Speciation of Arsenic under Dynamic Conditions

Arsenic mediated modifications in Bacillus aryabhattai and their biotechnological applications for arsenic bioremediation

The present study reports the arsenic (As) tolerance mechanism of bacteria Bacillus aryabhattai (NBRI014). The data explores the intracellular accumulation and volatilization of As from the culture medium after 48 h of exposure to 25,000 mg l^-1 arsenate As(V). The study also provides the evidence of presence of ars operon in bacteria, which may have played an important role in reducing As toxicity. Additionally, we found 7 differentially expressed proteins to be up-regulated in bacterial cells upon As exposure which may have role in reducing As toxicity inside bacterial cells. Furthermore, Fourier transform infrared (FTIR) spectroscopic techniques were useful to describe the structural and compositional alterations in bacterial cells after As treatment. It showed the changes in peak positions of the spectrum pattern when NBRI014 was grown in medium containing As, indicating that these functional groups viz. (amino, alkyl halides and hydroxyl) present on bacterial surface, which may be involved in As binding. The above results signify that biotechnological application of the isolate NBRI014 could be helpful in removal of As from polluted sites.

Characterization of Roseomonas and Nocardioides spp. for arsenic transformation

The metalloid arsenic predominantly exists in the arsenite [As(III)] and arsenate [As(V)]. These two forms are respectively oxidized and reduced by microbial redox processes. This study was designed to bioprospect arsenic tolerating bacteria from Lonar lake and to characterize their arsenic redoxing ability. Screening of sixty-nine bacterial species isolated from Lonar lake led to identification of three arsenic-oxidizing and seven arsenic-reducing species. Arsenite oxidizing isolate Roseomonas sp. L-159a being closely related to Roseomonas cervicalis ATCC 49957 oxidized 2mM As(III) in 60h. Gene expression of large and small subunits of arsenite oxidase respectively showed 15- and 17-fold higher expression. Another isolate Nocardioides sp. L-37a formed a clade with Nocardioides ghangwensis JC2055, exhibited normal growth with different carbon sources and pH ranges. It reduced 2mM As(V) in 36h and showed constitutive expression of arsenate reductase which increased over 4-fold upon As(V) exposure. Genetic markers related to arsenic transformation were identified and characterized from the two isolates. Moderate resistance against the arsenicals was exhibited by the two isolates in the range of 1-5mM for As(III) and 1-200mM for As(V). Altogether we provide multiple evidences to indicate that Roseomonas sp. and Nocardioides sp. exhibited arsenic transformation ability.

Unraveling the hydrodynamics of split root water uptake experiments using CT scanned root architectures and three dimensional flow simulations

Split root experiments have the potential to disentangle water transport in roots and soil, enabling the investigation of the water uptake pattern of a root system. Interpretation of the experimental data assumes that water flow between the split soil compartments does not occur. Another approach to investigate root water uptake is by numerical simulations combining soil and root water flow depending on the parameterization and description of the root system. Our aim is to demonstrate the synergisms that emerge from combining split root experiments with simulations. We show how growing root architectures derived from temporally repeated X-ray CT scanning can be implemented in numerical soil-plant models. Faba beans were grown with and without split layers and exposed to a single drought period during which plant and soil water status were measured. Root architectures were reconstructed from CT scans and used in the model R-SWMS (root-soil water movement and solute transport) to simulate water potentials in soil and roots in 3D as well as water uptake by growing roots in different depths. CT scans revealed that root development was considerably lower with split layers compared to without. This coincided with a reduction of transpiration, stomatal conductance and shoot growth. Simulated predawn water potentials were lower in the presence of split layers. Simulations showed that this was related to an increased resistance to vertical water flow in the soil by the split layers. Comparison between measured and simulated soil water potentials proved that the split layers were not perfectly isolating and that redistribution of water from the lower, wetter compartments to the drier upper compartments took place, thus water losses were not equal to the root water uptake from those compartments. Still, the layers increased the resistance to vertical flow which resulted in lower simulated collar water potentials that led to reduced stomatal conductance and growth.

Response of growth and superoxide dismutase to enhanced arsenic in two Bacillus species

Species differences in inorganic arsenic tolerance were investigated by comparing the responses of Bacillus subtilis (B. subtilis) and Bacillus thuringiensis (B. thuringiensis) to elevated concentrations of As(III) and As(V). The cell densities in treatments were always lower during the experiment compared to controls, with the exception of exposure to 1.0 mg As(V) l(-1) on the first day. It was also found that relative growth rate (RGR) of B. thuringiensis was lower than that of B. subtilis. Furthermore, RGR of each Bacillus species was negative correlation with toxicity of inorganic arsenic. However, total cell number still increased in each treatment according to cell density and RGR assays. Superoxide dismutase (SOD) activity of both Bacillus species was promoted by As(III) and As(V), especially under high arsenic concentration condition. In addition, SOD activity of B. thuringiensis was higher than that of B. subtilis during the same exposure time. In lipid peroxidation assay, thiobarbituric acid-reactive substances (TBARS) content of each Bacillus species had a significant increase with increment of arsenic concentration. Moreover, significant difference was observed between the two Bacillus species under high arsenic concentration. TBARS content of B. thuringiensis was higher than that of B. subtilis, indicating that effect of arsenic on cell membranes of B. thuringiensis was much more than that of B. subtilis. These results suggest that the two Bacillus species could adapt and live in high arsenic aquifers, although their growth and cell membranes were affected by As treatment in a way.

Arsenic transforming abilities of groundwater bacteria and the combined use of Aliihoeflea sp. strain 2WW and goethite in metalloid removal

Several technologies have been developed for lowering arsenic in drinking waters below the World Health Organization limit of 10 μg/L. When in the presence of the reduced form of inorganic arsenic, i.e. arsenite, one options is pre-oxidation of arsenite to arsenate and adsorption on iron-based materials. Microbial oxidation of arsenite is considered a sustainable alternative to the chemical oxidants. In this contest, the present study investigates arsenic redox transformation abilities of bacterial strains in reductive groundwater from Lombardia (Italy), where arsenite was the main arsenic species. Twenty isolates were able to reduce 75 mg/L arsenate to arsenite, and they were affiliated to the genera Pseudomonas, Achromobacter and Rhodococcus and genes of the ars operon were detected. Three arsenite oxidizing strains were isolated: they belonged to Rhodococcus sp., Achromobacter sp. and Aliihoeflea sp., and aioA genes for arsenite oxidase were detected in Aliihoeflea sp. strain 2WW and in Achromobacter sp. strain 1L. Uninduced resting cells of strain 2WW were used in combination with goethite for arsenic removal in a model system, in order to test the feasibility of an arsenic removal process. In the presence of 200 μg/L arsenite, the combined 2WW-goethite system removed 95% of arsenic, thus lowering it to 8 μg/L. These results indicate that arsenite oxidation by strain 2WW combined to goethite adsorption is a promising approach for arsenic removal from contaminated groundwater.

Changes in Sb speciation with waterlogging of shooting range soils and impacts on plant uptake

A pot experiment was conducted to investigate the solubility and redox species of antimony (Sb) in a relocated shooting range soil and its uptake by Lolium perenne L. and Holcus lanatus L. under different water regimes. After 1-week waterlogging, the total Sb concentration in soil solution decreased from ∼110 μg L(-1) to <20 μg L(-1), and slowly increased over the following 4 weeks, with the dissolution of Fe and Mn (hydr)oxides. In this process, half of the Sb in soil solution was reduced to Sb(III), which greatly affected the plant uptake of Sb. Waterlogging increased shoot Sb concentrations of L. perenne by ∼10 fold but decreased uptake in H. lanatus by 80%. Results indicate that Sb might primarily be taken up as Sb(III) by L. perenne and as Sb(V) by H. lanatus. Temporary waterlogging of soil may increase the risk of trace elements entering the food chain.

Removal processes for arsenic in constructed wetlands

Arsenic pollution in aquatic environments is a worldwide concern due to its toxicity and chronic effects on human health. This concern has generated increasing interest in the use of different treatment technologies to remove arsenic from contaminated water. Constructed wetlands are a cost-effective natural system successfully used for removing various pollutants, and they have shown capability for removing arsenic. This paper reviews current understanding of the removal processes for arsenic, discusses implications for treatment wetlands, and identifies critical knowledge gaps and areas worthy of future research. The reactivity of arsenic means that different arsenic species may be found in wetlands, influenced by vegetation, supporting medium and microorganisms. Despite the fact that sorption, precipitation and coprecipitation are the principal processes responsible for the removal of arsenic, bacteria can mediate these processes and can play a significant role under favourable environmental conditions. The most important factors affecting the speciation of arsenic are pH, alkalinity, temperature, dissolved oxygen, the presence of other chemical species--iron, sulphur, phosphate--,a source of carbon, and the wetland substrate. Studies of the microbial communities and the speciation of arsenic in the solid phase using advanced techniques could provide further insights on the removal of arsenic. Limited data and understanding of the interaction of the different processes involved in the removal of arsenic explain the rudimentary guidelines available for the design of wetlands systems.

Arsenic-resistant bacteria associated with roots of the wild Cirsium arvense (L.) plant from an arsenic polluted soil, and screening of potential plant growth-promoting characteristics

A rhizobacterial community, associated with the roots of wild thistle Cirsium arvense (L.) growing in an arsenic polluted soil, was studied by fluorescence in situ hybridization (FISH) analysis in conjunction with cultivation-based methods. In the bulk, rhizosphere, and rhizoplane fractions of the soil, the qualitative picture obtained by FISH analysis of the main phylogenetic bacterial groups was similar and was predominantly comprised of Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. The arsenic-resistant isolates belonged to 13 genera, the most abundant being those of Bacillus, Achromobacter, Brevundimonas, Microbacterium, and Ochrobactrum. Most bacteria grew in the presence of high arsenic concentrations (over 100mM arsenate and 10mM arsenite). Most strains possessed the ArsC, ArsB and ACR3 genes homologous to arsenate reductase and to the two classes of arsenite efflux pumps, respectively, peculiar to the ars operon of the arsenic detoxification system. ArsB and ACR3 were present simultaneously in highly resistant strains. An inconsistency between 16S rRNA phylogenetic affiliations and the arsenate reductase sequences of the strains was observed, indicating possible horizontal transfer of arsenic resistance genes in the soil bacterial community. Several isolates were able to reduce arsenate and to oxidise arsenite. In particular, Ancylobacter dichloromethanicum strain As3-1b possessed both characteristics, and arsenite oxidation occurred in the strain also under chemoautotrophic conditions. Some rhizobacteria produced siderophores, indole acetic acid and 1-amino-cyclopropane-1-carboxylic acid deaminase, thus possessing potential plant growth-promoting traits.