Iron in Cyanobacteria - Advances in Botanical Research - page 74

notably absent from many Synechocystis , Prochlorococcus and Synechococcus spe-
cies. Further bioinformatic analysis was done by Hopkinson and Morel
(2009) including the prevalence of NIS pathways for siderophore biosyn-
thesis as well as TonB-dependent transporters in cyanobacteria. Neither of
these pathways was found in marine unicellular picocyanobacteria ( Hop-
kinson & Morel, 2009 ). Several strains that have no known siderophore
biosynthesis genes do have TonB-dependent transporters encoded in their
genomes. The Synechocystis PCC 6803 genome contains four putative outer
membrane siderophore transporters (Katoh, 2001). Some ferrisiderophores
complexes are bioavailable to this organism but no siderophore secretion
was detected in CAS assays ( Kranzler et al., 2011 ).
An alternative Fe uptake strategy characterized in many plants, yeast
and eukaryotic algae is the extracellular reduction of Fe(III) or Fe(III) che-
lates by a reductase on the plasma membrane and subsequent uptake of
Fe′ ( Allen, del Campo, Kropat, & Merchant, 2007 ; Kustka, Allen, & Morel,
2007 ; Lesuisse & Labbe, 1989 ; Morrissey & Guerinot, 2009 ; Schmidt,
1999 ; Shaked, Kustka, & Morel, 2005 ). Currently, there is some evidence
for extracellular reduction of Fe(III) species before transport in a number
of non-photosynthetic prokaryotes ( Schröder, Johnson, & De Vries, 2003 ;
Vartivarian & Cowart, 1999 ).
2.6.2. Uptake of free, unchelated, inorganic iron
The existence of an iron uptake pathway for cyanobacteria that involves a
reductive step has been targeted in a few studies ( Kranzler et al., 2011 ; Lis
& Shaked, 2009 ; Rose et al., 2005 ). Rose et al. (2005) demonstrated that
superoxide-mediated extracellular iron reduction enhances Fe uptake in
the marine cyanobacterium Lyngbya majuscula . Using a chemiluminescence
system, extracellular Fe(II) production by L. majuscula was demonstrated.
Reduction of iron by superoxide increases iron availability in this organism
and superoxide production seems to be metabolically related. When iron
was applied in the form of FeEDTA, the addition of superoxide dismutase
inhibited Fe uptake by 94% suggesting that superoxide is involved in the
iron uptake pathway. However, the application of the Fe(II)-specific ligand,
ferrozine (FZ), did not inhibit Fe uptake suggesting that organically com-
plexed ferrous iron may be directly transported (Rose et al., 2005).
Both natural Synechococcus populations in the Gulf of Aqaba as well as
axenic Synechococcus WH8102 cultures may also employ reductive Fe uptake
in order to acquire organically bound iron ( Lis & Shaked, 2009 ). Both
were able to acquire iron bound to the strong Fe(III) chelator, ferroxiamine