The key role of biogenic arsenic sulfides in the removal of soluble arsenic and propagation of arsenic mineralizing communities.

Biogeochemical processes govern the transport and availability of arsenic in sediments. However, little is known about the transition from indigenous communities to cultivable consortia when exposed to high arsenic concentrations. Such cultivable communities could be exploited for arsenic bioremediation of waste streams and polluted sites. Thus, it is crucial to understand the dynamics and selective pressures that shape the communities during the development of customized bacterial consortia. First, from the arsenic partitioning of two sediments with high arsenic concentrations, we found that up to 55% of arsenic was bioavailable because it was associated with the soluble, carbonate, and ionically exchangeable fractions. Next, we prepared sediment enrichment cultures under arsenate- and sulfate-reducing conditions to precipitate arsenic sulfide biominerals and analyze the communities. The produced biominerals were used as the inoculum to develop bacterial consortia via successive transfers. Tracking of the 16S rRNA gene in the fresh sediments, sediment enrichments, biogenic minerals, and bacterial consortia revealed differences in the bacterial communities. Removing the sediment caused a substantial decrease in diversity and shifts toward the dominance of the Firmicutes phylum to the detriment of Proteobacteria. In agreement with the 16S rRNA gene results, the sequencing of the arrA gene confirmed the presence of phylotypes closely related to Desulfosporosinus sp. Y5 (100% similarity), highlighting the pivotal role of this genus in the removal of soluble arsenic. Here, we demonstrated for the first time that besides being important as arsenic sinks, the biogenic arsenic sulfide minerals are reservoirs of arsenic resistant/respiring bacteria and can be used to culture them.

Presence of Arsenic in Potential Sources of Drinking Water Supply Located in a Mineralized and Mined Area of the Sierra Madre Oriental in Mexico.

Mine wastes from the La Aurora mine in the state of Guanajuato were generated by the flotation process and placed in four tailing dumps on the local stream while the plant operated. Given that these wastes contain toxic elements, it is important to establish their impact on the quality of several surrounding natural sources of water that are considered potential drinking water supplies. This study identified four water source types, in which the contents of arsenic (As), mercury (Hg), and thallium (Tl) were exceeded, according to international guideline values for drinking water quality. The first type of aqueous sample corresponded to leachates produced by rainwater infiltration in tailings and water-mineral waste interactions. The second type corresponded to surface water along the Xichú and La Laja Streams, and the third and fourth types involved two groundwater well samples and spring samples, respectively. The Chiquito Stream was used as a reference area that had not been impacted by the mine wastes. The isotopic signatures associated with δ34Ssulfate and δ18Osulfate compositions from the El Ojo de Agua spring are similar to those of the Santa María River and are different from those of the mine waste leachates. This study shows evidence of the presence of As in the El Ojo de Agua spring, which results from dissolution of secondary mineral phases that were produced by alteration of the mine wastes, which then migrated along the Xichú Stream system until reaching the spring. These As-bearing fine particles are prone to dissolution when in contact with this water source. Principal component analysis revealed that the observed As, Tl, and Hg can be attributed to weathering of the mine wastes. However, the results suggest that a natural contribution of these elements could be associated with rainwater-igneous rock interactions.

Effect of heavy metals on seed germination and seedling development of Nama aff. stenophylla collected on the slope of a mine tailing dump.

Nama aff. stenophylla plants grow on mining waste abandoned 100 years ago, exposed to high concentrations of heavy metals. Accumulation of heavy metals in plant biomass has been related to the phyto-accessible fractions of these, so we assessed the effect of those heavy metals in different concentrations on the germination and development of seedlings. The seed traits were characterized with optical and scanning electron microscope. The seeds were assessed for dormancy by pre-germinative treatments, germination percentage, tolerance index regarding heavy metals, and the elongation of seedlings under three concentrations of arsenic (As), cadmium (Cd), Iron (Fe), lead (Pb), and zinc (Zn) (phyto-accessible [Ph], five times higher [Hi] and lower [Lo]). The seeds have no dormancy, and the heavy metals did not affect the embryo. Pb, Cd, and As, affected the germination percentage more (p < 0.005). The treatments that most affected seedling elongation were Zn [Hi], Cd [Hi], Pb [Lo], Zn [Ph], Pb [Hi], Zn [Lo] (p < 0.005). The seedlings cells alterations were associated with the reduction in length, although larger cortical cells may be due to heavy metal compartmentalization in vacuoles. The seeds and seedlings showed tolerance to high concentrations of Fe and As, and to phyto-accessible of As, Cd, Pb, and Fe.

Dissolution and final fate of arsenic associated with gypsum, calcite, and ferrihydrite: Influence of microbial reduction of As(V), sulfate, and Fe(III).

Several studies have demonstrated that gypsum (CaSO4·2H2O) and calcite (CaCO3) can be important hosts of arsenic in contaminated hydrogeological systems. However, the extent to which microbial reducing processes contribute to the dissolution and transformation of carbonate and sulfate minerals and, thereby, to arsenic mobilization is poorly understood. These processes are likely to have a strong impact on arsenic mobility in iron-poor environments and in reducing aquifers where iron oxyhydroxides become unstable. Anoxic batch bioassays with arsenate (As(V)) coprecipitated with calcite, gypsum, or ferrihydrite (Fe(OH)3) were conducted in the presence of sulfate or molybdate to examine the impact of bioprocesses (i.e. As(V), sulfate, and Fe(III)-reduction) on arsenic dissolution, speciation, and eventual remineralization. Microbial reduction of As(V)-bearing calcite caused an important dissolution of arsenite, As(III), which remained in solution up to the end of the experiment (30 days). The reduction of As(V) from gypsum-As(V) also led to the release of As(III), which was subsequently remineralized, possibly as arsenic sulfides. The presence of sulfate triggered arsenic dissolution in the bioassays with ferrihydrite-As(V). This study showed that although gypsum and calcite have a lower capacity to bind arsenic, compared to iron oxides, they can play a critical role in the biogeochemical cycle of arsenic in natural calcareous and gypsiferous systems depleted of iron since they can be a source of electron acceptors for reducing bioprocesses.

Role of indigenous microbiota from heavily contaminated sediments in the bioprecipitation of arsenic.

High arsenic concentrations have been detected in alluvial aquifers of arid and semi-arid zones in Mexico. This work describes the potential of microbial arsenate reduction of the indigenous community present in sediments from an arsenic contaminated aquifer. Microcosms assays were conducted to evaluate arsenate and sulfate-reducing activities of the native microbiota. Two different sediments were used as inoculum in the assays amended with lactate (10mM) as electron donor and with sulfate and arsenate (10mM each) as electron acceptors. Sediments were distinguished by their concentration of total arsenic 238.3±4.1mg/kg or 2263.1±167.7mg/kg, which may be considered as highly contaminated sediments with arsenic. Microbial communities present in both sediments were able to carry out arsenate reduction, accomplished within 4days, with the corresponding formation of arsenite; sulfate reduction took place as well. Both reducing activities occurred without previous acclimation period or enrichment, even at potential inhibitory concentrations of arsenate as high as 750mg/L (10mM). The formation of a yellowish colloidal precipitate was evident when both reducing processes occurred in the microcosm, which contributed to remove between 52 and 90.9% of As(III) from the liquid phase by bioprecipitation of arsenic as arsenic sulfide.

Optimal conditions for chlorothalonil and dissolved organic carbon in horizontal subsurface flow constructed wetlands.

The most efficient system of horizontal subsurface flow constructed wetlands (HSSFCW) for removing dissolved organic carbon (DOC) in the presence of chlorothalonil pesticide (CLT) present in synthetic domestic wastewater was determined using the macrophyte Phragmites australis. Two concentrations of CLT (85 and 385 µg L-1) and one concentration of glucose (20 mg L-1) were evaluated in four pilot scale horizontal surface flow constructed wetlands coupled with two sizes of silica gravel, igneous gravel, fine chalky gravel (3.18-6.35 mm), coarse gravel (12.70-25.40 mm) and two water surface heights (20 and 40 cm). For a month, wetlands were acclimated with domestic wastewater. Some groups of bacteria were also identified in the biofilm attached to the gravel. In each treatment periodic samplings were conducted in the influent and effluent. Chlorothalonil was quantified by gas chromatography (GC-ECD m), DOC by an organic carbon analyzer and bacterial groups using conventional microbiology in accordance with Standard Methods. The largest removals of DOC (85.82%-85.31%) were found when using fine gravel (3.18-6.35 mm) and the lower layer of water (20 cm). The bacterial groups quantified in the biofilm were total heterotrophic, revivable heterotrophic, Pseudomonas and total coliforms. The results of this study indicate that fine grain gravel (3.18-6.35 mm) and both water levels (20 to 40 cm) can be used in the removal of organic matter and for the treatment of agricultural effluents contaminated with organo-chloride pesticides like CLT in HSSFCW.

Arsenic mobility controlled by solid calcium arsenates: a case study in Mexico showcasing a potentially widespread environmental problem.

An As-contaminated perched aquifer under an urban area affected by mining was studied over a year to determine the contamination source species and the mechanism of As mobilization. Results show that the dissolution of calcium arsenates in residues disposed on an inactive smelter has caused high levels of As pollution in the adjoining downgradient 6-km perched aquifer, reaching up to 158 mg/L of dissolved As, and releasing a total of ca. 7.5 tons of As in a year. Furthermore, free calcium ion availability was found to control As mobility in the aquifer through the diagenetic precipitation of calcium arsenates (Ca5H2(AsO4)4·cH2O) preventing further mobilization of As. Results shown here represent a model for understanding a highly underreported mechanism of retention of arsenate species likely to dominate in calcium-rich environments, such as those in calcareous sediments and soils, where the commonly reported mechanism of adsorption to iron(III) oxyhydroxides is not the dominant process.

Inhibition of sulfate reduction by iron, cadmium and sulfide in granular sludge.

This study investigated the inhibition effect of iron, cadmium and sulfide on the substrate utilization rate of sulfate reducing granular sludge. A series of batch experiments in a UASB reactor were conducted with different concentrations of iron (Fe2+, 4.0-8.5 mM), cadmium (Cd2+, 0.53-3.0 mM) and sulfide (4.2-10.6 mM), the reactor was fed with ethanol at 1g chemical oxygen demand (COD)/L and sulfate to yield a COD/SO4(2-) (g/g) ratio of 0.5. The addition of iron, up to a concentration of 8.1mM, had a positive effect on the substrate utilization rate which increased 40% compared to the rate obtained without metal addition (0.25 g COD/gVSS-d). Nonetheless, iron concentration of 8.5 mM inhibited the specific substrate utilization rate by 57% compared to the substrate utilization rate obtained in the batch amended with 4.0 mM Fe2+ (0.44 g COD/gVSS-d). Cadmium had a negative effect on the specific substrate utilization rate at the concentrations tested; at 3.0 mM Cd2+ the substrate utilization rate was inhibited by 44% compared with the substrate utilization rate without metal addition. Cadmium precipitation with sulfide did not decrease the inhibition of cadmium on sulfate reduction. These results could have important practical implications mainly when considering the application of the sulfate reducing process to treat effluents with high concentrations of sulfate and dissolved metals such as iron and cadmium.

Phytochelatin-cadmium-sulfide high-molecular-mass complexes of Euglena gracilis.

High-molecular-mass PC complexes (PC-HMWCs) constituted by phytochelatins (PCs), cadmium and sulfide are synthesized by several organisms after exposure to cadmium. In this study, PC-HMWCs were isolated from photoheterotrophic Euglena gracilis and purified to homogeneity, resulting in compounds of molecular mass 50-380 kDa depending on the CdCl2 and sulfate concentrations in the culture medium. In contrast with plants and some yeasts, PC-HMWCs from E. gracilis mainly comprise (57-75%) monothiol molecules (Cys, gamma-glutamylcysteine, GSH) and, to a lesser extent (25-43%), PCs. A similar acid-soluble thiol compound composition was found in whole cell extracts. The -SH/Cd2+ and S2-/Cd2+ ratios found in purified PC-HMWCs were 1.5 and 1.8, respectively; the (-SH + S2-)/Cd2+ ratio was 3.2. PC-HMWCs of molecular mass 60 and 100 kDa were also localized inside Percoll-purified chloroplasts, in which cadmium and PCs were mainly compartmentalized. Cadmium and sulfur-rich clusters with similar sulfur/cadmium stoichiometries to those of the purified PC-HMWCs were detected in the chloroplast and throughout the cell by energy dispersive microanalysis and atomic resolution electron microscopy. The presence of PC-HMWCs in primitive photosynthetic eukaryotes such as the protist, E. gracilis, suggests that their function as the final cadmium-storage-inactivation process is widespread. Their particular intracellular localization suggests that chloroplasts may play a major role in the cadmium-resistance mechanism in organisms lacking a plant-like vacuole.