Environmental Engineering Reference
In-Depth Information
when these complexes are stored intracellularly, MtIII metal complex formation
may not be ideal for bioremediation purposes. This is mainly owing to the fact that
MtIII metal complexes can be exported and dissociated when cytosolic heavy metal
concentrations become elevated [70, 83]. Regrettably, under these circumstances the
freed ionic metals return to bioavailable forms.
3.5 Toxicity of Heavy Metals
Metals in their elemental ionic forms elicit quite variable effects on cells. Each
metal has been shown to have its means of entry, however how toxic stress is
caused remains largely unclear. Heavy metals have been demonstrated to cause
membrane depolarization and acidification of the cytoplasm, disrupting homeosta-
sis [84]. Depolarization alters ion gradients required for the proper function of
within cellular compartments including organelles. In addition, heavy metals pro-
mote oxidative stress by causing an increase in the concentration of reactive oxygen
species [85] and suppressing cellular antioxidation mechanisms [86]. This is the
case for Fe(II) and Cu(II) in sunflower where these metal ions decrease the activi-
ties of antioxidizing enzymes [87]. It is evident that a paradox exists with respect
to cellular maintenance of metal concentrations because high levels of even essen-
tial metal ions can cause widespread oxidative damage. The paradox arises from
these metals also playing crucial roles in enzymes that are responsible for the
removal of reactive oxygen species, including Cu-, Zn-, and Fe-superoxide dismu-
tases [47]. Elevated levels of these three metals actually induce oxidative stress.
Although antioxidant production has been thoroughly studied in higher plants [88],
the study of antioxidant synthesis at the molecular level has not been investigated in
photosynthetic microbes to the same extent [89, 90].
Several pieces of evidence suggest that the presence of heavy metal ions of Cd,
Hg, Pb, and Cu in high concentrations disrupts mitochondrial and chloroplast func-
tion [91]. The intense electron fluxes within these cellular compartments of algae
elevate oxygen and metal ion concentrations, putting these organelles at particular
risk of oxidative damage [47]. Cadmium has been extracted from chloroplasts and
mitochondria in relatively high concentrations by comparison to that in other cel-
lular organelles [29]. Oxidative damage can severely impede algal photosynthetic
and metabolic activity as well as affect the overall electrical gradient of the cell.
Furthermore, an abundance of light can cause an increase in the amount of reac-
tive oxygen species within cells by causing a proportional increase in the number of
excited molecules, such as triplet state chlorophyll and singlet state oxygen with the
latter being able to strongly oxidize other molecules [47].
3.6 Genetic Transformation Studies
The first gene transfer study in plants used cauliflower mosaic virus as the vector
for a mammalian MtI gene in turnip leaves that became resistant to elevated Cd(II)
[92]. In another study using photoautotrophic organisms,
Anabaena
PCC 7120,
