Tải bản đầy đủ - 0 (trang)
C. Iron Complexes with Low-Molecular Weight Organic Acids

C. Iron Complexes with Low-Molecular Weight Organic Acids

Tải bản đầy đủ - 0trang



oxides over the whole pH range commonly found in soils. Known 1:1

stability constants of Fe(III)–siderophore complexes range from 1023 to

1052 compared to 1013.2 (Fe–citrate), 1027.7 (Fe–EDTA), and 1021.7 (Fe–

HEDTA) (Martell et al., 2001). Also, most siderophores have a high specificity for iron so that they bind iron in the presence of high concentrations

of other cations. For example, Ca2ỵ cannot compete with protonation of

hydroxamate groups of the microbial siderophore DFOB (Fig. 5) even at

high Ca2ỵ concentrations in carbonatecontaining soils. Powell et al. (1980)

estimated microbial siderophore concentrations in soil solutions of 10À7 to

10À8 M. Microbial siderophore concentrations are higher in the rhizosphere

compared to bulk soil (Reid et al., 1984b).

Due to their high aYnity and specificity for iron, the presence of microbial

siderophores has a strong eVect on the solubility and transport of iron oxides.

Therefore, a number of studies have focused on Fe complexes with microbial

siderophores as iron sources for plant nutrition. Various strategy I plants were

able to take up microbial siderophore complexes (Cline et al., 1984; Crowley

et al., 1987, 1991; Johnson et al., 2002). Iron uptake from iron complexes of

hexadentate microbial siderophores by strategy II plants has been demonstrated (Crowley et al., 1988; Reid et al., 1984a), but various studies have

concluded that it is rather ineYcient (Bar‐Ness et al., 1991; Crowley et al.,

1992; Hoărdt et al., 2000; Roămheld and Marschner, 1986, 1990). The reasons for

the low availability of microbial siderophore complexes to strategy II plants

may include their high thermodynamic and kinetic stability that can inhibit

ligand‐exchange reactions with mugineic acids as discussed later.

An unusual microbial siderophore with regard to its structure and relatively

low aYnity and specificity for iron (compared to other microbial siderophores)

is rhizoferrin (Fig. 5). Rhizoferrin is a fungal tetradentate hydroxyl‐carboxylate

siderophore. Its iron complexes are available to strategy I and II plants

(Shenker et al., 1995; Yehuda et al., 1996). Equilibrium speciation calculations

based on published stability constants of Ca–rhizoferrin complexes suggest

that competition between Ca and Fe will inhibit the formation of significant

concentrations of iron–siderophore complexes in calcareous soils. However, it

was shown that rhizoferrin eYciently promotes dissolution of ferrihydrite

at pH 8.7 with net dissolution rates comparable to mugineic acid (Shenker

et al., 1996, 1999) to concentration levels which seem to indicate that the

stability of iron complexes has been underestimated relative to the stability of

Ca complexes.

Microbial siderophore complexes undergo biological and chemical degradation reactions in soils and aqueous systems (Pierwola et al., 2004;

Warren and Neilands, 1964, 1965; Winkelmann et al., 1999). The degradation products usually have a lower denticity and aYnity for iron and an

increased bioavailability to plant species. For example, iron complexes of

hydrolysis products of coprogen have a higher reduction potential, and a



Figure 5 Selection of fungal and bacterial siderophores and the synthetic aminocarboxylate

ligand EDTA. Rhizoferrin is a carboxylate siderophore, exuded by the fungus Rhizopus and

related mucorales (Drechsel et al., 1991). DFO‐B, DFO‐D, coprogen, and desferriferrichrome

are trihydroxamate siderophores, rhodotorulic acid is a dihydroxamate siderophore.

lower thermodynamic and kinetic stability, compared to the parent compound, resulting in improved uptake by strategy I and II plants (Hoărdt et al.,

2000). Similarly, the photolysis of iron–siderophore complexes possessing

a‐hydroxycarboxylate groups leads to the reduction of Fe(III) and the



formation of lower aYnity iron complexes, increasing the bioavailability of

dissolved iron to phytoplankton (Barbeau et al., 2001; Borer et al., 2005).

In light of these observations, it seems likely the plant availability of microbial siderophores in long‐term uptake experiments and in the field may be

related to their microbial or chemical degradation rates.


Natural organic matter in soils including humic acids and fulvic acids is

usually associated with iron. Low‐molecular weight organic acids and siderophores that have been discussed earlier are usually considered part of the

NOM pool. Therefore, the role of NOM in plant iron nutrition that has been

observed may be in part due to low‐molecular weight organic acids as

discussed earlier. A number of studies have demonstrated that this iron

bound to NOM (or fractions of it) can serve as source for strategy I and

strategy II plant iron acquisitions. It is diYcult to draw generalizations on the

plant availability of iron bound to NOM from diVerent sources due to the

contrasting properties regarding transport (immobile, soluble, or colloidal),

and thermodynamics and kinetics of iron binding and release. However, in

field and pot experiments it has been demonstrated that fulvic acid, humic

acid, and water‐ and pyrophosphate‐extractable fractions of humic substances as well as preparations of peat, manure, compost, coal, lignite, and

sewage sludge improve iron uptake by strategy I and/or strategy II plants

(Barak and Chen, 1982; Cesco et al., 2002; Chen, 1996; Chen and Aviad,

1990; Chen et al., 1999; Garcia‐Mina et al., 2004; Linehan and Shepherd,

1979; Pinton et al., 1999). Besides its function as iron source, NOM has a

range of eVects on the physical, chemical, and biological properties of the soil

and influences the physiology of plant roots. All of these factors can directly

or indirectly influence plant iron acquisition.

EXAFS studies of iron binding by NOM at natural abundance levels and

after addition of FeCl3 demonstrated that organic complexation inhibits iron

polymerization even in the neutral pH range (Rose et al., 1998; Vilge‐Ritter

et al., 1999). Important ligating groups for iron in humic substances are carboxyl (–COOH), carbonyl (¼CO), phenolic hydroxyl (–OH), amine (–NH2),

imine (¼NH), and sulfhydryl (–SH) (Blume et al., 2002). Adjacent organic

ligating groups can chelate iron in five‐ or six‐membered rings. Electron spin

resonance studies of Fe–fulvic acid complexes showed complexation by carboxylic and phenolic OH groups (Senesi, 1990). A fraction of the iron content of

purified humic substances can be bound to minerals that are tightly associated

with the organic matter (Davies et al., 1997). Due to the range of possible

binding configuration within humic substances, iron reacts with sites of varying

thermodynamic and kinetic stability (Burba, 1994; Tipping et al., 2002).



The solubility of iron in equilibrium with iron oxides is strongly influenced by the presence of dissolved organic matter (DOM). Tipping et al.

(2002) modeled equilibrium speciation of iron in the presence of ferrihydrite

and 10 mg literÀ1 fulvic acid (representing freshwater DOM) at 3‐mM Ca

concentrations. In the pH range between 6.5 and 7.5, they predicted iron

solubilities in the micromolar range. The addition of up to 7 mg literÀ1

humic acid to 0.7 N NaCl solutions at pH 8 increased the solubility of iron

in equilibrium with freshly precipitated ferrihydrite to 10À8 to 10À7 M (Liu

and Millero, 1999). Cesco et al. (2000) demonstrated that water‐extractable

humic substances increased the dissolution rate and solubility of freshly

precipitated ferrihydrite. Also, it served to extract iron from various soil

types including carbonatic soils in column experiments.

Soil solutions contain several ten to several hundred mg literÀ1 DOM,

including organic acids and higher molecular weight fractions (Zsolnay,

1996). Therefore, significant concentrations of Fe could be associated with

DOM in soils. DOM as the mobile fraction of soil organic matter is of particular interest for plant nutrition as it can act as an iron shuttle for strategy I and II

plants. However, phytosiderophores themselves can act as iron shuttles and

also scavenge iron from the immobile soil organic matter fractions.



The exudation of phytosiderophores has a strong eVect on the geochemistry of iron in the rhizosphere. In calcareous soils, phytosiderophore

complexes become the dominant soluble iron species. This increases the

bioavailability of iron for graminaceous plants that take up the phytosiderophore complexes and for rhizosphere microorganisms that can scavenge

iron from the complexes. Phytosiderophores increase the solubility of iron‐

bearing minerals and accelerate their dissolution. Also, phytosiderophores

can serve as iron shuttles so that immobile iron pools (solid, adsorbed, or

complexed by organic matter) that are not in direct contact with the plant

root can be scavenged.


Mugineic acid and its derivatives (commonly called mugineic acids or

MAs) are hexadentate ligands with aminocarboxylate and hydroxycarboxylate functional groups (Fig. 6). The ligands are synthesized by hydroxylation

of the parent compound nicotianamine (Ma and Nomoto, 1993; Mori and



Figure 6 Structure of mugineic acid derivatives and nicotianamine.

Nishizawa, 1987). Their molecular weight ranges from 278 (DDA‐A) to 336

(HMA, epi‐HMA).






In hydroponic culture studies, overall phytosiderophore release rates are

a function of the nutritional status of the plant. The nutritional status is not

only a function of the free iron activity in solution but also of plant age

(Gries et al., 1995). The observed release rates of phytosiderophore under

strong iron deficiency greatly exceed iron‐uptake rates required for normal

growth. Phytosiderophore exudation by plant roots is highly restricted in

time and space. Diurnal exudation for only a few hours during the light

period is commonly observed (Marschner et al., 1986a; Takagi et al., 1984).

Exceptional continuous exudation by Z. mays L. cv. Alice has been observed

(Yehuda et al., 1996). The release of siderophores varies along the root and

is most pronounced in apical root zones (Marschner et al., 1987). These

restrictions along with known exudation rates and estimated microbial

degradation rates have been used as parameters of a radial diVusion model

to estimate the local distribution of phytosiderophores in the rhizosphere

(Roămheld, 1991). The model predicts a strong gradient of phytosiderophore

concentrations away from the root surface with average concentrations of

1 mM within the first 0.25 mM during the period of maximum exudation rates.

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

C. Iron Complexes with Low-Molecular Weight Organic Acids

Tải bản đầy đủ ngay(0 tr)