Environmental Engineering Reference
In-Depth Information
mercury eventually accumulates in watershed ecosystems where organisms must
cope with it. The widespread assumption that Hg(II), as determined in animals and
higher plants, is bound to thiol chelates such as the metallothioneins discussed previ-
ously, does not appear to be universal. Kelly and colleagues [75] discovered that the
main mercury compound in several algae, cyanobacteria and fungi was mercury sul-
fide. It was concluded that these organisms can convert mercury into meta-cinnabar
and Hg(0) [101]. However, the pathway for the former conversion remains to be elu-
cidated [75] and at lower exposure rates the production of Hg(0) became negligible.
The cyanobacterial species
Limnothrix planctonica
,
Synechococcus leopoliensis
and
Phormidium limnetica
biotransformed Hg(II) under pH stable and aerated condi-
tions to meta-cinnabar as well as a relatively small amount of the volatile Hg(0) [24].
Furthermore, these species did not produce methyl-mercury under these conditions.
Biotransformation studies with several fresh water eukaryotic algae revealed
similar results demonstrating the synthesis of mercury sulfide.
Selenastrum min-
utum
,
Chlorella fusca
var.
fusca
,
Galdieria sulphuraria
and
Navicula pellicosa
were all tested for their ability to biotransform mercury provided as HgCl
2
[75].
All of the cultures were capable of biotransformation of the mercury into meta-
cinnabar, however the rates at which the transformations occurred was dramatically
different among the species. For
S. minutum
,
C. fusca
var
. fusca
, and the diatom
N. pelluclosa,
all of the mercury within the cultures was biotransformed in a period
of hours, whereas
G. sulphuraria
completed the transformation within a matter of
minutes [95].
When
G. sulphuraria
was exposed to 100 ppb Hg(II) it transformed 90% into
β
-HgS within 20 min [95]. This species is the only eukaryotic algae tested thus far
that can convert Hg(II) at such a rapid rate and this may be a testament to the condi-
tions to which the species is adapted, including volcanic and acidic areas throughout
the world [102, 103]. Volcanic activity can be associated with the release of high
amounts of mercury [102], and extremophiles such as
G. sulphuraria
would thus be
required to exhibit high Hg(II) biotransformation rates in order to survive.
The aerobic production of metal sulfides occurs in two apparent phases after
metal exposure [75]. When first exposed to the metal ions, there is a rapid phase of
metal sulfide formation. This rapid phase has been proposed to be dependent on a
readily available endogenous pool of sulfur present within the cell for direct sulfide
production [95]. Following this rapid phase, the production of metal sulfides slows
considerably and may be proportional to the rate by which such a pool is synthesized
by the organism. It is interesting to speculate that the sulfur in the metal sulfide may
be that which has been shown to be associated with and possibly derived from the
high molecular weight form of phytochelatin [56].
3.8 Metal Bioremediation
The cost for conventional remediation of metal-contaminated environments is high,
especially when dealing with removal of low concentrations in order to satisfy
regulatory requirements. The advantage of using biological organisms to treat
