Christian Mustin

Evolution of size distribution and transfer of mineral particles between flocs in activated sludges: an insight into floc exchange dynamics

The aggregation behavior of activated sludge flocs was investigated by monitoring the size distribution of flocs and transfer of mineral particles between flocs, under various conditions of agitation and dilution. The results showed that (i) the shape of the floc size distribution can be fitted with a gamma function, (ii) a steady-state mean floc size is reached for a given stirring rate, (iii) this stable floc size is shifted towards floc growth as sludge concentration is increased, (iv) under cycled-shear conditions, microbial aggregates break up and re-form in an almost reversible manner, (v) blending of raw sludge and sludge spiked with Aquatal mineral particles results in particle exchange between flocs and (vi) the detailed study of exchange kinetics indicates that some flocs do not participate to the aggregation dynamics. These experimental results suggest that the activated sludge floc size is governed by a flocculation/deflocculation balance, implying an exchange of floc constituents between microbial aggregates.

Biocompatible and stable ZnO quantum dots generated by functionalization with siloxane-core PAMAM dendrons

Despite the growing interest of quantum dots (QDs) in biological applications, there are many concerns regarding the potential accumulation and toxic effects of Cd-containing QDs in animals and humans. Zinc oxide QDs are promising alternatives for diagnosis and imaging but their aqueous instability has markedly limited their use. Generations 1, 2 and 3 (noted G1, G2, and G3, respectively) of new poly(amidoamine) (PAMAM) dendrons bearing a siloxane group at the focal point were prepared from 3-aminopropyltrimethoxysilane. Using tetramethylammonium hydroxide as cross-linking agent, hydrophobic oleate-capped ZnO QDs were functionalized with G1 or G2 dendrons, as evidenced by FT-IR, UV-visible and XPS analyses, and were successfully transferred in aqueous solution. AFM and TEM images show that ZnO@G1 and ZnO@G2 QDs have a spherical shape with average crystalline sizes of 5.3 and 5.1 nm, respectively. Immediately after dispersion in water, ZnO@G1 and ZnO@G2 QDs exhibit a broad and strong visible emission peak centered at 550 nm with a quantum yield of ca. 18%. A strong increase of photoluminescence quantum yields was observed over time and values up to 59% could be reached after ca. 20 days of storage in water at room temperature. The good quantum yields and the stabilities of PAMAM-dendron capped ZnO QDs ensured their potential applications in cell imaging. ZnO@G2 was successfully used for the labelling of the Gram+ bacterium Staphylococcus aureus. The biocompatibility of these QDs is markedly improved compared to Cd-based ones as growth inhibition tests showed that ZnO@G2 QDs could be used with concentrations up to 1 mM without altering the cell growth of the Escherichia coli bacterium while most Cd-containing QDs already exhibit cytotoxicity at the nM level.

Composition, structure and size distribution of suspended particulates from the Rhine River

Fluvial suspended particulates collected from the Rhine River were investigated in terms of composition, structure and size distribution. Elemental analysis and Diffuse Reflectance Infrared Spectroscopy reveal that most particulate organic matter is formed from material derived from microorganisms. Transmission Electron Microscopy observations on resin-embedded samples and structural characterization from break-up experiments, show that fluvial particulate matter should be viewed as fractal aggregates organized by bacterial exopolymeric substances. The shape of particulate size distribution suggests that the formation and dynamics of suspended particulate matter are controlled mainly by physical processes. Finally, particulate growth and structure are consistent with a cluster-cluster aggregation scheme.

Surface-engineered quantum dots for the labeling of hydrophobic microdomains in bacterial biofilms.

Quantum dots (QDs) nanoprobes are emerging as alternatives to small-molecule fluorescent probes in biomedical technology. This paper reports an efficient and rapid method of producing highly dispersed and stable CdSe-core QDs with a hydrophobic gradient. Amphiphilic core/shell CdSe/ZnS QDs were prepared by ligand exchange at the surface of lipophilic CdSe/ZnS QDs using the dihydrolipoic acid (DHLA) dithiol ligand linked to leucine or phenylalanine amino acids. Contact angle relaxations on a hydrophobic surface and surface tension measurements indicated that aqueous dispersions of CdSe/ZnS@DHLA-Leu or CdSe/ZnS@DHLA-Phe QDs exhibit increased hydrophobicity compared to CdSe-core QDs capped by the hydrophilic 3-mercaptopropionic acid (MPA) ligand. We found that the surface functional groups and the ligand density at the periphery of these QDs significantly dictated their interactions with a complex biological matrix called biofilm. Using fluorescence confocal microscopy and an autocorrelation function (semi-variogram), we demonstrated that MPA-capped QDs were homogeneously associated to the biopolymers, while amphiphilic CdSe/ZnS@DHLA-Leu or CdSe/ZnS@DHLA-Phe QDs were specifically confined allowing identification of hydrophobic microdomains of the biofilms. Results obtained clearly point out that the final destination of QDs in biofilms can properly be controlled by an appropriate design of surface ligands.

Bacterial and iron oxide aggregates mediate secondary iron mineral formation: green rust versus magnetite

In the presence of methanoate as electron donor, Shewanella putrefaciens, a Gram-negative, facultative anaerobe, is able to transform lepidocrocite (gamma-FeOOH) to secondary Fe (II-III) minerals such as carbonated green rust (GR1) and magnetite. When bacterial cells were added to a gamma-FeOOH suspension, aggregates were produced consisting of both bacteria and gamma-FeOOH particles. Recently, we showed that the production of secondary minerals (GR1 vs. magnetite) was dependent on bacterial cell density and not only on iron reduction rates. Thus, gamma-FeOOH and S. putrefaciens aggregation pattern was suggested as the main mechanism driving mineralization. In this study, lepidocrocite bioreduction experiments, in the presence of anthraquinone disulfonate, were conducted by varying the [cell]/[lepidocrocite] ratio in order to determine whether different types of aggregate are formed, which may facilitate precipitation of GR1 as opposed to magnetite. Confocal laser scanning microscopy was used to analyze the relative cell surface area and lepidocrocite concentration within the aggregates and captured images were characterized by statistical methods for spatial data (i.e. variograms). These results suggest that the [cell]/[lepidocrocite] ratio influenced both the aggregate structure and the nature of the secondary iron mineral formed. Subsequently, a [cell]/[lepidocrocite] ratio above 1 x 10(7) cells mmol(-1) leads to densely packed aggregates and to the formation of GR1. Below this ratio, looser aggregates are formed and magnetite was systematically produced. The data presented in this study bring us closer to a more comprehensive understanding of the parameters governing the formation of minerals in dense bacterial suspensions and suggest that screening mineral-bacteria aggregate structure is critical to understanding (bio)mineralization pathways.

Isotopic tracing of clear water sources in an urban sewer: A combined water and dissolved sulfate stable isotope approach

This paper investigates the potential of stable isotopes of both water (deltaD and deltaOH(2)O18) and dissolved sulfate (delta(34)S and deltaOSO(4)18) for determining the origin and the amount of clear waters entering an urban sewer. The dynamics of various hydrological processes that commonly occur within the sewer system such as groundwater infiltration, rainwater percolation, or stormwater release from retention basins, can be readily described using water isotope ratios. In particular, stable water isotopes indicate that the relative volumes of infiltrated groundwater and sewage remain approximately constant and independent of wastewater flow rate during the day, thus demonstrating that the usual quantification of parasitic discharge from minimal nocturnal flow measurements can lead to completely erroneous results. The isotopic signature of dissolved sulfate can also provide valuable information about the nature of water inputs to the sewage flow, but could not be used in our case to quantify the infiltrating water. Indeed, even though the microbial activity had a limited effect on the isotopic composition of dissolved sulfate at the sampling sites investigated, the dissolved sulfate concentration in sewage was regulated by the formation of barite and calcium-phosphate mineral species. Sulfate originating from urine was also detected as a source using the oxygen isotopic composition of sulfate, which suggests that deltaOSO(4)18 might find use as a urine tracer.

Deformation of Lactococcus lactis surface in atomic force microscopy study

The surface of the bacterium Lactococcus lactis was investigated under water by contact mode atomic force microscopy (AFM) while varying the imaging parameters (imaging force, scanning velocity, scanning direction). Height images were three-dimensionally (3-D) reconstructed using GOCAD(C) software; this revealed grooves oriented along the scanning direction. Although the grooves were created by the scanning probe, they were not due to a single passage of the probe; the periodicity of the grooves was indeed always three to four times larger than the scanning line periodicity (ratio between the image size and the number of scanning lines). Upon repeated imaging at low force, the grooves were re-created each time with the same morphology; groove depth was increased at higher imaging force. The groove formation is tentatively explained by a deformation of the surface, due to compression or to adhesion, and its slow relaxation. While the grooves reveal a perturbation of the cell surface by the AFM probe, they provide pertinent information about the nanomechanical properties of the cell surface

Bio-dissolution of colloidal-size clay minerals entrapped in microporous silica gels.

Four colloidal-size fractions of strongly anisotropic particles of nontronite (NAu-2) having different ratios of basal to edge surfaces were incubated in the presence of heterotrophic soil bacteria to evaluate how changes in mineral surface reactivity influence microbial dissolution rate of minerals. To avoid any particle aggregation, which could change the reactive surface area available for dissolution, NAu-2 particles were immobilized in a biocompatible TEOS-derived silica matrix. The resulting hybrid silica gels support bacterial growth with NAu-2 as the sole source of Fe and Mg. Upon incubation of the hybrid material with bacteria, between 0.3% and 7.5% of the total Fe included in the mineral lattice was released with a concomitant pH decrease. For a given pH value, the amount of released Fe varied between strains and was two to twelve-fold higher than under abiotic conditions. This indicates that complexing agents produced by bacteria play an important role in the dissolution process. However, in contrast with proton-promoted NAu-2 dissolution (abiotic incubations) that was negatively correlated with particle size, bacterial-enhanced dissolution was constant for all size fractions used. We conclude that bio-dissolution of nontronite particles under acidic conditions seems to be controlled by bacterial metabolism rather than by the surface reactivity of mineral.

Dissolution of anisotropic colloidal mineral particles: evidence for basal surface reactivity of nontronite.

Anisotropic textural and crystallographic properties of phyllosilicate particles often influence the mineral weathering rate. The purpose of this study was to investigate how the changes in mineral surfaces (basal vs. edge) as a result of changes in crystal size control the dissolution of the mineral. Different nano-size fractions of Na-exchanged nontronites (NAu2 and NAu1) were immobilized in a silica gel and then incubated under acidic conditions using HNO(3) at 28 degrees C for 5 days. For each sample, the dissolution behavior was analyzed by measuring the amount of iron released from the mineral lattice. The results showed that for a given pH, a decrease in particle size significantly increased NAu2 and NAu1 dissolution. At pH 1.5, 7.2% of the total iron of the highest size sample of NAu2 was released in solution whereas this proportion increased up to 25% for the smallest size fraction. The percentage of total iron extracted from NAu1 at the same pH (1.5) was less important: 3.5% and 6.5% for higher and smaller size fractions, respectively. The observed increase in dissolution was not directly correlated to the increase in the amount of edge faces, suggesting that all mineral surfaces contributed to mineral dissolution. In the present case this may be related to the fact that 8% and 2% of total iron of NAu2 and NAu1, respectively, are located in the tetrahedral sheet. In conclusion, the basal surface of nontronites plays an important role in the weathering process.

Perforative Corrosion of Pyrite Enhanced by Direct Attachment of Acidithiobacillus ferrooxidans

This study aimed to determine the relationship between cell adhesion and pyrite corrosion. In order to determine cell adhesion rates, a new in vivo imaging procedure was developed with CLSM. Total bacteria coverage rates remained low (< 14%). Adsorption isotherms pointed out two types of cell adhesion sites. Bacteria adhered mainly to corrosion pittings walls with a higher affinity. They favour the formation of deep corrosion pits by maintaining short-range electronic circulations and they avoid surface passivation. This study completes the common model of bio-corrosion of pyrite and we propose to refine the direct/indirect model by proximal and distal approaches.