194
C.M.G. van den Berg
out the water column of the Mediterranean, to about -0.5 pM in the Pacific, and to
5 pM in the Mediterranean. An oceanic cyanobacterium in an EDTA-controlled culture has been shown to be limited by inorganic iron levels around 1 pM (Brand et al.
1983), suggesting that the organic complexation in combination with the low iron concentrations (0.1 nM) in the upper water column may well be the cause of growth limitation of the algae by iron.
9.3
Sources of Ligands
The distributions of the ligands in the oceanic water column (Fig. 9.1 and 9.2) tend to
show a maximum in the upper water column, suggesting that microorganisms are the
origin. Marine as well as fresh-water algae in cultures are known since a long time to
produce ligands for copper and other metals (van den Berg et al. 1979; Morelli et al.
1989). Fungi and bacteria are also known to produce ligands for copper and iron
(Neilands 1967; Sunda and Gessner 1989). The function of the ligands for iron produced by bacteria, fungi and some plants is reasonably clear: they are siderophores
and improve the availability of iron to the organisms. This function has not (yet) been
demonstrated for the marine system. It is likely that the ligands change the availability of metals to microorganisms, usually making them less available but sometimes
increasing their availability, for instance if the complexes are lipid-soluble (Florence
and Stauber 1986; Florence et al. 1992).
9.4
Composition of Ligands
Little is known about the composition of the ligands in the oceanic water column related to difficulties in their recovery. The free ligands are known to adsorb poorly on
hydrophobic surfaces such as those in C-18 cartridges whilst they adsorb reasonably
well once saturated with copper (Buckley and van den Berg 1986; Donat et al. 1986):
this suggests that the ligands are quite hydrophilic. Several attempts have been made
to get closer to some identification (Zhou and Wangersky 1989; Donat et al.1997) but
with no success besides general classifications. On the other hand several ligands have
been proposed as potential models for the ligands occurring in the natural system.
Phytochelatins are glutathione-rich peptides which are known to be produced by plants
and marine algae in response to high metal concentrations (Ahner et al. 1995; Scarano
and Morelli 1996). Algae are rich in thiol compounds like glutathione and cysteine,
which are known to form very stable complexes with copper(I) (Leal and van den Berg
1998). Glutathione is known to occur throughout the oceanic water column at levels
as high as 10 nM (Le Gall and van den Berg 1998) and, along with other thiol compounds, is therefore a strong candidate for the identity of copper binding ligands.
9.5
Calculation of Metal Complexation by the Ligands
The concentrations of the organic complexing ligands are much less than those of the
major ions but usually greater than that of the trace metals. The ligands participate in
C.M.G. van den Berg
out the water column of the Mediterranean, to about -0.5 pM in the Pacific, and to
5 pM in the Mediterranean. An oceanic cyanobacterium in an EDTA-controlled culture has been shown to be limited by inorganic iron levels around 1 pM (Brand et al.
1983), suggesting that the organic complexation in combination with the low iron concentrations (0.1 nM) in the upper water column may well be the cause of growth limitation of the algae by iron.
9.3
Sources of Ligands
The distributions of the ligands in the oceanic water column (Fig. 9.1 and 9.2) tend to
show a maximum in the upper water column, suggesting that microorganisms are the
origin. Marine as well as fresh-water algae in cultures are known since a long time to
produce ligands for copper and other metals (van den Berg et al. 1979; Morelli et al.
1989). Fungi and bacteria are also known to produce ligands for copper and iron
(Neilands 1967; Sunda and Gessner 1989). The function of the ligands for iron produced by bacteria, fungi and some plants is reasonably clear: they are siderophores
and improve the availability of iron to the organisms. This function has not (yet) been
demonstrated for the marine system. It is likely that the ligands change the availability of metals to microorganisms, usually making them less available but sometimes
increasing their availability, for instance if the complexes are lipid-soluble (Florence
and Stauber 1986; Florence et al. 1992).
9.4
Composition of Ligands
Little is known about the composition of the ligands in the oceanic water column related to difficulties in their recovery. The free ligands are known to adsorb poorly on
hydrophobic surfaces such as those in C-18 cartridges whilst they adsorb reasonably
well once saturated with copper (Buckley and van den Berg 1986; Donat et al. 1986):
this suggests that the ligands are quite hydrophilic. Several attempts have been made
to get closer to some identification (Zhou and Wangersky 1989; Donat et al.1997) but
with no success besides general classifications. On the other hand several ligands have
been proposed as potential models for the ligands occurring in the natural system.
Phytochelatins are glutathione-rich peptides which are known to be produced by plants
and marine algae in response to high metal concentrations (Ahner et al. 1995; Scarano
and Morelli 1996). Algae are rich in thiol compounds like glutathione and cysteine,
which are known to form very stable complexes with copper(I) (Leal and van den Berg
1998). Glutathione is known to occur throughout the oceanic water column at levels
as high as 10 nM (Le Gall and van den Berg 1998) and, along with other thiol compounds, is therefore a strong candidate for the identity of copper binding ligands.
9.5
Calculation of Metal Complexation by the Ligands
The concentrations of the organic complexing ligands are much less than those of the
major ions but usually greater than that of the trace metals. The ligands participate in