Environmental Engineering Reference
In-Depth Information
Thalassiosira weissflogii
, that efflux of phytochelatins resulted in a physiological
removal of Cd(II) from the cells.
Cytosolic fractions taken from species of cyanobacterial
Nostoc
after exposure to
Cd and Zn can contain up to 30% of the metals bound to proteins [71]. Their findings
correspond with those of Bierkens et al. [72] and Torres et al. [73] in algae of met-
allothioneins binding to Cd and converting it to a less harmful form. Furthermore,
Class III metallothioneins can exist as low and high molecular weight variants. In
low molecular weight forms the metal is bound to thiol groups, whereas in the high
molecular weight forms, additional inorganic sulfur is incorporated into the MtIII
complexes. This sulfur forms particles that are in the range of nanometres in diame-
ter [56]. The presence of the inorganic sulfur appears to stabilize the MtIII complex
and improve detoxifying capabilities, though their origin is not known. Vande Weigh
and Ow [74] have proposed that in
Schizosaccharomyces pombe
the supply of inor-
ganic sulfur is controlled by a sulfide oxidoreductase that maintains a sulfide to
metal equilibrium. This is of particular interest with respect to the recent findings
concerning mercury sulfide production in cyanobacteria and algae [75].
3.4.4 Sequestration and Compartmentalization of Phytochelatins
Metal-MtIII complexes can be sequestration into vacuoles has been observed in the
microalga
Dunaliella bioculata
[76] and
Schizosaccharomyces pombe
[77]. In the
green alga,
Tetraselmis suecica
, heavy metals have also been shown to accumulate
in the cell wall and within organelles, with precipitation of Cd(II), Ca(II) and S(II)
being detected in the vacuole [78]. This can also happen in other species exposed to
Cd (II), Cu (II), Hg (II) and Cr (II) [61, 71, 79, 80]). Metals can accumulate in the
mitochondria and chloroplasts of species devoid of vacuoles [81, 82].
To add to the complexity of metallothionein partitioning into organelles, there are
three possibilities for the formation of MtIII compounds present in chloroplasts and
mitochondria. Firstly, the MtIIIs may be synthesized in the cytosol where they bind
to heavy metals. These are then transported into the mitochondria and chloroplasts.
Secondly, MtIIIs may be synthesized inside the chloroplasts and mitochondria,
where they then sequester metals, forming heavy molecular weight complexes with
inorganic sulfur. Thirdly, both of these pathways may co-exist and MtIII may be syn-
thesized in each of the three cellular compartments. Interestingly, cDNAs encoding
MtIIIs have been identified in
Chlamydomonas reinhardtii
where 60% of Cd(II) was
found within the chloroplast, however it is worth noting that the chloroplasts of this
species are large structures that compose most of the cell [82].
3.4.5 Cellular Exportation of Phytochelatins
Phytochelatin-metal complexes can be exported from the cell via exocytosis. Once
exported, however, these complexes do not appear to remain stable as studies have
shown that Cd(II) and Pb(II) disassociate to their free ionic forms in the media
[70, 83]. Although the process of stabilization of heavy metals by MtIIIs is adequate
