JP 1 Arsenic - David Grauer Arsenic uptake speciation and...

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David Grauer October 19, 2007 Arsenic uptake, speciation and tolerance in the hyperaccumulator Pteris vittata Introduction Arsenic is a metalloid element present throughout the world’s soils in varying concentrations. Typically, the concentrations range from less than 10 ppm naturally to >30,000 ppm in anthropogenically contaminated soils (Gozaga, M., et al., 2006). Anthropogenic contamination is typically the result of pesticides and fungicides used in many treated woods and, until recently, on farms. Some anthropogenic As contamination is also derived from the combustion of some fossil fuels as well as the smelting of some metallic ores that contain residual As concentrations (Gonzaga, M., et al., 2006). Once into the natural system, As typically invades soil and groundwater in one of two forms, oxidized arsenate (AsO 4 3- , As[V]) or reduced arsenite (As 2 O 3 , As[III]). The two forms typically correspond to whether the soil is aerobic (arsenate) or anaerobic (arsenite). As in both forms is quite harmful to humans as a known carcinogen among other dangerous properties. In plants, arsenate’s likeness to inorganic phosphate causes metabolic disruption while arsenite disrupts enzyme function and eventually leads to cell death (Rathinasabapathi et. al., 2006 and Wang et al., 2002). Thus, As soil and groundwater contamination is of particular concern. Until recently, there were no known hyperaccumulators of arsenic. This changed with a report regarding the Chinese brake fern’s, Pteris vittata , unique ability to accumulate extraordinary quantities of As from contaminated soil (Ma et al, 2001). The fern has the ability to accumulate more than 1% its dry weight mass of As. This ability makes it an interesting candidate for remediation of As contaminated soils (Gonzaga et al., 2006). As yet, Pteris vittata’s mechanism for this hyperaccumulation has not been elucidated completely. Still, a general picture has emerged and that mechanism is quite different from other species known to tolerate As. This paper will review the mechanisms associated with the initial uptake, transportation, transformations and sequestration of As in the fern, Pteris vittata. Hopefully, an examination of these mechanisms will provide some insight into the extraordinary tolerance that P. vittata has for arsenic. Arsenic Uptake in P. vittata As mentioned, arsenate, the most abundant form of arsenate in most soils, is an analog of phosphate. Phosphate, unlike As, is an essential element for all plants. Plants, as a result, all have active phosphate import systems in their roots. Given arsenate’s structural similarity to phosphate (it has the same period and oxidation state as inorganic phosphate), all plants, inadvertently or not, actively import arsenate via their phosphate transporter proteins. P. vittata is suspected to be no different. Given these assumptions, one would expect competition between phosphate and arsenate uptake in P. vittata since both compounds use the same import mechanism and this is seen in the uptake kinetics of As; increasing phosphate soil concentrations decreases the rate at which As is
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accumulated (Wang, J., et al. 2002).
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