controlled by MTB as other potentially formed iron oxide phases do not provide the
magnetic properties required for magnetotaxis.
1.3.1.1 Iron Source
Ferrous and ferric iron oxides are sparingly soluble in water around neutral pH and
thus the total soluble iron without any complexing or reducing agents at pH 4–10
lies below micromolar concentrations (Cornell and Schwertmann 2003). Most
bacteria, also non-magnetotactic organisms, require micromolar levels for growth,
yet many aquatic environments provide much less. Thus, organisms have developed means to accumulate the necessary amounts of iron from their respective
environments (Sandy and Butler 2009). Natural MTB habitats are freshwater or
marine sediments with typically micromolar concentrations of soluble iron (Flies
et al. 2005). Due to the biomineralization of magnetite, MTB acquire several orders
of magnitude more iron than non-magnetotactic organisms, reaching up to 4% of
their cell dry weight (Sch€ uler and Baeuerlein 1996). Cultivated MTB strains are
grown in medium supplied with iron (typically as citrate or quinate) in similar
amounts as within their natural habitats. It has been demonstrated that they can only
tolerate concentrations up to the millimolar range. Higher quantities were shown to
be toxic rather than beneficial for growth (Nakamura et al. 1993; Sch€ uler and
Baeuerlein 1996; Sch€ uler and Baeuerlein 1998; Faivre et al. 2007).
1.3.1.2 Iron Uptake and Transport
MTB have been proclaimed to possess specific iron uptake systems coupled to
magnetosome synthesis in order to obtain the necessary quantities for biomineralization. Iron must be taken up from the surrounding environment and transported
across the outer membrane into the periplasm. Two alternative routes of further
processing are possible as it has not been fully resolved yet whether magnetosomes
remain interconnected with the periplasm or are completely detached (Komeili
et al. 2006; Faivre et al. 2007). Iron must be transported either directly from
periplasm into the magnetosome vesicle or across the cytoplasmic membrane and
subsequently the magnetosome membrane in order to be concentrated and form
magnetite (Fig. 1.2). Both proposed pathways would involve different transporters,
proteins that provide energy, and regulators that control uptake and transport.
It was shown that MTB are able to take up iron in both its divalent and trivalent
(ferrous and ferric) forms (Sch€ uler and Baeuerlein 1996; Faivre et al. 2007), and
this uptake involves siderophores in some cases, similarly to non-magnetotactic
organisms. Siderophores are low-molecular-weight iron chelators that function to
enable uptake of ferric iron (Sandy and Butler 2009). They are produced and
released by bacterial cells into the surrounding medium to complex available ferric
iron. Iron–siderophore complexes are then internalized by specific transporters such
as TonB-dependent receptors. Intracellular iron release is enabled by reduction to
10
J. Baumgartner and D. Faivre
magnetic properties required for magnetotaxis.
1.3.1.1 Iron Source
Ferrous and ferric iron oxides are sparingly soluble in water around neutral pH and
thus the total soluble iron without any complexing or reducing agents at pH 4–10
lies below micromolar concentrations (Cornell and Schwertmann 2003). Most
bacteria, also non-magnetotactic organisms, require micromolar levels for growth,
yet many aquatic environments provide much less. Thus, organisms have developed means to accumulate the necessary amounts of iron from their respective
environments (Sandy and Butler 2009). Natural MTB habitats are freshwater or
marine sediments with typically micromolar concentrations of soluble iron (Flies
et al. 2005). Due to the biomineralization of magnetite, MTB acquire several orders
of magnitude more iron than non-magnetotactic organisms, reaching up to 4% of
their cell dry weight (Sch€ uler and Baeuerlein 1996). Cultivated MTB strains are
grown in medium supplied with iron (typically as citrate or quinate) in similar
amounts as within their natural habitats. It has been demonstrated that they can only
tolerate concentrations up to the millimolar range. Higher quantities were shown to
be toxic rather than beneficial for growth (Nakamura et al. 1993; Sch€ uler and
Baeuerlein 1996; Sch€ uler and Baeuerlein 1998; Faivre et al. 2007).
1.3.1.2 Iron Uptake and Transport
MTB have been proclaimed to possess specific iron uptake systems coupled to
magnetosome synthesis in order to obtain the necessary quantities for biomineralization. Iron must be taken up from the surrounding environment and transported
across the outer membrane into the periplasm. Two alternative routes of further
processing are possible as it has not been fully resolved yet whether magnetosomes
remain interconnected with the periplasm or are completely detached (Komeili
et al. 2006; Faivre et al. 2007). Iron must be transported either directly from
periplasm into the magnetosome vesicle or across the cytoplasmic membrane and
subsequently the magnetosome membrane in order to be concentrated and form
magnetite (Fig. 1.2). Both proposed pathways would involve different transporters,
proteins that provide energy, and regulators that control uptake and transport.
It was shown that MTB are able to take up iron in both its divalent and trivalent
(ferrous and ferric) forms (Sch€ uler and Baeuerlein 1996; Faivre et al. 2007), and
this uptake involves siderophores in some cases, similarly to non-magnetotactic
organisms. Siderophores are low-molecular-weight iron chelators that function to
enable uptake of ferric iron (Sandy and Butler 2009). They are produced and
released by bacterial cells into the surrounding medium to complex available ferric
iron. Iron–siderophore complexes are then internalized by specific transporters such
as TonB-dependent receptors. Intracellular iron release is enabled by reduction to
10
J. Baumgartner and D. Faivre