Iron is of great importance for many metabolic processes since the redox potential
between its two valence states Fe2+ and Fe3+ lies within the range of physiological
processes. Actually, iron is not a rare element, it is fourth in abundance in the earth
crust, but it is not readily available for microorganisms. In the soil ferric oxide hydrates
are formed at pH values around seven and the concentration of free Fe3+ is at best
10
17 mol/dm3 while about 10
6 mol/dm3 would be needed. In living organisms iron
is usually strongly bound to peptidic substances such as transferrins. To increase the
supply of soluble iron microorganisms other than those living in an acidic habitat may
circumvent the problem by reduction of Fe3+ to Fe2+ (182), which seems to be of major
importance for marine phytoplankton (151); see also amphiphilic marine bacteria
(Sect. 2.8) and Fe2+ binding ligands (Sect. 7) below. An important alternative is the
production of Fe3+ chelating compounds, so-called siderophores. Siderophores are
secondary metabolites with masses below 2,000 Da and a high affinity to Fe3+. Small
iron-siderophore complexes can enter the cell via unspecific porins, larger ones need a
transport system that recognizes the ferri-siderophore at the cell surface. In the cell,
iron is released mostly by reduction to the less strongly bound Fe2+ state (137), and the
free siderophore is re-exported (“shuttle mechanism”); for a modified shuttle system
see pyoverdins (Sect. 2.1) and amonabactins (Sect. 2.7). Rarely the siderophore is
degraded in the periplasmatic space as, e.g. enterobactin (Sect. 2.7). Alternatively Fe3+
is transferred at the cell surface from the ferri-siderophore to a trans-membrane
transport system (“taxi mechanism”). A probably archaic and unspecific variety
of the taxi mechanism comprises the reduction of Fe3+ at the cell surface (see
ferrichrome A, Sect. 2.6 (99, 105)). The terms “shuttle” and “taxi mechanism” were
coined by Raymond and Carrano (296).