Mathieu Pédrot

Insights into colloid-mediated trace element release at the soil/water interface

Organic or inorganic colloids play a major role in the mobilization of trace elements in soils and waters. Environmental physicochemical parameters (pH, redox potential, temperature, pressure, ionic strength, etc.) are the controlling factors of the colloidal mobilization. This study was dedicated to follow the colloid-mediated mobilization of trace elements through time at the soil/water interface by means of an experimental approach. Soil column experiments were carried out using percolating synthetic solutions. The percolated solutions were ultrafiltrated with various decreasing cutoff thresholds to separate the different colloidal phases in which the dissolved organic carbon and trace element concentrations were measured. The major results which stem from this study are the following: (i) The data can be divided into different groups of organic compounds (microbial metabolites, fulvic acids, humic acids) with regard to their respective aromaticity and molecular weight. (ii) Three groups of elements can be distinguished based on their relationships with the colloidal phases: the first one corresponds to the so-called truly dissolved group (Li, B, K, Na, Rb, Si, Mg, Sr, Ca, Mn, Ba, and V). The second one can be considered as an intermediate group (Cu, Cd, Co, and Ni), while the third group gathers Al, Cr, U, Mo, Pb, Ti, Th, Fe, and rare earth elements (REE) carried by the organic colloidal pool. (iii) The data demonstrate that the fulvic acids seem to be a major organic carrier phase for trace elements such as Cu, Cd, Co, and Ni. By contrast, the trace elements belonging to the so-called colloidal pool were mostly mobilized by humic acids containing iron nanoparticles. Lead, Ti, and U were mobilized by iron nanoparticles bound to these humic acids. Thus, humic substances allowed directly or indirectly a colloidal transport of many insoluble trace elements either by binding trace elements or by stabilizing a ferric carrier phase. (iv) Finally, the results demonstrated also that REE were mostly mobilized by humic substances. The REE normalized patterns showed a middle REE downward concavity. Therefore, as previously shown elsewhere humic substances are a major control of REE speciation and REE fractionation patterns as well since the humic substance/metal ratio was the key parameter controlling the REE pattern shape.

How does organic matter constrain the nature, size and availability of Fe nanoparticles for biological reduction?

Few studies have so far examined the kinetics and extent of the formation of Fe-colloids in the presence of natural organic ligands. The present study used an experimental approach to investigate the rate and amount of colloidal Fe formed in presence of humic substances, by gradually oxidizing Fe(II) at pH 6.5 with or without humic substances (HS) (in this case, humic acid--HA and fulvic acid--FA). Without HS, micronic aggregates (0.1-1 μm diameter) of nano-lepidocrocite is obtained, whereas, in a humic-rich medium (HA and FA suspensions at 60 and 55 ppm of DOC respectively), nanometer-sized Fe particles are formed trapped in an organic matrix. A proportion of iron is not found to contribute to the formation of nanoparticles since iron is complexed to HS as Fe(II) or Fe(III). Humic substances tend to (i) decrease the Fe oxidation and hydrolysis, and (ii) promote nanometer-sized Fe oxide formation by both inhibiting the development of hydroxide nuclei and reducing the aggregation of Fe nanoparticles. Bioreduction experiments demonstrate that bacteria (Shewanella putrefaciens CIP 80.40 T) are able to use Fe nanoparticles associated with organic matter about eight times faster than in the case of nano-lepidocrocite. This increase in bioreduction rate appears to be related to the presence of humic acids that (i) indirectly control the size, shape and density of oxyhydroxides and (ii) directly enhance biological reduction of nanoparticles by electron shuttling and Fe complexation. These results suggest that, in wetlands but also elsewhere where mixed organic matter-Fe colloids occur, Fe nanoparticles closely associated with organic matter represent a bioavailable Fe source much more accessible for microfauna than do crystallized Fe oxyhydroxides.

Dynamic structure of humic substances: Rare earth elements as a fingerprint

Whereas humic substances are known to play a key role in controlling metal speciation and trace element mobility within soils and waters, the understanding of their structure is still unclear and remains a matter of debate. Several models of humic substance structure have been proposed, where humic substances were composed of either: (i) macromolecular polyelectrolytes that can form molecular aggregates or (ii) supramolecular assemblies (molecular aggregates) of small molecules without macromolecular character, joined together by weak attraction forces. This experimental study was designed and dedicated: (i) to follow the size of organic molecules versus ionic strength or pH by the combined means of ultrafiltration and aromaticity data and rare earth element (REE) fingerprinting, and (ii) to investigate the pH and ionic strength effect on the distribution of associated rare earth elements in soil solution. This study supports the presence of supramolecular associations of small molecules and probably the presence of macromolecules in the bulk dissolved organic matter. By contrast to ionic strength, pH appeared to be the major parameter playing on the stability of the humic substance structure. Humic substances displayed dynamic structures, which evolved with regard to pH. Low pH led to a destabilization of the humic substance conformation. This destabilization had an impact on the trace element distribution in soil solution, as assessed by REE data, and conversely, the destabilization degree of humic substances seemed to be influenced by the metal ion charge.

Double pH control on humic substance-borne trace elements distribution in soil waters as inferred from ultrafiltration

Colloidal dissolved organic carbon (DOC) is an important carrier phase for trace elements (TE) in subsurface environments. As suggested by previously published field observations, preferential sorption of DOC onto mineral surfaces tends to enrich the solid phase in humic acids. This DOC fractionation may affect the mobility of TE. pH is known to play an important role in the stability of colloids. This study was therefore dedicated to identifying the influence of DOC fractionation on TE mobility. Sequential extraction has been used to provide information on the possible TE carriers within soil (as exchangeable, weak acid soluble, reducible, oxidizable, and nonextractible metal fractions). Batch experiments were carried out to investigate the influence of pH on the detachment of colloids and associated TE. Different groups of elements were identified according to TE behavior during pH changes. Several elements displayed increasing concentrations with decreasing pH. These concentrations can represent an important fraction of the total soil concentration. By contrast, other elements showed increasing concentrations following increasing pH, in association with an increasing amount of colloids in soil solution. Concerning this latter group, two colloidal carrier phases were identified during the pH increase: (i) the first one concerned the majority of elements, which were associated with humic substances remaining in solution, and (ii) the second one involved several TE rather associated with nanooxides. Therefore, DOC fractionation plays a key role in the TE concentration in soil solution during pH changes.

Unraveling the Stratification of an Iron-Oxidizing Microbial Mat by Metatranscriptomics

A metatranscriptomic approach was used to study community gene expression in a naturally occurring iron-rich microbial mat. Total microbial community RNA was reversely transcribed and sequenced by pyrosequencing. Characterization of expressed gene sequences provided accurate and detailed information of the composition of the transcriptionally active community and revealed phylogenetic and functional stratifications within the mat. Comparison of 16S rRNA reads and delineation of OTUs showed significantly lower values of metatranscriptomic-based richness and diversity in the upper parts of the mat than in the deeper regions. Taxonomic affiliation of rRNA sequences and mRNA genome recruitments indicated that iron-oxidizing bacteria affiliated to the genus Leptothrix, dominated the community in the upper layers of the mat. Surprisingly, type I methanotrophs contributed to the majority of the sequences in the deep layers of the mat. Analysis of mRNA expression patterns showed that genes encoding the three subunits of the particulate methane monooxygenase (pmoCAB) were the most highly expressed in our dataset. These results provide strong hints that iron-oxidation and methane-oxidation occur simultaneously in microbial mats and that both groups of microorganisms are major players in the functioning of this ecosystem.