exclusively at the magnetosome membrane, namely, MamB and MamM (Gr€ unberg
et al. 2004). These proteins have been attributed to the so-called CDF3 subfamily
which is supposed to comprise iron transporters (Nies 2003). Deletion mutants of
these genes in M. gryphiswaldense are nonmagnetic, supporting the hypothesis of
their involvement in the magnetite biomineralization process (Sch€ uler, personal
communication). Both genes are found in genomes of all other sequenced
magnetotactic bacterial species as well, emphasizing their potentially essential
role. So far, various proteins have been shown to be involved in iron uptake, but
it remains unclear which are related to biomineralization and whether separate and
specific uptake mechanisms exist besides the one for metabolic needs. Only the
proteins MagA, MamB, and MamM seem to be specific to MTB and thus are
candidates for further studies which will hopefully lead to insights into how iron
transport is coupled to biogenic magnetite synthesis.
1.3.1.3 Magnetite Formation
Magnetite formation in Magnetospirillum spp. requires microaerobic or anaerobic
conditions and higher oxygen levels can suppress its biomineralization (Heyen and
Sch€ uler 2003). This is consistent with abiotic synthesis conditions, which require
low or no oxygen to prevent oxidation of ferrous to ferric iron and thus formation of
oxidized iron oxide phases such as maghemite, hematite, or goethite. Generally,
magnetite forms under slightly reducing conditions (E h ~ À0.2 to À0.4 V) and
alkaline pH (>8) (Winklhofer and Petersen 2007; Faivre and Sch€ uler 2008). Thus,
MTB have to form specific intracellular compartments for the synthesis of the
magnetite phase as normal physiological conditions do not allow for the formation
of this iron oxide phase. The pH level present within magnetosome vesicles is still
unclear; however, foraminifera, marine calcite-forming unicellular organisms have
been shown to be able to regulate pH above 9 within intracellular vesicles (de
Nooijer et al. 2009). This finding suggests that other single-cell aquatic organisms,
and therefore MTB, may be able to regulate pH within this range.
Two alternative magnetosome formation pathways have been proposed since the
discovery of bacterial magnetite biomineralization. One suggestion involves invagination and detachment of magnetosome vesicles from the cytoplasmic membrane
before nucleation and growth of crystals. Another proposed mechanism involves
the formation of small crystal nuclei at the cytoplasmic membrane and simultaneous or consecutive vesicle invagination (Komeili 2007). Various magnetite
mineral precipitation mechanisms have also been proposed. Initial assumptions
after the discovery of magnetite biomineralization involved an amorphous or low
crystallinity mineral precursor such as an amorphous ferrous oxyhydroxide or
ferrihydrite, which is subsequently transformed into magnetite by oxidation or
reduction, respectively (Frankel et al. 1983). Recent findings, however, seem to
refute the existence of such precursors as they have not been detectable in cultivable
MTB. A possible mechanism without mineral precursors is the coprecipitation
of Fe
II and Fe
III ions under alkaline conditions within the magnetosome vesicle
1 Magnetite Biomineralization in Bacteria
13
et al. 2004). These proteins have been attributed to the so-called CDF3 subfamily
which is supposed to comprise iron transporters (Nies 2003). Deletion mutants of
these genes in M. gryphiswaldense are nonmagnetic, supporting the hypothesis of
their involvement in the magnetite biomineralization process (Sch€ uler, personal
communication). Both genes are found in genomes of all other sequenced
magnetotactic bacterial species as well, emphasizing their potentially essential
role. So far, various proteins have been shown to be involved in iron uptake, but
it remains unclear which are related to biomineralization and whether separate and
specific uptake mechanisms exist besides the one for metabolic needs. Only the
proteins MagA, MamB, and MamM seem to be specific to MTB and thus are
candidates for further studies which will hopefully lead to insights into how iron
transport is coupled to biogenic magnetite synthesis.
1.3.1.3 Magnetite Formation
Magnetite formation in Magnetospirillum spp. requires microaerobic or anaerobic
conditions and higher oxygen levels can suppress its biomineralization (Heyen and
Sch€ uler 2003). This is consistent with abiotic synthesis conditions, which require
low or no oxygen to prevent oxidation of ferrous to ferric iron and thus formation of
oxidized iron oxide phases such as maghemite, hematite, or goethite. Generally,
magnetite forms under slightly reducing conditions (E h ~ À0.2 to À0.4 V) and
alkaline pH (>8) (Winklhofer and Petersen 2007; Faivre and Sch€ uler 2008). Thus,
MTB have to form specific intracellular compartments for the synthesis of the
magnetite phase as normal physiological conditions do not allow for the formation
of this iron oxide phase. The pH level present within magnetosome vesicles is still
unclear; however, foraminifera, marine calcite-forming unicellular organisms have
been shown to be able to regulate pH above 9 within intracellular vesicles (de
Nooijer et al. 2009). This finding suggests that other single-cell aquatic organisms,
and therefore MTB, may be able to regulate pH within this range.
Two alternative magnetosome formation pathways have been proposed since the
discovery of bacterial magnetite biomineralization. One suggestion involves invagination and detachment of magnetosome vesicles from the cytoplasmic membrane
before nucleation and growth of crystals. Another proposed mechanism involves
the formation of small crystal nuclei at the cytoplasmic membrane and simultaneous or consecutive vesicle invagination (Komeili 2007). Various magnetite
mineral precipitation mechanisms have also been proposed. Initial assumptions
after the discovery of magnetite biomineralization involved an amorphous or low
crystallinity mineral precursor such as an amorphous ferrous oxyhydroxide or
ferrihydrite, which is subsequently transformed into magnetite by oxidation or
reduction, respectively (Frankel et al. 1983). Recent findings, however, seem to
refute the existence of such precursors as they have not been detectable in cultivable
MTB. A possible mechanism without mineral precursors is the coprecipitation
of Fe
II and Fe
III ions under alkaline conditions within the magnetosome vesicle
1 Magnetite Biomineralization in Bacteria
13