Environmental Engineering Reference
In-Depth Information
mechanisms of metal decontamination to occur between these micro-organisms and
plants, and there is a wealth of information on the latter, examples of higher plant
studies have been included in this article.
3.2 Membrane Transport of Heavy Metals
There are several known mechanisms for heavy metal absorption into cells. These
vary depending on the species of metal as well as the compounds with which metals
may be associated. Metals can be imported into cells through their natural transport
processes or by molecular and ionic mimicry through competition for binding sites
within carrier proteins for other ions or compounds, such as Ca(II) channels [9].
Negative correlations between intracellular concentrations of metals have shown
that absorption of different metals can occur through the same process. For example,
the red alga,
Cyanidium caldarium
, has been shown to be extremely tolerant when
grown in media with Al(III) concentrations ranging from 50 to 100 mM [31]. Even
though it can grow in such high concentrations of aluminum, internal concentrations
remain relatively low, thus suggesting that there is selective exclusion of aluminum.
However, at higher temperatures the ratio of absorption between Fe and Al shifts
towards the latter, suggesting a loss of metal specificity in Fe transporters.
Table 3.1 presents a list of transport processes for heavy metals other than in
mammalian species.
Heavy metals can also bind extracellularly with low molecular weight thiols
such as glutathione and, as a consequence, can be transported across the membrane
through the thiol transporters [44, 45]. Silver has been shown to be transported read-
ily into the algae,
Chlamydomonas reinhardtii
and
Pseudokirchneriella subcapitata
,
when it is bound to thiosulfate [46], apparently crossing the plasma membrane intact
[38]. Metals bound with thiols are more stable and less likely to cause oxidative
damage, although high levels can still be deleterious.
Heavy metals can also bind with metallothioneins, including phytochelatins, fol-
lowed by entry into cells through endocytosis. In a reverse process, these chelators
can become a means of metal excretion because they release metals into the exterior
medium by dissociation. In addition, to prevent the build up of heavy metals within
cells, active metal efflux occurs through metal specific ATPase pumps [47].
3.3 Uptake and Assimilation of Sulfate
Sulfur is an essential component of the amino acids, cysteine and methionine. It is
taken up from the environment by organisms in the form of inorganic sulfate that
is absorbed by active transport systems - bacteria [39], algae [48], yeast [49], and
higher plants [50, 51]. After uptake by the cell, sulfate is transferred through the
cytoplasm into organelles and, in plant cells, excesses become stored in vacuoles.
