Biology Reference
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significantly to this fraction and require large amounts of the essential micronutrient
iron in order to maintain their Fe-rich photosynthetic apparatus. Cyanobacterial iron
requirements exceed non-photosynthetic prokaryotes by
∼
10-fold and are exception-
ally high even among other photosynthetic organisms. The genomes of cyanobac-
terial species code for a multitude of iron transporters, iron storage complexes and
iron-responsive elements involved in maintaining homeostasis in a highly variable
environment. In this chapter, we will review iron transport strategies, the maintenance
of intracellular homeostasis and iron limitation responses of cyanobacteria while taking
into account the chemistry and environmental bioavailability of iron species.
1. IRON BIOGEOCHEMISTRY IN WATER BODIES
1.1. Iron Chemistry and the Evolution of Oxygen
Evolution
Iron is the fourth most abundant element in Earth's crust, yet Fe bioavailabil-
ity has been shown to limit primary productivity in large regions of the ocean
as well as in many freshwater environments (
Boyd et al., 2007
). This is due
to both Fe concentration and chemistry. In aqueous solutions, iron has two
environmentally relevant oxidation states: Fe(II) and Fe(III) (
Frausto da Silva
& Williams, 2001
). Before the evolution of oxygenic photosynthesis, reducing
environmental conditions resulted in iron existing primarily in its reduced,
lower valency form, Fe(II). Fe(II) is relatively soluble at the circumneutral pH
range and, therefore, considered readily bioavailable. It is thus likely that the
iron-rich photosynthetic electron transport chain evolved between ∼2.3 and
2.2 10
9
years ago during the Proterozoic era when Fe(II) was in abundance
(
Falkowski, 2006
). Upon the evolution of oxygenic photosynthesis, molecular
oxygen buildup in the atmosphere led to the oxygenation of aquatic environ-
ments (
Bekker et al., 2004
;
Allen & Vermaas, 2001
). As a result, Fe(II) species
were no longer thermodynamically stable and Fe(II) was rapidly oxidized
to Fe(III). Relative to its ferrous counterpart, Fe(III) is poorly soluble at the
circumneutral pH range (with concentrations of 0.08-0.2 nM in seawater;
Liu & Millero, 2002
) and precipitates out of solution as ferric oxyhydroxides
which are not considered bioavailable (e.g.
Rich & Morel, 1990
).
Nonetheless, dissolved iron concentrations (where dissolved refers to
the fraction passing through a 0.2 µm or 0.45 µm filter) are higher than
expected because of complexation by organic ligands which maintain Fe
in solution (Fig.
3
.
1
A). It was demonstrated that over 99% of the dissolved
Fe(III) in the ocean and freshwater environments is bound by a heteroge-
neous pool of organic ligands, buffering picomolar equilibrium concentra-
tions of free inorganic Fe(III) species (
Nagai, Imai, Matsushige, Yokoi, &
