B. Sulzberger
1995; Van den Berg 1995; Wu and Luther 1995; Kuma et al. 1996; Luther and Wu 1997;
Rue and Bruland 1997). These studies have used ligand-exchange methods in combination with voltammetric detection, which give information about the conditional
stability constants and the concentrations of the organic ligands involved. These organic ligands, for which no structures are yet known, are likely to play an essential
role in regulating the dissolved iron concentration in the oceans. The occurrance of
organic complexes of iron in fresh waters has so far not been systematically investigated. Measured "dissolved" iron concentrations (determined by filtration) are usually in excess of the theoretical solubility of Fe(III)-hydroxides. The discrepancy between measured "dissolved" iron concentrations and the solubility is generally attributed to the presence of colloidal Fe(III)-hydroxides (Davison 1993; Perret et al. 1990).
The presence of organic Fe(III) complexes may however also enhance the solubility
of Fe(III) (Kuma et al. 1996; Millero 1997).
Hitherto, it is not known whether the major organic ligands of Fe(III) in marine
and fresh waters are humic acids or low molecular-weight organic compounds formed,
e.g. from microbial degradation of organic matter. This may not matter regarding Fe(lI)
production, since the rate coefficients of photolysis of Fe(III)-humate or -fulvate complexes may be similar to those of photolysis of Fe(III) bound to low molecular-weight
ligands, e.g. simple carboxylic acids (Faust and Zepp 1993; King et al.1993; Voelker et al.
1997). From the point of view of the effect of iron on the photo oxidation of biologically refractory humic substances, however, the type of organic Fe(III) ligands present
in the eupotic zone of surface waters is essential.
An important class of low molecular-weight organic molecules which specifically
chelate Fe(III) are siderophores. Siderophores are excreted by bacteria, e.g. by bluegreen algae, to take up iron. They are highly selective for Fe(III). Their chelating structures include typically hydroxamates and catecholates that bind Fe(III) in octahedral
coordination. The stability constants of these chelates are very high (Winkelmann et al.
1987; Crumbliss 1991; Reid et al. 1993). Employing voltammetric methods, Lewis and
co-workers (1995) have estimated thermodynamic stability constants of approximately
10 40 M- 1 for the complexation ofFe 3 + with catecholate-type siderophores isolated from
the marine bacterium Alteromonas luteoviolacea and from the marine cyanobacterium
Synechoccus sp. PCC 7002. Production of siderophores under iron-limiting conditions
is considered to be a frequent strategy among both marine and fresh-water cyanobacteria and eubacteria (Reid and Butler 1991; Wilhelm and Trick 1994). Because siderophores are excreted by bacteria to take up iron, they may be present at low levels in
natural waters. Whether Fe(III)-siderophore complexes are subject to photolysis is a
still unanswered question.
3.5.1
Roles of Ligands in the Oxidation of Iron(ll) in the Euphotic Zone of Surface
Waters
The kinetics of Fe(lI) oxidation largely depend on Fe(lI) speciation (Wehrli 1990). For
example, the net rate of Fe(lI) oxidation by O2 can be enhanced (Liang et al. 1993) or
retarded (Theis and Singer 1974) by complexation with organic ligands, depending on
the relative stability of the Fe(II)- and Fe(III)-organic complexes and on the reducing
power of the ligands. Emmenegger and co-workers (1998) have investigated the Fe(lI)
1995; Van den Berg 1995; Wu and Luther 1995; Kuma et al. 1996; Luther and Wu 1997;
Rue and Bruland 1997). These studies have used ligand-exchange methods in combination with voltammetric detection, which give information about the conditional
stability constants and the concentrations of the organic ligands involved. These organic ligands, for which no structures are yet known, are likely to play an essential
role in regulating the dissolved iron concentration in the oceans. The occurrance of
organic complexes of iron in fresh waters has so far not been systematically investigated. Measured "dissolved" iron concentrations (determined by filtration) are usually in excess of the theoretical solubility of Fe(III)-hydroxides. The discrepancy between measured "dissolved" iron concentrations and the solubility is generally attributed to the presence of colloidal Fe(III)-hydroxides (Davison 1993; Perret et al. 1990).
The presence of organic Fe(III) complexes may however also enhance the solubility
of Fe(III) (Kuma et al. 1996; Millero 1997).
Hitherto, it is not known whether the major organic ligands of Fe(III) in marine
and fresh waters are humic acids or low molecular-weight organic compounds formed,
e.g. from microbial degradation of organic matter. This may not matter regarding Fe(lI)
production, since the rate coefficients of photolysis of Fe(III)-humate or -fulvate complexes may be similar to those of photolysis of Fe(III) bound to low molecular-weight
ligands, e.g. simple carboxylic acids (Faust and Zepp 1993; King et al.1993; Voelker et al.
1997). From the point of view of the effect of iron on the photo oxidation of biologically refractory humic substances, however, the type of organic Fe(III) ligands present
in the eupotic zone of surface waters is essential.
An important class of low molecular-weight organic molecules which specifically
chelate Fe(III) are siderophores. Siderophores are excreted by bacteria, e.g. by bluegreen algae, to take up iron. They are highly selective for Fe(III). Their chelating structures include typically hydroxamates and catecholates that bind Fe(III) in octahedral
coordination. The stability constants of these chelates are very high (Winkelmann et al.
1987; Crumbliss 1991; Reid et al. 1993). Employing voltammetric methods, Lewis and
co-workers (1995) have estimated thermodynamic stability constants of approximately
10 40 M- 1 for the complexation ofFe 3 + with catecholate-type siderophores isolated from
the marine bacterium Alteromonas luteoviolacea and from the marine cyanobacterium
Synechoccus sp. PCC 7002. Production of siderophores under iron-limiting conditions
is considered to be a frequent strategy among both marine and fresh-water cyanobacteria and eubacteria (Reid and Butler 1991; Wilhelm and Trick 1994). Because siderophores are excreted by bacteria to take up iron, they may be present at low levels in
natural waters. Whether Fe(III)-siderophore complexes are subject to photolysis is a
still unanswered question.
3.5.1
Roles of Ligands in the Oxidation of Iron(ll) in the Euphotic Zone of Surface
Waters
The kinetics of Fe(lI) oxidation largely depend on Fe(lI) speciation (Wehrli 1990). For
example, the net rate of Fe(lI) oxidation by O2 can be enhanced (Liang et al. 1993) or
retarded (Theis and Singer 1974) by complexation with organic ligands, depending on
the relative stability of the Fe(II)- and Fe(III)-organic complexes and on the reducing
power of the ligands. Emmenegger and co-workers (1998) have investigated the Fe(lI)