October 19, 2007
Arsenic uptake, speciation and tolerance in the hyperaccumulator
Arsenic is a metalloid element present throughout the world’s soils in varying
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).
into the natural system, As typically invades soil and groundwater in one of two forms,
oxidized arsenate (AsO
, As[V]) or reduced arsenite (As
The two forms
typically correspond to whether the soil is aerobic (arsenate) or anaerobic (arsenite).
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.
with a report regarding the Chinese brake fern’s,
, unique ability to
accumulate extraordinary quantities of As from contaminated soil (Ma et al, 2001).
fern has the ability to accumulate more than 1% its dry weight mass of As.
makes it an interesting candidate for remediation of As contaminated soils (Gonzaga et
mechanism for this hyperaccumulation has not been
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,
Hopefully, an examination of these
mechanisms will provide some insight into the extraordinary tolerance that
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.
as a result, all have active phosphate import systems in their roots.
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
is suspected to be no different.
Given these assumptions,
one would expect competition between phosphate and arsenate uptake in
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