Biology Reference
In-Depth Information
reductive iron uptake may be applied to a variety of Fe sources. This gen-
eralist-type strategy would confer an obvious advantage to organisms living
in a dilute and heterogeneous aquatic environment, in which iron is bound
by many different organic chelators, siderophores included (
Hopkinson &
Morel, 2009
).
This was demonstrated in a numerical model that quantified the effec-
tiveness of endogenous siderophore-mediated uptake and reductive Fe
uptake (
Völker & Wolf-Gladrow, 1999
). High-affinity transport of specific
ferrisiderophore complexes is effective in densely populated environments
where cell numbers offset diffusive losses. In a dilute aqueous environment
where diffusive losses are imminent, endogenous siderophore uptake is inef-
ficient. In this type of environment, reductive Fe uptake is an effective strat-
egy in the acquisition of organically bound iron. It is important to note that
these two transport systems can co-exist in one organism.
A recent study of iron transporters in marine prokaryote genomes and
metagenomes identified uptake mechanisms for Fe(III), Fe(II) and organi-
cally bound iron. Bioinformatic analysis of environmentally abundant
microbes was used to identify iron uptake strategies prevalent in the marine
environment. Fe(III) ABC transporters were most commonly found among
marine bacteria. Among most known free-living bacteria such as picocya-
nobacteria, TonB-dependent transporters were absent (
Hopkinson and Bar-
beau, 2012
). The authors also tested for statistical associations between Fe
transport genes and found a negative correlation between ABC transport
systems and FeoB transporters (Hopkinson and Barbeau, 2011).
3. INTRACELLULAR FE HOMEOSTASIS
Once transported into the cell, iron can be utilized for the assembly
of active cofactors such as haeme, cytochromes or Fe-S clusters. A major
factor in the production of Fe-S cluster assembly is the SufBCDE system.
This system is regulated by the iron status of the cell but also by other
environmental inputs such as light intensity (
Balasubramanian, Shen,
Bryant, & Golbeck, 2006
;
Seki et al., 2006
). Additionally, the involvement of
monothiol Grx proteins as Fe-S clusters donors was suggested (
Picciocchi,
Saguez, Boussac, Cassier-Chauvat, & Chauvat, 2007
). Cytochrome assembly
takes place by a number of routes and exhibits strong divergence between
different organisms (review by Kranz et al., 2002).
Alternatively, iron can be stored in ferritin complexes, which form
multimeric spherical complexes inside which ferric oxide is deposited
