Biology Reference
In-Depth Information
(
Kranzler et al., 2011
;
Lis & Shaked, 2009
;
Maldonado & Price, 2001
). Other
organisms - to date exclusively freshwater autotrophs - are known to trans-
port the entire Fe siderophore into the cell where it undergoes decomplex-
ation.
Particulate and colloidal iron is present in both organic and inorganic
forms. Although not classically considered bioavailable in and of themselves,
colloidal dissolution can replenish supplies of Fe′ and organically com-
plexed iron (
Fig. 3.1
).This may occur by means of thermal or photochemical
processes (
Rich & Morel, 1990
) and can be siderophore mediated (
Krae-
mer, 2004
). Whether colloid dissolution actually occurs is a function of
colloid structure and thermodynamic stability (
Wells et al., 1983
). Recently,
work with the filamentous, dinitrogen fixing cyanobacterium,
Trichodes-
mium
, demonstrated that while only dissolved iron is transported, this
organism facilitates dissolution of iron oxides and dust (
Rubin, Berman-
Frank, & Shaked, 2011
). This dissolution was most effectively accom-
plished by puff-shaped colonies, surrounding dust particles where active
shuttling of particulate iron by these colonies was documented (
Rubin
et al., 2011
).
2. IRON UPTAKE
2.1. Siderophores in Cyanobacteria
Siderophores are the strongest Fe(III) chelators secreted by microorgan-
isms and plants. Siderophore production and secretion occurs, especially
under iron starvation, when the intracellular iron concentration drops
under a certain threshold required for functionality. Depending on the
chemical nature of the organic ligand that coordinates iron, siderophores
can be divided into three main classes, the catecholates, the hydrox-
amates or mixed-types that contain another iron complexing group
such as α-hydroxy-carboxylate next to the hydroxamate or catecholate
group (
Fig. 3.2
;
Miethke & Marahiel, 2007
). Once bound to Fe(III), the
Figure 3.2
Building blocks for iron binding molecules.
