The mobility of arsenic commonly increases as reducing conditions are established within sediments or flooded soils. Although the reduction of arsenic increases its solubility at circumneutral pH, hydrous ferric oxides (HFO) strongly sorb both As(V) (arsenate) and As(III) (arsenite), the two primary inorganic species. Thus, in the presence of excess HFO, reductive dissolution of iron may be the dominant mechanism by which As is released into solution.In this paper, we report that the dissimilatory ironreducing bacterium Shewanella alga strain BrY promoted As mobilization from a crystalline ferric arsenate as well as from sorption sites within whole sediments. S. alga cells released arsenate from the mineral scorodite (FeAsO 4 ‚2H 2 O) as a result of dissimilatory (i.e., respiratory) reduction of Fe(III) to Fe(II). Solid-phase analysis with SEM-EDS and XAFS (X-ray absorption fine structure) spectroscopy revealed that the valence states of Fe and As in the solidphase product were identical to those in solution, i.e., Fe(II) and As(V). Additionally, As(V) sorbed to sediments from Lake Coeur d'Alene, ID, a mining-impacted environment enriched in both Fe and As, was solubilized by the activity of S. alga BrY. In neither experiment was As(III) detected. We conclude that arsenic mobility can be enhanced by the activity of dissimilatory iron-reducing bacteria in the absence of arsenic reduction.

When microbes evolve in a continuous, nutrient-limited environment, natural selection can be predicted to favor genetic changes that give cells greater access to limiting substrate. We analyzed a population of baker's yeast that underwent 450 generations of glucose-limited growth. Relative to the strain used as the inoculum, the predominant cell type at the end of this experiment sustains growth at significantly lower steady-state glucose concentrations and demonstrates markedly enhanced cell yield per mole glucose, significantly enhanced high-affinity glucose transport, and greater relative fitness in pairwise competition. These changes are correlated with increased levels of mRNA hybridizing to probe generated from the hexose transport locus HXT6. Further analysis of the evolved strain reveals the existence of multiple tandem duplications involving two highly similar, high-affinity hexose transport loci, HXT6 and HXT7. Selection appears to have favored changes that result in the formation of more than three chimeric genes derived from the upstream promoter of the HXT7 gene and the coding sequence of HXT6. We propose a genetic mechanism to account for these changes and speculate as to their adaptive significance in the context of gene duplication as a common response of microorganisms to nutrient limitation.

Dissimilatory arsenate-reducing bacteria have been implicated in the mobilization of arsenic from arsenicenriched sediments. An As(V)-reducing bacterium, designated strain GBFH, was isolated from arsenic-contaminated sediments of Lake Coeur d'Alene, Idaho. Strain GBFH couples the oxidation of formate to the reduction of As(V) when formate is supplied as the sole carbon source and electron donor. Additionally, strain GBFH is capable of reducing As(V), Fe(III), Se(VI), Mn(IV) and a variety of oxidized sulfur species. 16S ribosomal DNA sequence comparisons reveal that strain GBFH is closely related to Desulfitobacterium hafniense DCB-2 T and Desulfitobacterium frappieri PCP-1 T . Comparative physiology demonstrates that D. hafniense and D. frappieri, known for reductively dechlorinating chlorophenols, are also capable of toxic metal or metalloid respiration. DNA-DNA hybridization and comparative physiological studies suggest that D. hafniense, D. frappieri, and strain GBFH should be united into one species. The isolation of an Fe(III)-and As(V)-reducing bacterium from Lake Coeur d'Alene suggests a mechanism for arsenic mobilization in these contaminated sediments while the discovery of metal or metalloid respiration in the genus Desulfitobacterium has implications for environments cocontaminated with arsenious and chlorophenolic compounds.Arsenic is the 20th most abundant element in the Earth's crust (56) and is widely distributed throughout nature as a result of weathering, dissolution, fire, volcanic activity, and anthropogenic input (13). The last includes the use of arsenic in pesticides, herbicides, wood preservatives, and dye stuffs as well as production of arsenic-containing wastes during smelting and mining operations (56). In arsenic-enriched environments, a major concern is the potential for mobilization and transport of this toxic element to groundwater and drinking water supplies. In Bangladesh, an estimated 57 million people have been exposed to arsenic through contaminated wells (9). This incident serves as an unfortunate reminder of the toxic consequences of arsenic mobilization and underscores the need to understand the factors controlling the mobility and solubility of arsenic in aquatic systems (60).Coeur d'Alene Lake (CDAL) is the second largest lake in Idaho. As a result of a century of mining along the Coeur d'Alene River, one of two rivers feeding CDAL, lake sediments are highly enriched in trace elements including Ag, As, Cd, Pb, Sb, and Zn (31). Sediment pore waters are also trace element enriched with mean total arsenic and lead concentrations exceeding 160 and 250 g/liter (28), respectively. Nevertheless, CDAL surface waters comply with current federal drinking water standards (28) (50 and 15 g/liter for As and Pb [75,76], respectively). Because residents of Northern Idaho use these waters for recreation and fishing and as a source of drinking water (82), concern remains over the possibility that contaminants could be mobilized from the sediment to the water column.Iron is the dominant met...