Arsenic(V) Reduction in Relation to Iron(III) Transformation and Molecular Characterization of the Structural and Functional Microbial Community in Sediments of a Basin-Fill Aquifer in Northern Utah

Abstract

Basin-fill aquifers of the Southwestern United States are associated with elevated concentrations of arsenic (As) in groundwater. Many private domestic wells in the Cache Valley Basin, UT, have As concentrations in excess of the U.S. EPA drinking water limit. Thirteen sediment cores were collected from the center of the valley at the depth of the shallow groundwater and were sectioned into layers based on redoxmorphic features. Three of the layers, two from redox transition zones and one from a depletion zone, were used to establish microcosms. Microcosms were treated with groundwater (GW) or groundwater plus glucose (GW؉G) to investigate the extent of As reduction in relation to iron (Fe) transformation and characterize the microbial community structure and function by sequencing 16S rRNA and arsenate dissimilatory reductase (arrA) genes. Under the carbon-limited conditions of the GW treatment, As reduction was independent of Fe reduction, despite the abundance of sequences related to Geobacter and Shewanella, genera that include a variety of dissimilatory iron-reducing bacteria. The addition of glucose, an electron donor and carbon source, caused substantial shifts toward domination of the bacterial community by Clostridium-related organisms, and As reduction was correlated with Fe reduction for the sediments from the redox transition zone. The arrA gene sequencing from microcosms at day 54 of incubation showed the presence of 14 unique phylotypes, none of which were related to any previously described arrA gene sequence, suggesting a unique community of dissimilatory arsenate-respiring bacteria in the Cache Valley Basin.A rsenic (As) is one of the most frequently detected contaminants in private domestic wells used for household drinking water (1) and public water supplies (2). In the United States, As concentrations in excess of the drinking water maximum contaminant level (MCL; 10 g/liter) in public supply wells are distributed across the country, but 3/4 of these wells are in the western United States (2). In basin-fill aquifers in California, Nevada, New Mexico, Arizona (1), and Utah (3), 10% of the domestic wells tested had As in excess of the MCL. Modeling efforts by the USGS predicted that 43% of the area in the basin-fill aquifers of the Southwest might have groundwater that equals or exceeds the MCL for As (4). While public water suppliers are required to treat water to meet drinking water standards, there are no such requirements for private wells.Common aspects of the basin-fill aquifers in the western United States are geothermal activity and volcanic rock. The source of As in the Cache Valley Basin, UT, is presumed to be the hydrothermal sulfide and arsenide deposits in the surrounding mountains. Due to low rainfall and high evapotranspiration, groundwater recharge does not result from precipitation in the basin but from precipitation in the surrounding mountains. The Cache Valley Basin is located 128 km northeast of Salt Lake City and is on the eastern edge of the Basin and Range Province. A sur...

New Arsenate Reductase Gene ( arrA ) PCR Primers for Diversity Assessment and Quantification in Environmental Samples

“…S1) from the study by Mirza et al (17), as well as amplification from soil and groundwater samples collected from the CVB (Fig. S4).…”

“…Growth in arrA gene databases since 2009 has allowed us to design new PCR primers that amplify arrA gene fragments of the optimum size directly from environmental samples. The specificity of these new primers was confirmed by (i) amplification of previously generated arrA gene clones from diverse phylogenetic groups (17) and (ii) direct amplification and high-throughput sequencing of arrA gene amplicons from soil and groundwater from the Cache Valley Basin (CVB) in Utah. These primers were also evaluated for the quantification of arrA genes in soil and groundwater well samples through qPCR.…”

“…These dissimilatory arsenate-respiring bacteria (DARB) have a conserved arsenate reductase gene (arrA) that encodes an arsenate reductase enzyme, an important component of the microbial arsenate reduction system. This gene has been widely used as a biomarker to assess variations in the arsenic-reducing microbial taxa in different environments (11,17,(19)(20)(21)(22)(23)(24)(25)(26).…”

“…Arsenate (As V ) and arsenite (As III ) are the two dominant forms of naturally occurring inorganic arsenic (2,4); the latter has been reported to be more toxic and labile in the subsurface, but the relative mobilities of the two oxidation states are dependent on a number of environmental factors, such as pH and mineral composition (5). In nature, although abiotic reduction of As V to As III is possible in the presence of sulfide under strongly acidic conditions (6, 7), microbial reduction of As V to As III is more prevalent and is one of the major causes of arsenic contamination in groundwater (8)(9)(10)(11)(12)(13)(14)(15)(16)(17).…”

“…The length of the amplified PCR product is 630 bp. These primers have been used extensively (11,17,(21)(22)(23)(24)(25) to generate a useful arrA gene database. However, these primers have three potential limitations to their widespread application in studying the ecology of arsenatereducing microorganisms.…”