wheat, maize), for which better resistance to Al toxicity is associated with root exudation of citric acid,
succinic acid, and other organic acids [47–50].
It has been reported that in seawater, Cu, Cd, and Zn can also be strongly complexed by organic lig-
ands which could have originated from organisms [51,52]. The ligands can be phytochelatins (PCs) (for-
merly metallothionein class III [53]; according to Zenk [54], the term PC as used here includes cadystins),
usualy described as occasional intracellular low-molecular-weight proteins containing a high proportion of
SH groups [55]. However, the role of PCs outside the cell remains to be conclusively demonstrated because
up to the present the only example is the release of Cd-PC complexes from phytoplankton. In addition, these
complexes are not very stable and as a consequence Cd is reabsorbed by the cells [56]. Nevertheless, Cd ex-
trusion may be an important adaptative mechanism for lowering the concentration of free Cd in the cells.
Metals can also be trapped by polysaccharides, alginates, and other slimes secreted by algae [57–59].
Metal ion captured by algal polysaccharides is governed by ion-exchange selectivity [60] and is propor-
tional to the total carboxylic acid content [45]. Again, the capacity to bind these metal ions depends on
the chemical composition of secreted compounds, which unfortunately remains too often not determined.
Usually, Pb is not chelated [61]. However exceptions have been described [62,63]. According to these au-
thors, the capacity to bind Pb could be due to the presence of large amounts of proteinaceous substances
in the compounds extruded by the algae.
B. Metal Excretion and Volatilization
When comparing a Cu-tolerant and a Cu-nontolerant strain of the green alga Chlorella vulgaris, Foster
[12] found the first strain capable of growing in a medium containing 1 mg Cu L^1 , whereas the growth
of the sensitive strain was already completely inhibited at 0.3 mg Cu L^1 This was explained by excre-
tion of Cu by the tolerant strain. Chlorellais also able to exclude Zn [64].
On transferring the brown macroalgae Ascophyllum nodosumfrom a Zn-polluted zone to a nonpol-
luted one, Eide et al. [65] noted a decrease in the Zn concentration in the algal thallus. This decrease was
interpreted as the result of Zn excretion, possibly bound to phenols.
The mechanism of Hg and phenyl-Hg-acetate resistance in Chlorellaappears to be similar to that ob-
served with bacteria and yeast; i.e., they are volatilized [64]. The enzymatic system partially purified from
Hg-resistant cells [64] is able to volatilize both Hg and phenyl-Hg-acetate in vitro and is very similar to
that isolated from an Hg-resistant strain of Escherichia coli.
C. Intracellular Metal Sequestration
- Proteins
Metallothioneins (MTs) and phytochelatins (PCs) are two protein families capable of sequestering metals.
They are both cystein-rich polypeptides having the ability to form metal-thiolate clusters. MTs have been
characterized as gene-encoded proteins [66,67] with a molecular weight of 5000–20,000, whereas PCs are
smaller enzymically synthesized polypeptides (molecular weight in the range 500–2300) [53]. Initial anal-
ysis of the organism responses to HMs has identified MTs as proteins rather typical of vertebrates [68,69]
and PCs as rather typical of plants [54,55]. Actually, MTs have also been found in lower organisms. For
fungi, Kneer and Zenk [70] generalized the original finding of Mehra et al. [71] that HMs induce both MT
and PC synthesis. However, the role of MTs does not appear to be restricted to metal detoxification [66],
and the complete roles of both metalloproteins are not fully understood.
METALLOTHIONEINS The relatively high expression of MTs in diverse plant tissues indicates a
fundamental role of these proteins. They are involved in homeostasis, i.e., the mechanism regulating the
availability of metal ions in cells. This statement is supported by the fact that MTs and MT-like proteins
are also expressed in plants growing in the absence of metal excess. In 1996, Zenk [54] noted that there
is no experimental evidence that these “plant MTs” are involved in the detoxification of HMs. Conse-
quently, plant MT-like proteins will not be described further here. The interested reader may read the
comprehensive reviews on MTs [68,69].
PHYTOCHELATINS The ability to synthesize PCs in response to HM pollution is a general feature
of the plant kingdom, including algae [72]. A few organisms such as Viola calaminaria,Thlaspi
caerulescens, or Brassica junceaare naturally capable of growing in an environment extremely enriched
754 BERTRAND ET AL.