Environmental Engineering Reference
In-Depth Information
Synechocystis
PCC 6803, and
Synechococcus
PCC 7942 were transformed with a
MtI cDNA gene under the regulation of a metal inducible promoter [93]. These
transgenic cyanobacteria were tolerant to up to 6 times the Cd(II) concentration of
the wildtypes.
A phytochelatin synthase-like protein has been detected in the cyanobacterium
Nostoc
PCC 7120 and, when its gene was expressed in
E. coli,
it yielded a protein
with MtIII homology [94]. Furthermore, NCBI-BLAST search enquiries revealed
that multiple MtIII synthase genes exist in this species. Despite this, cyanobacteria
do not actually appear to have their MtIII production enhanced through exposure
to metals. The fact that cyanobacteria possess these phytochelatin synthase genes
supports the notion that they may be constitutively produced at effective metal
detoxifying levels. Tsuji and colleagues [94] suggested that other organisms sub-
sequently evolved regulatory processes for activating these genes in the presence of
excess metals.
3.7 Metal Sulfide Biotransformation
Certain photosynthetic microbes [24, 95] and anaerobic bacteria [2] share the ability
to biotransform metal ions into sulfides, apparently through a common ability to
reduce sulfate into sulfide.
3.7.1 Anaerobic Metal-Sulfide Production
Sulfate reducing bacteria possess the ability to form hydrogen sulfide in the anoxic
zone of wetlands that, in turn, can act to precipitate metal ions into insoluble metal
sulfides [2]. This heavy metal binding process has already been incorporated into
bioremediation efforts using up-flow anaerobic packed bed reactors and other indus-
trial decontamination procedures [96, 97]. These anaerobic bacteria form sulfides
with Fe(III), U(VI), Cr(VI), Te(VII), Mo(VI) and Pd(II) [98]. Interestingly, conver-
sion into insoluble metal sulfides can inhibit further sulfur metabolism by sterically
preventing sulfate and organic compounds from coming into contact with relevant
enzymes [99].
One drawback to the use of these bacteria in bioremediation lies in the conun-
drum that even low levels of free metals, such as Cd(II), Zn(II) or Ni(II) in
concentrations as low as 20
M, can be toxic [100]. Furthermore, these organisms
require anoxic environments in order to function properly and maintaining these
conditions in an open system can be problematic.
μ
3.7.2 Aerobic Metal-Sulfide Biotransformation
Aerobic metal-sulfide biotransformation has been studied for mercury in algae,
cyanobacteria, and fungi [24, 101]. The spread of mercury has resulted from
industrial processes acting as point sources, the volatile mature of Hg(0), and the
fact that rainfall precipitation is a driving force in mercury's mobility. Therefore,
