface, whereas at pH 4.5 a uranium precipitation occurred on the cell wall and within the extracellular polysaccharides (EPS) characteristic of this bacterium. Dead/live staining studies showed that 30% and 50% of the uranium treated cell populations were alive at pH 2 and 4.5, respectively. The precipitation of U(VI) as mineral meta-autunite-like phase is possibly due to the bacterial acidic phosphatase activity. The precipitation of uranium as mineral phases may lead to more stable U(VI) sequestration that may be suitable for remediation purposes. These observations, combined with the very high uranium accumulation capability of the studied bacterial cells indicate that M. xanthus may significantly influence the fate of uranium in soil environments where these bacterial species are mainly found.

Lanthanum fixation by Myxococcus xanthus: cellular location and extracellular polysaccharide observation.

Myxococcus xanthus is a soil bacterium of the myxobacteria group and is abundant in almost all soils. Its role in soil ecology is considered significant. One noteworthy characteristic of the bacterium is that it produces large quantities of extracellular polymeric substances (EPS). It is also known that its biomass has the capacity to fix heavy metals. Here it is reported that M. xanthus was able to accumulate 0.6 mmol of La per g of wet biomass and/or 0.99 mmol per g of dry biomass. Transmission Electron Microscopy (TEM) observation of M. xanthus cells treated with La showed that a substantial amount of this cation was fixed in the EPS and in the cell wall. Smaller amounts were also observed in the cytoplasm. Fixed La appeared as phosphate in all cellular locations. The results given here also show that the use of La enables TEM observation of the M. xanthus EPS as a dense fibrillar net surrounding the cells. This technique is relatively easy and prevents EPS collapse, which occurs frequently during the fixation and dehydration procedures commonly used in preparations for TEM observations. Since antibodies are no longer required, the La stain can be carried out without delaying bacterial cell cultivation or isolation. In addition, the presence of La in cell cytoplasm without cell degeneration suggests that this microorganism could be used as a model in the study of bacteria-lanthanide interactions.

Bio-precipitation of uranium by two bacterial isolates recovered from extreme environments as estimated by potentiometric titration, TEM and X-ray absorption spectroscopic analyses.

This work describes the mechanisms of uranium biomineralization at acidic conditions by Bacillus sphaericus JG-7B and Sphingomonas sp. S15-S1 both recovered from extreme environments. The U-bacterial interaction experiments were performed at low pH values (2.0-4.5) where the uranium aqueous speciation is dominated by highly mobile uranyl ions. X-ray absorption spectroscopy (XAS) showed that the cells of the studied strains precipitated uranium at pH 3.0 and 4.5 as a uranium phosphate mineral phase belonging to the meta-autunite group. Transmission electron microscopic (TEM) analyses showed strain-specific localization of the uranium precipitates. In the case of B. sphaericus JG-7B, the U(VI) precipitate was bound to the cell wall. Whereas for Sphingomonas sp. S15-S1, the U(VI) precipitates were observed both on the cell surface and intracellularly. The observed U(VI) biomineralization was associated with the activity of indigenous acid phosphatase detected at these pH values in the absence of an organic phosphate substrate. The biomineralization of uranium was not observed at pH 2.0, and U(VI) formed complexes with organophosphate ligands from the cells. This study increases the number of bacterial strains that have been demonstrated to precipitate uranium phosphates at acidic conditions via the activity of acid phosphatase.

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.

BIOSORPTION OF URANIUM BY MYXOCOCCUS XANTHUS

This paper deals with uranium biosorption by M~.YO~OL.~US xanthus biomass in which dry biomass, accumulating up to 2.4 mM of uranium gP ‘. is demonstrated to be a more efficient biosorbent than wet biomass. For uranium concentrations of 0.1-0.3 mM, between 95.79% and 95.99% of the uranium was taken up from the solution. Dry biomass biosorption was found to be relatively rapid, reaching equilibrium after 5-l0min. In addition, the pH influenced biosorption, pH 4.5 promoting maximum uptake. It was also established that the biosorbed uranium is located on the cellular wall and within the extracellular mucopolysaccharide of this microorganism. Furthermore, using sodium carbonate as a desorbent agent, 80.82% of the biosorbed uranium could be recovered. The results obtained indicate the possible utilization of M. xunthus biomass to solve some problems of the water contaminated by uranium. En este trabajo se estudia la bioadsorcion de uranio por la biomasa de M_t.~ococcu.s santhus, y se demuestra que la biomasa seca es mejor bioadsorbente que la himeda. siendo capaz de acumular hasta 2.4mM de uranio g- ’ Se ha comprobado que para concentraciones de uranio comprendidas entre 0. I y 0.3 mM se puede eliminar de la solution entre el 95.79% y el 95.99%. Ademis. se ha puesto de manifiesto que la bioadsorcion por la biomasa seca es un proceso relativamente rapido, alcanzandose el equilibrio entre 5 y IO minutos. Esta bioadsorcion se afecta por el pH, siendo mas efectiva a pH 4.5. El estudio de la localization celular de1 uranio captado por la biomasa indica que se deposita a nivel de la pared celular y del mucopolisadrido extracelular. De otra parte. utilizando carbonato sodico, puede recuperarse el 80.82% del uranio acumulado por la biomasa seca. En resumen, 10s resultados obtenidos indican que la biomasa de M. xanthus podria tener una posible utilization coma bioadsorbente para la resolution de algunos problemas de aguas contaminadas con uranio. ‘(‘I 1998 Elsevier Science Ltd. All rights reserved

Myxococcus xanthus' killed cells as inducers of struvite crystallization. Its possible role in the biomineralization processes

We studied the possibility of struvite formation using killed cells of Myxococcus xanthus. Cells were killed by heat, UV light and sonication. In all cases, we show that struvite crystallization occurs and we propose that the dead cells or cell debris can act by themselves as heterogeneous crystallization nuclei. We also show that the slime, produced by Myxococcus in a large quantities, is not involved in this process.

Biotransformation of 6β-Eudesmanolides Functionalized at C-3 with Curvularia lunata and Rhizopus nigricans Cultures

Abstract A series of biotransformations of 6β-santonin and its derivatives with functions at C-3, were carried out with Curvularia lunata and Rhizopus nigricans cultures. Rhizepus nigricans was more active in the biotransformation process against these substrates. The biotransformation of 6β-santonin yielded its 2α-hydroxy-1,2-dihydro derivative. The biotransformation of ketones at C-3 obtained by partial or total hydrogenation of double bonds in ring A led to 3S alcohols. Incubation of the 3S-hydroxyl-4S-13S- 6α-eudesmanolide with Rhizopus nigricans produced epimerization at C-4 and hydroxylation at C-8, C-1 or C-4, in decreasing order. This epimerization is probably produced with the participation of the hydraxyl goup at C-3. Microbial functionalization at C-8 can provide access to the synthesis of 8,12-eudesmanolides.