Design of a Whole-Cell Biosensor Based on Bacillus subtilis Spores and the Green Fluorescent Protein To Monitor Arsenic.

A green fluorescent protein (GFP)-based whole-cell biosensor (WCB-GFP) for monitoring arsenic (As) was developed in Bacillus subtilis. To this end, we designed a reporter gene fusion carrying the gfpmut3a gene under the control of the promoter/operator region of the arsenic operon (Pars::gfpmut3a) in the extrachromosomal plasmid pAD123. This construct was transformed into B. subtilis 168, and the resultant strain was used as a whole-cell biosensor (BsWCB-GFP) for the detection of As. The BsWCB-GFP was specifically activated by inorganic As(III) and As(V), but not by dimethylarsinic acid [DMA(V)], and exhibited high tolerance to the noxious effects of arsenic. Accordingly, after 12 h exposure, B. subtilis cells carrying the Pars::gfpmut3a fusion exhibited 50 and 90% lethal doses (LD50 and LD90) to As(III) of 0.89 mM and As 1.71 mM, respectively. Notably, dormant spores from the BsWCB-GFP were able to report the presence of As(III) in a concentration range from 0.1 to 1,000 µM 4 h after the onset of germination. In summary, the specificity and high sensitivity for As, as well as its ability to proliferate under concentrations of the metal that are considered toxic in water and soil, makes the B. subtilis biosensor developed here a potentially important tool for monitoring environmental samples contaminated with this pollutant. IMPORTANCE Arsenic (As) contamination of groundwater is associated with serious worldwide health risks. Detection of this pollutant at concentrations that are established as permissible for water consumption by WHO is a matter of significant interest. Here, we report the generation of a whole-cell biosensor for As detection in the Gram-positive spore former B. subtilis. This biosensor reports the presence of inorganic As, activating the expression of the green fluorescent protein (GFP) under the control of the promoter/operator of the ars operon. The biosensor can proliferate under concentrations of As(III) that are considered toxic in water and soil and detect this ion at concentrations as low as 0.1 µM. Of note, spores of the Pars-GFP biosensor exhibited the ability to detect As(III) following germination and outgrowth. Therefore, this novel tool has the potential to be directly applied to monitor As contamination in environmental samples.

Comparative assessment of water quality indices-a case study to evaluate water quality for drinking water supply and irrigation in Northern Mexico.

Water quality indices (WQIs) are numerical measures used by researchers and water managers to communicate water quality status to the public. This study analyzes the official databases from the CONAGUA monitoring network of the main tributary rivers in the middle basin of the San Pedro-Mezquital River in Durango, Mexico, for a 6-year period (2013-2018). We applied three WQIs to 432 data (18 sampling sites, four samples per year, 6 years): Canadian Council of Ministers of the Environment (CCME) WQI, National Sanitation Foundation (NSF) WQI, and Secretariat of Urban Development and Ecology (SEDUE) WQI. The Canadian index proved to be a flexible, appropriate, and rigorous methodology for assessing water quality based on its use. Results classify the water quality in the studied reservoirs as good, while river water was rated for public use, as marginal to very poor. No statistical significant differences in the quality of water between the rainy (June-October) and dry (November-May) seasons were found. However, tendency shows that in the rainy season the water quality was lower, mainly attributed to agricultural runoffs and bank erosion. The main contamination problem was the presence of fecal coliforms in high concentrations, which is associated to the high population density in the area, low sanitation efficiency, and multiple non-point discharges.

Seven potential sources of arsenic pollution in Latin America and their environmental and health impacts.

This review presents a holistic overview of the occurrence, mobilization, and pathways of arsenic (As) from predominantly geogenic sources into different near-surface environmental compartments, together with the respective reported or potential impacts on human health in Latin America. The main sources and pathways of As pollution in this region include: (i) volcanism and geothermalism: (a) volcanic rocks, fluids (e.g., gases) and ash, including large-scale transport of the latter through different mechanisms, (b) geothermal fluids and their exploitation; (ii) natural lixiviation and accelerated mobilization from (mostly sulfidic) metal ore deposits by mining and related activities; (iii) coal deposits and their exploitation; (iv) hydrocarbon reservoirs and co-produced water during exploitation; (v) solute and sediment transport through rivers to the sea; (vi) atmospheric As (dust and aerosol); and (vii) As exposure through geophagy and involuntary ingestion. The two most important and well-recognized sources and mechanisms for As release into the Latin American population's environments are: (i) volcanism and geothermalism, and (ii) strongly accelerated As release from geogenic sources by mining and related activities. Several new analyses from As-endemic areas of Latin America emphasize that As-related mortality and morbidity continue to rise even after decadal efforts towards lowering As exposure. Several public health regulatory institutions have classified As and its compounds as carcinogenic chemicals, as As uptake can affect several organ systems, viz. dermal, gastrointestinal, peptic, neurological, respiratory, reproductive, following exposure. Accordingly, ingesting large amounts of As can damage the stomach, kidneys, liver, heart, and nervous system; and, in severe cases, may cause death. Moreover, breathing air with high As levels can cause lung damage, shortness of breath, chest pain, and cough. Further, As compounds, being corrosive, can also cause skin lesions or damage eyes, and long-term exposure to As can lead to cancer development in several organs.

An overview of nitrate sources and operating processes in arid and semiarid aquifer systems.

Nitrate concentration in most aquifers in arid and semi-arid areas has increased in the past several decades as a result of human activities. Under the predominantly oxic conditions of these aquifers, denitrification is inhibited, allowing nitrate, a soluble and stable form of nitrogen (N), to accumulate. Because of its close association with municipal and agricultural wastes, nitrate is commonly used as an indicator of anthropogenic contamination. Aquifers affected by agricultural waste may contain salts from irrigation returns and herbicides in addition to nitrates. Preventing leakage from soil to deeper parts of the aquifer is thus a priority in the sustainable management of aquifers in arid and semiarid areas. Studies report a wide range of nitrate concentrations distributed non-uniformly within the aquifer, with roughly 40% and 20% of sampled wells exceeding 50mg/L nitrate in shallow and deep parts of the aquifer respectively. In aquifers at risk of becoming contaminated, nitrate isotopes (δ15N, δ18O, Δ17O) can be used to identify the source of nitrogen as mineral or organic fertilizer, sewage, or atmospheric deposition. A variety of mathematical models (crop, hydrological, geochemical, or a combination of them) have been successful in identifying best practices that minimize N leakage without negatively affecting crop yield. In addition, field research in crop management, e.g., conservation agriculture, has yielded promising results in determining the adequate dosage and time of application of fertilizers to reduce N losses. Examples of key dryland aquifers impacted by nitrate are discussed, and some of the most pressing challenges to achieve sustainability are presented.

Arsenic and fluoride variations in groundwater of an endorheic basin undergoing land-use changes.

The salt content of soil and water in endorheic basins within arid areas greatly restrict agricultural activities. Despite this limitation, these lands are increasingly used to accommodate new settlements and/or agricultural practices. This study focuses on the Laguna El Cuervo closed basin of northern Mexico and its underlying aquifer, which has been found to contain high concentrations of arsenic (As) and fluoride (F). The spatial distribution of As and F, their variations with time, and the impact of drought conditions and land-use changes were investigated using well data collected from a total of 27 wells in 2007, 2010, and 2011 (As data also collected in 2005). Four of these wells were used as monitoring wells. Data also included the As content of 140 surface sediments. Results showed that 54.5 % of the wells surpassed the As limit for drinking water of 0.025 mg L(-1) and that 89.0 % surpassed he F limit of 1.5 mg L(-1). Spatial analyses identified the areas in the center of the basin with the highest content of contaminants. Principal component and correlation analyses showed a co-occurrence of As and F with r = 0.55 for the 2011 data and 0.59 for the combined data. In contrast, the relationship of As and F concentrations to droughts and changes in land use were not as clearly shown, possibly because of the short time this area has been monitored. The high As and F concentrations in the groundwater may be limiting the availability of water within this basin, especially considering the greater groundwater demand foreseen for the future. Water-conservation practices, such as drip irrigation and artificial groundwater recharge, should be considered to maintain groundwater levels supportive of agricultural practices.

Modelling of arsenic retention in constructed wetlands.

A new model was developed in order to simulate the most significant arsenic retention processes that take place in constructed wetlands (CWs) treating high arsenic waters. The present contribution presents the implementation phases related to plants (arsenic uptake and accumulation, root arsenic adsorption, and root oxygen release), showing the first simulation results of the complete model. Different approaches with diverse influent configurations were simulated. In terms of total arsenic concentrations in effluent, the simulated data closely matched the data measured in all evaluated cases. The iron and arsenic species relationships, and the arsenic retention percentages obtained from simulations, were in agreement with the experimental data and literature. The arsenic retention efficiency increased whenever a new phase was implemented, reaching a maximum efficiency range of 85-95%. According to the quality of the obtained results, it can be considered that the implementation of all steps of RCB-ARSENIC provided reasonably good response values.

Co-occurrence of arsenic and fluoride in groundwater of semi-arid regions in Latin America: genesis, mobility and remediation.

Several million people around the world are currently exposed to excessive amounts of arsenic (As) and fluoride (F) in their drinking water. Although the individual toxic effects of As and F have been analyzed, there are few studies addressing their co-occurrences and water treatment options. Several studies conducted in arid and semi-arid regions of Latin America show that the co-occurrences of As and F in drinking water are linked to the volcaniclastic particles in the loess or alluvium, alkaline pH, and limited recharge. The As and F contamination results from water-rock interactions and may be accelerated by geothermal and mining activities, as well as by aquifer over-exploitation. These types of contamination are particularly pronounced in arid and semi-arid regions, where high As concentrations often show a direct relationship with high F concentrations. Enrichment of F is generally related to fluorite dissolution and it is also associated with high Cl, Br, and V concentrations. The methods of As and F removal, such as chemical precipitation followed by filtration and reverse osmosis, are currently being used at different scales and scenarios in Latin America. Although such technologies are available in Latin America, it is still urgent to develop technologies and methods capable of monitoring and removing both of these contaminants simultaneously from drinking water, with a particular focus towards small-scale rural operations.

Performance of Eleocharis macrostachya and its importance for arsenic retention in constructed wetlands.

INTRODUCTION: Arsenic (As) can be removed from water via rhizofiltration using phytostabilizing plants. The aim of this study was to investigate the performance of Eleocharis macrostachya in constructed wetland prototypes, as well as the plant's arsenic mass retention and the distribution of As along the wetland flow gradient and the soil in the wetland mesocosmos. MATERIALS AND METHODS: Experiments were carried out in laboratory-scale wetland prototypes, two planted with E. macrostachya and one without plants. Samples of water were taken at the inlet and outlet of the wetlands during the 33-week test period. At the end of the experiment, plants and soil (silty-sand) from each prototype were divided in three equal segments (entrance, middle and exit) and analyzed for their arsenic content. Results revealed that the planted wetlands have a higher As-mass retention capacity (87-90% of the total As inflow) than prototypes without plants (27%). RESULTS: As mass balance in the planted wetlands revealed that 78% of the total inflowing As was retained in the soil bed. Nearly 2% was absorbed in the plant roots, 11% was flushed as outflow, and the fate of the remaining 9% is unknown. In the prototype without plants, the soil retained 16% of As mass, 72% of the arsenic was accounted for in the outflow, and 12% was considered unknown. Although E. macrostachya retained only 2% of the total arsenic mass in their roots, its presence was a determining factor for arsenic retention in the wetland soil medium. CONCLUSION: Hence, planted wetlands might be a suitable option for treating As-contaminated water.

Small-scale and household methods to remove arsenic from water for drinking purposes in Latin America.

Small-scale and household low-cost technologies to provide water free of arsenic for drinking purposes, suitable for isolated rural and periurban areas not connected to water networks in Latin America are described. Some of them are merely adaptation of conventional technologies already used at large and medium scale, but others are environmentally friendly emerging procedures that use local materials and resources of the affected zone. The technologies require simple and low-cost equipment that can be easily handled and maintained by the local population. The methods are based on the following processes: combination of coagulation/flocculation with adsorption, adsorption with geological and other low-cost natural materials, electrochemical technologies, biological methods including phytoremediation, use of zerovalent iron and photochemical processes. Examples of relevant research studies and developments in the region are given. In some cases, processes have been tested only at the laboratory level and there is not enough information about the costs. However, it is considered that the presented technologies constitute potential alternatives for arsenic removal in isolated rural and periurban localities of Latin America. Generation, handling and adequate disposal of residues should be taken into account in all cases.

Occurrence and treatment of arsenic in groundwater and soil in northern Mexico and southwestern USA.

This review focuses on the occurrence and treatment of arsenic (As) in the arid region of northern Mexico (states of Chihuahua and Coahuila) and bordering states of the southwestern US (New Mexico, Arizona, and Texas), an area known for having high As concentrations. Information assembled and assessed includes the content and probable source of As in water, soil, and sediments and treatment methods that have been applied in the area. High As concentrations were found mainly in groundwater, their source being mostly from natural origin related to volcanic processes with significant anthropogenic contributions near mining and smelting of ores containing arsenic. The affinity of As for solid phases in alkaline conditions common to arid areas precludes it from being present in surface waters, accumulating instead in sediments and shifting its threat to its potential remobilization in reservoir sediments and irrigation waterways. Factors such as oxidation and pH that affect the mobility of As in the subsurface environment are mentioned. Independent of socio-demographic variables, nutritional status, and levels of blood lead, cognitive development in children is being affected when exposed to As. Treatments known to effectively reduce As content to safe drinking water levels as well as those that are capable of reducing As content in soils are discussed. Besides conventional methods, emergent technologies, such as phytoremediation, offer a viable solution to As contamination in drinking water.