Table from Fitz and Wenzel 2002
During evolution, plants have developed two strategies to help them survive
in arsenic-enriched environments, exclusion (or tolerance) and accumulation.
(Tu et.al 2002) Tolerant plants restrict transfer of arsenic to the roots
and then from the roots to the shoots, and are therefore less affected by
it. (Fitz and Wenzel 2002) Accumulators actively take up, transport, and then
sequester arsenic in vacuoles or cell walls, removing it from the soil. (Fitz
and Wenzel 2002)
|
Table from Fitz and Wenzel 2002
|
Although tolerators may be useful in revegetation of polluted sites, this
web site will deal mainly with accumulators, as they are more useful for most
phytoremediation strategies of arsenic-contaminated sites.
It is believed that arsenate is taken up from the soil by the phosphate
transporter. This has been suggested by exclusion experiments in which inorganic
arsenate competed with inorganic phosphate for transport by a phosphate transporter.
(Csanaky and Gregus 2001) It is believed that the high affinity phosphate
uptake system is involved, because it has a lower selectivity than the low
affinity system, and suppression of the high affinity system results in smaller
arsenic uptake and accumulation. (Fitz and Wenzel 2002) Once inside the plant,
the arsenate travels with the transpiration flow through the xylem and into
the stems and leaves. (Tu et.al 2002) Here it is reduced, possibly by an arsenate
reductase enzyme into its more toxic form, arsenite. (Dhankher et.al 2002)
Arsenite has a great affinity for thiol groups and consequently binds to
peptide-thiol molecules, also known as phytochelatins. (Dhankher et.al 2002)
Phytochelatin production appears to be activated by the presence of heavy
metals and arsenic. (Sauge-Merle et.al 2003) This complex between phytochelatin
and arsenic is then transported via a specific transport mechanism into a
vacuole or into the cell wall. (Pickering et.al 2000)
| Pteris vittata or brake fern was the first naturally
occurring arsenic hyperaccumulator discovered. It was found in Central Florida,
growing on a chromated copper arsenic-contaminated site. (Ma, et.al 2001)
The plants found on the site had arsenic concentrations between 1442-7526
ppm. (Ma, et.al 2001) In greenhouse experiments, the fern was able to grow
on soils up to 1500 ppm in concentration, and over a six week period accumulated
22,630 ppm arsenic in its fronds. (Ma, et.al 2001) Further studies with this
species showed that it had a great translocation ability, concentrating 42
times more arsenic in shoots than in roots within eight weeks of growth. (Tu
et.al 2002) Within 20 weeks of growth, each plant had removed an average of
38 mg of arsenic, 90% of which was translocated to the shoots. (Tu et.al 2002)
Also, 90% of this arsenic was taken from the insoluble arsenic pools exchangeable
with the soil particles, rather than dissolved arsenic which is generally
more available. (Tu et.al 2002) It is fast growing, has a large biomass,
and grows well in most mild climates, and is perennial, making it very useful
for phytoremediation. (Ma et.al 2001) |
Photo from http://www.horticulturist.com |
Photo from http://moorea.berkeley.edu |
Pityrogramma calomelanos is another
brake fern with hyperaccumulating abilities. Its growth was actually enhanced
by the addition of arsenic to its growth medium. (Francesconi et.al 2002)
The fern was able to accumulate arsenic in concentrations of up to 8350 micrograms
of arsenic per gram dry weight from soils with arsenic concentrations up to
510 micrograms per gram. (Francesconi et.al 2002) Interestingly, it showed
the greatest accumulation from the least contaminated soil, only 135 micrograms
per gram, which suggests that it may be able to remove arsenic from less contaminated
soils, reducing arsenic contamination to very low levels. (Francesconi et.al
2002) |
| Ceratophyllum demersum, Egeria
densa, and Lepidium sativum are arsenic accumulating
watercress species used as a food source in New Zealand. (Robinson et.al 2003)
They were found near a geothermal power station, and the arsenic was thought
to be a result of geothermal activity and the use of that power for commercialization. (Robinson
et.al 2003) They were found to have a arsenic concentration of 412 mg per
kg dry weight, which is well in excess of the World Health Organization's
limit of only 2 mg/kg fresh weight. (Robinson et.al 2003) In greenhouse studies,
the plants were able to accumulate arsenic up to 106 mg/kg, and this concentration
was positively correlated with the arsenic concentrations of the surrounding
water. (Robinson et.al 2003) This plant has the ability to remove arsenic
from contaminated water as well as soils, decreasing the arsenic concentrations
by about 7.3%. (Robinson et.al 2003) |
Photo from http://www.rnzih.org.nz |
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