Impact of microbial communities from tropical soils on the mobilization of trace metals during dissolution of cinnabar ore.

Biodissolution experiments on cinnabar ore (mercury sulphide and other sulphide minerals, such as pyrite) were performed with microorganisms extracted directly from soil. These experiments were carried out in closed systems under aerobic and anaerobic conditions with 2 different soils sampled in French Guyana. The two main objectives of this study were (1) to quantify the ability of microorganisms to mobilize metals (Fe, Al, Hg) during the dissolution of cinnabar ore, and (2) to identify the links between the type and chemical properties of soils, environmental parameters such as season and the strategies developed by indigenous microorganisms extracted from tropical natural soils to mobilize metals. Results indicate that microbial communities extracted directly from various soils are able to (1) survive in the presence of cinnabar ore, as indicated by consumption of carbon sources and, (2) leach Hg from cinnabar in oxic and anoxic dissolution experiments via the acidification of the medium and the production of low molecular mass organic acids (LMMOAs). The dissolution rate of cinnabar in aerobic conditions with microbial communities ranged from 4.8×10-4 to 2.6×10-3µmol/m2/day and was independent of the metabolites released by the microorganisms. In addition, these results suggest an indirect action by the microorganisms in the cinnabar dissolution. Additionally, because iron is a key element in the dynamics of Hg, microbes were stimulated by the presence of this metal, and microbes released LMMOAs that leached iron from iron-bearing minerals, such as pyrite and oxy-hydroxide of iron, in the mixed cinnabar ore.

Evaluation of trace metal accumulation in six vegetable crops intercropped with phytostabilizing plant species, in a French urban wasteland.

The extensive development of agriculture in urban and peri-urban wastelands polluted with several trace elements (TE) poses risks to human health through contaminated food products. The objective was to explore the accumulation of TE in the various parts of vegetable crop plants (tomato, French bean, radish, potato, spinach, and leek) intercropped with phytostabilizing plant species (ryegrass and white clover, respectively). Field studies were conducted in a multicontaminated French urban wasteland with Cd, Cu, Pb and Zn, and an alkaline soil pH. Analyses of the respective non-edible parts of monocultured vegetable crops showed accumulation of all TE, mostly Zn, then Pb and Cu, and finally Cd. The corresponding TE accumulation factors (soil to plant) were all below 0.25. In the edible parts, average concentrations for TE were above the limit values, according to European and Chinese standards. TE contents in the phytostabilizing species chosen were in the same orders of magnitude and the same ranking as described for vegetable crops and most accumulation was in the roots. Unexpectedly, the presence of the phytostabilizing plants had a very strong positive impact on the soil to plant accumulation factor. Moreover, the edible plant parts were poorly impacted by the co-cropping with phytostabilizing plants.

Seasonality of Hg dynamics in the Ebrié Lagoon (Côte d'Ivoire) ecosystem: influence of biogeochemical factors.

This study addresses the different biogeochemical parameters that control the dynamics of Hg, which is a less-studied metal in the Ebrié Lagoon. During two hydrological seasons, the dry season and the rainy season, we regularly sampled and analysed various compartments (e.g. sediments and fishes (Tilapia sp.)) of the lagoon. Thus, the physicochemical parameters were measured in situ (e.g. temperature, pH, salinity, redox potential and dissolved oxygen, total dissolved organic carbon, nitrates and sulphates), and the microbiological parameters (e.g. cultivable cells, total enzymatic activity and catabolic activity) were measured to establish the seasonal variations in the links between Hg and biogeochemical parameters through multivariate statistical analyses. The bioavailability of Hg from an unpolluted site was studied by comparing the ratios of fish and sediment. The results indicated that the seasons influenced the different biogeochemical factors, although for some factors, the variations were not significant. This influence was more pronounced in the dry season than in the rainy season. The impact of microbial activities and organic matter on Hg dynamics was observed in all seasons. However, other factors, such as pH, temperature, salinity, Eh and sulphates, influenced the dynamics of Hg only in the dry season.

Mercury behaviour and C, N, and P biogeochemical cycles during ecological restoration processes of old mining sites in French Guiana.

Several decades of gold mining extraction activities in the Amazonian rainforest have caused deforestation and pollution. While ecological rehabilitation is essential for restoring biodiversity and decreasing erosion on deforested lands, few studies note the behaviour or toxicity of trace elements during the rehabilitation process. Our original study focused on the potential use of microbial activity and Hg speciation and compared them with As, Cu, Zn and Cr speciation in assessing the chemical and biological quality of ecological restoration efforts. We sampled two sites in French Guyana 17 years after rehabilitation efforts began. The former site was actively regenerated (R) with the leguminous species Clitoria racemosa and Acacia mangium, and the second site was passively regenerated with spontaneous vegetation (Sv). We also sampled soil from a control site without a history of gold mining (F). We performed microcosm soil experiments for 30 days, where trace element speciation and enzyme activities (i.e., FDA, dehydrogenase, ß-glucosidase, urease, alkaline and acid phosphatase) were estimated to characterise the behaviour of trace elements and the soil microbial activity. As bioindicators, the use of soil microbial carbon biomass and soil enzyme activities related to the carbon and phosphorus cycles seems to be relevant for assessing soil quality in rehabilitated and regenerated old mining sites. Our results showed that restoration with leguminous species had a positive effect on soil chemical quality and on soil microbial bioindicators, with activities that tended toward natural non-degraded soil (F). Active restoration processes also had a positive effect on Hg speciation by reducing its mobility. While in Sv we found more exchangeable and soluble mercury, in regenerated sites, Hg was mostly bound to organic matter. These results also suggested that enzyme activities and mercury cycles are sensitive to land restoration and must be considered when evaluating the efficiency of restoration processes.

Interactions of three soil bacteria species with phyllosilicate surfaces in hybrid silica gels.

To simulate iron consumption in soils, iron leaching from silicate minerals due to three heterotrophic bacterial strains and a chemical treatment was studied using hybrid silica gel (HSG) doped with two phyllosilicates, nontronite (NAu-2) or low-iron-content montmorillonite (SWy-2). HSG methodology, a novel way of separating bacteria cells from a colloidal mineral source, consisted in embedding colloidal mineral particles into an amorphous porous silica matrix using a classical sol-gel procedure. Pantoae agglomerans PA1 and Rahnella aquatilis RA1 were isolated from silicate-rich soils, that is, beech and wheat rhizospheres (Vosges, France); Burkholderia sp. G5 was selected from acidic and nutrient-poor podzol soils (Vosges, France). Fe release from clay minerals and production of bacterial metabolites, that is, low molecular weight organic acids (LMWOA) and siderophores, were monitored. Two LMWOA profiles were observed with major gluconate production (> 9000 µM) for Burkholderia sp. G5 and moderate production of lactate, acetate, propionate, formate, oxalate, citrate, and succinate (< 300 µM) for R. aquatilis RA1 and P. agglomerans PA1. HSG demonstrated its usefulness in revealing clay mineral-microorganisms interactions. The effect of bacterial exsudates was clearly separated from physical contact effect.