Alexandre Gélabert

Interaction between Escherichia coli and TiO2 nanoparticles in natural and artificial waters.

Seine River water was used as a natural environmental medium to quantify the ecotoxicological impact of three types of manufactured titanium dioxide (TiO(2)) nanoparticles toward the model bacterium Escherichia coli. Under ambient light, a significant toxicity starting at 10 ppm of TiO(2) in water was observed. Presence of the anatase polymorph slightly increased the toxicity in comparison to pure rutile samples. Furthermore, the toxicity was found to be lower at pH 5 compared to Seine water (pH 8). To assess the nanoparticles state of dispersion and their interactions with bacteria, cryogenic transmission electron microscopy (TEM) and zeta potential measurements were performed. A higher sorption of nanoparticle aggregates on cells is observed at pH 5 compared to Seine water. This allows concluding that the observed toxicity is not directly linked to the particles sorption onto the cell surfaces. In spite of stronger interaction between cells and nanoparticles at pH 5, a bacterial subpopulation apparently non-interacting with nanoparticles is evidenced by both TEM and zeta potential measurements. Such heterogeneities in cell populations can increase global bacterial resistance to TiO(2) nanoparticles.

Exopolysaccharides protect Synechocystis against the deleterious effects of titanium dioxide nanoparticles in natural and artificial waters.

We have studied the effect of TiO2 nanoparticles (NPs) on the model cyanobacteria Synechocystis PCC6803. We used well-characterized NPs suspensions in artificial and natural (Seine River, France) waters. We report that NPs trigger direct (cell killing) and indirect (cell sedimentation precluding the capture of light, which is crucial to photosynthesis) deleterious effects. Both toxic effects increase with NPs concentration and are exacerbated by the presence of UVAs that increase the production of Reactive Oxygen Species (hydroxyl and superoxide radicals) by TiO2 NPs. Furthermore, we compared the responses of the wild-type strain of Synechocystis, which possesses abundant exopolysaccharides surrounding the cells, to that of an EPS-depleted mutant. We show, for the first time, that the exopolysaccharides play a crucial role in Synechocystis protection against cell killing caused by TiO2 NPs.

Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria.

An isotope record of magnetic bacteria Microorganisms have shaped Earths oceans and atmosphere over billions of years. Ancient microbes left very little direct morphological evidence of their existence in the rock record, thereby requiring geochemical clues for evidence of their activity. Amor et al. show that magnetotactic bacteria impart a distinct isotopic signature to their internal iron nanoparticles. Cultures of a modern magnetic bacterium fractionated 57Fe isotopes independent of their mass, in contrast to fractionation patterns often observed for other isotopes. Because this signature is not produced abiotically or by other iron-metabolizing bacteria, it could serve as a reliable biomarker of this ancient magnetic microbial lifestyle. Science, this issue p. 705 The iron isotope fractionation patterns of magnetotactic bacteria hint at a reliable biomarker of ancient microbes. Magnetotactic bacteria perform biomineralization of intracellular magnetite (Fe3O4) nanoparticles. Although they may be among the earliest microorganisms capable of biomineralization on Earth, identifying their activity in ancient sedimentary rocks remains challenging because of the lack of a reliable biosignature. We determined Fe isotope fractionations by the magnetotactic bacterium Magnetospirillum magneticum AMB-1. The AMB-1 strain produced magnetite strongly depleted in heavy Fe isotopes, by 1.5 to 2.5 per mil relative to the initial growth medium. Moreover, we observed mass-independent isotope fractionations in 57Fe during magnetite biomineralization but not in even Fe isotopes (54Fe, 56Fe, and 58Fe), highlighting a magnetic isotope effect. This Fe isotope anomaly provides a potential biosignature for the identification of magnetite produced by magnetotactic bacteria in the geological record.

Zn Isotope Fractionation during Sorption onto Kaolinite

In this study, we quantify zinc isotope fractionation during its sorption onto kaolinite, by performing experiments under various pH, ionic strength, and total Zn concentrations. A systematic enrichment in heavy Zn isotopes on the surface of kaolinite was measured, with Δ(66)Znadsorbed-solution ranging from 0.11‰ at low pH and low ionic strength to 0.49‰ at high pH and high ionic strength. Both the measured Zn concentration and its isotopic ratio are correctly described using a thermodynamic sorption model that considers two binding sites: external basal surfaces and edge sites. Based on this modeling approach, two distinct Zn isotopic fractionation factors were calculated: Δ(66)Znadsorbed-solution = 0.18 ± 0.06‰ for ion exchange onto basal sites, and Δ(66)Znadsorbed-solution = 0.49 ± 0.06‰ for specific complexation onto edge sites. These two distinct factors indicate that Zn isotope fractionation is dominantly controlled by the chemical composition of the solution (pH, ionic strength).

Chemical signature of magnetotactic bacteria

Significance Magnetite precipitates through either abiotic or biotic processes. Magnetotactic bacteria synthesize nanosized magnetite intracellularly and may represent one of the most ancient biomineralizing organisms. Thus, identifying bacterial magnetofossils in ancient sediments remains a key point to constrain life evolution over geological times. Although electron microscopy and magnetic characterizations allow identification of recent bacterial magnetofossils, sediment aging leads to variable dissolution or alteration of magnetite, potentially yielding crystals that barely preserve their structural integrity. Thus, reliable biosignatures surviving such modifications are still needed for distinguishing biogenic from abiotic magnetite. Here, we performed magnetotactic bacteria cultures and laboratory syntheses of abiotic magnetites. We quantified trace element incorporation into both types of magnetite, which allowed us to establish criteria for biomagnetite identification. There are longstanding and ongoing controversies about the abiotic or biological origin of nanocrystals of magnetite. On Earth, magnetotactic bacteria perform biomineralization of intracellular magnetite nanoparticles under a controlled pathway. These bacteria are ubiquitous in modern natural environments. However, their identification in ancient geological material remains challenging. Together with physical and mineralogical properties, the chemical composition of magnetite was proposed as a promising tracer for bacterial magnetofossil identification, but this had never been explored quantitatively and systematically for many trace elements. Here, we determine the incorporation of 34 trace elements in magnetite in both cases of abiotic aqueous precipitation and of production by the magnetotactic bacterium Magnetospirillum magneticum strain AMB-1. We show that, in biomagnetite, most elements are at least 100 times less concentrated than in abiotic magnetite and we provide a quantitative pattern of this depletion. Furthermore, we propose a previously unidentified method based on strontium and calcium incorporation to identify magnetite produced by magnetotactic bacteria in the geological record.

Metals in the Aquatic Environment—Interactions and Implications for the Speciation and Bioavailability: A Critical Overview

In most case scenarios, individual metals exist as components in mixtures with organic and inorganic substances and/or particulate matter. While the concepts encompassing mixture toxicity and modeling have been around for decades, only recently have new approaches (dynamic speciation techniques and fate and bioavailability models) been expanded to consider metal mixture scenarios. For example, the kinetic features of humic substances and inorganic colloids on the complexation of metals are generally considered. Although current environmental regulations rarely require an assessment of chemicals mixtures, research on these mixtures in the environment is essential for future regulatory demands and is vital for ensuring adequate environmental protection. Interpretation of speciation and bioavailability data from metal mixtures can be very complex and demanding, due to the existence of kinetic physicochemical transformations of the dynamic components. This kinetic effect largely affects metals’ dynamic speciation, culminating in different transformed metal-containing products with different contributions for the metal uptake by a consuming interface. This manuscript is focused on the environmental fate of metal mixtures, which determines how the mixture is biogeochemically processed and which receptors are most exposed (organisms and exposure route), with a special focus on their dynamic speciation, including a critical evaluation of the current challenges and available dynamic speciation techniques as well as computer codes and models.

Testing nanoeffect onto model bacteria: Impact of speciation and genotypes

Abstract The gram-negative bacteria Escherichia coli (E. coli) is a very useful prokaryotic model for testing the toxicity of ZnO nanoparticles (nano-ZnO). This toxicity is often linked to Zn2+ released from nanoparticles in the culture medium, and nano-ZnO dissolution in different media is clearly established. Here, two model E. coli strains MG1655 and W3110 both descendant from the original K-12 showing slight differences in their genome were submitted to nano-ZnO or Zn2+ in order 1 > to refine the nano-ZnO toxicity mechanisms to E. coli, and 2 > to investigate whether toxicity resulted from a real “nanoparticle” effect or from the release of Zn2+ in solution. To do so, both strains were submitted to various concentrations (i.e., 0.1–1 mM) of nano-ZnO or Zn2+ in Luria Bertani (LB) medium. These toxicity studies take into account the nano-ZnO solubility in the culture medium by specifically monitoring the Zn2+ release in our experimental systems. In our experimental conditions, differences in tolerance to nano-ZnO or Zn2+ between both strains were clearly evidenced. W3110 is generally more tolerant to metal than MG1655, the latter showing no real difference in its sensitivity to the two zinc added forms unlike W3110. The differences in behavior between both strains could be attributed to differences in the two genomes as a mutation named “amber” in W3110. Moreover, by using these two closely E. coli strains, a real “nano” effect is here clearly demonstrated providing a model to study the toxicity of ZnO nanoparticles.

Trace metals dynamics under contrasted land uses: contribution of statistical, isotopic, and EXAFS approaches.

Three sub-basins of the Seine River (France) under contrasted land uses (i.e., forested, agricultural, and urban) have been investigated in order to assess the origin and seasonal variation of trace metals, and evaluate their geochemical background and dynamics. Our results highlight a high anthropogenic impact on all elements for both the dissolved and particulate fractions. The main source for each element in the dissolved phase was determined and shows that transition and post-transition metals mainly originate from forested areas, while alkali and alkaline earth elements, metalloids, and halogens rather originate from agricultural land use. Conversely, for the particulate phase, most of the elements cannot be associated with a specific land use. Seasonal variation of elements was assessed according to the forested and agricultural land uses, and geochemical backgrounds were determined using average export rates, highlighting that the geochemical background for the forested land use is higher than the agricultural one for most of the elements. Finally, to confirm those results, Zn dynamics in the three characteristic sub-basins and between the different land uses was investigated using a combination of Zn speciation, Zn isotopic ratio, and Zn export rates.