Biology Reference
In-Depth Information
and supported uptake rates similar to those measured for FeDFB. This find-
ing is consistent with data from work with eukaryotic algae demonstrating
that free iron is the most biologically available form for uptake (
Hudson
& Morel, 1989
;
Morel et al., 2008
) and that Fe′ uptake rates are 100-200
times faster than uptake of FeDFB (
Maldonado & Price, 2001
;
Shaked et al.,
2005
). Nevertheless, the data also exhibited considerable rates of FeDFB
and Feaerobactin transport. Ferrated siderophore uptake was inhibited by
FZ. These results suggested that the reductive uptake system identified in
Synechocystis
PCC 6803 is able to utilize organically bound Fe of an exog-
enous source.
2.7. Fe
′
Transport through the Plasma Membrane
The reduction of a variety of inorganic and organic iron sources, before
transport through the plasma membrane, eliminates the need for specific
ferrisiderophore transporters. For internalization of Fe, this type of mecha-
nism requires only free iron transporters in the plasma membrane. Follow-
ing reduction, iron may be transported as Fe(II) or, in some cases, reoxidized
to Fe(III) and then transported (
Askwith et al., 1994
;
Maldonado et al.,
2006
;
Stearman, Yuan, Yamaguchi-Iwai, Klausner, & Dancis, 1996
). These
possibilities are consistent with two of the transport systems identified in
the plasma membrane of
Synechocystis
PCC 6803: FutABC and FeoB, sug-
gested to transport free Fe(III) and Fe(II), respectively (
Badarau et al., 2008
;
Katoh, Hagino, Grossman et al. 2001
;
Katoh, Hagino, & Ogawa, 2001
). Dis-
ruption mutants in the
fut
and
feoB
genes are viable; however, a homo-
plasmic mutant in which both transport systems are inactivated could not
be segregated (
Katoh, Hagino, & Ogawa, 2001
) suggesting that these two
systems are responsible for most of the iron accumulation in this organism.
While the reduction of organically bound iron results in its release from
the complex, thus enabling uptake, reduction of Fe′ may seem redundant in
the presence of both Fe(II) and Fe(III) transporters. Nonetheless, this strat-
egy is well documented in many eukaryotic microorganisms such as
Sac-
charomyces cerevisiae
(
Stearman et al., 1996
), the centric diatoms
Thalassiosira
weissflogii
and
Thalassiosira pseudonana
(
Shaked et al., 2005
), and the green
algae
Chlamydomonas reinhardtii
(
La Fontaine et al., 2002
).
2.8. Environmental Implications
Thus far, two iron uptake strategies have been demonstrated for cyanobac-
teria: siderophore-mediated transport and reductive iron uptake. Given its
broad range of iron substrates found within natural aquatic environments,
