1.2.1 Ecology
MTB have been ubiquitously found in aquatic environments such as fresh- and
marine water columns or the upper layers of the respective sediments. Their
habitats are usually chemically stratified with respect to oxygen, which diffuses
from the water–air surface downward, and sulfide generated by sulfate-reducing
bacteria, which diffuses upward from the anaerobic zone. This stratification
establishes a respective double gradient within the water column. MTB preferably
move to and inhabit the so-called oxic–anoxic transition zone (OATZ), where they
find optimal conditions and can reach cell densities of 10
5 –10
6 per mL (Blakemore
1982). It is thought that magnetotaxis aids them in finding the OATZ by simplifying
their search problem to one dimension, as the magnetic field lines of Earth are
inclined in the northern and southern hemispheres and align bacteria mostly
vertically in the water column.
All MTB discovered thus far are microaerophiles and/or anaerobes, and the
biomineralization of intracellular magnetite crystals is favored under the low
oxygen concentrations found in their optimal habitats (Heyen and Sch€ uler 2003;
Flies et al. 2005). Typical iron concentrations in such environments are around
0.01–1 mg/L, i.e., 0.2–20 mM, and higher abundance in lab experiments does not
lead to increasing numbers of bacteria but rather has been shown to be toxic
(Sch€ uler and Baeuerlein 1996). MTB have also been detected in highly alkaline
and saline environments where available iron is even scarcer due to its low
solubility at high pH (Nash 2004). Thus, they must have developed means to
accumulate iron against a large concentration gradient. Studied strains have been
isolated by taking sediment samples from aquatic habitats, storage in microcosms,
and subsequent cell harvest with a magnet (Schleifer et al. 1991). Only a few strains
have been obtained in pure culture so far, which is probably due to difficulties in
providing similarly complex environmental conditions in the lab as MTB inhabit in
nature.
1.2.2 Diversity
MTB are polyphyletic, meaning they represent a heterogeneous group of species as
determined by 16 S rRNA analysis. Known strains have very different
morphologies including rods, vibrios, spirilla, cocci and more complex forms
such as giant, barbell-shaped, and multicellular bacteria (see Fig. 1.1). They have
been attributed to the Gram-negative Alpha-, Gamma-, Deltaproteobacteria, and
the Nitrospira phylum (Amann et al. 2007). Best studied is the Magnetospirillum
genus as several species can be cultured in the laboratory, namely, M. magnetotacticum (Blakemore et al. 1979), M. magneticum (Matsunaga et al. 1991), and
M. gryphiswaldense (Schleifer et al. 1991). The vibrio strain MV-1 from the
Alphaproteobacteria, Desulfovibrio magneticus (Sakaguchi et al. 2002) from the
6
J. Baumgartner and D. Faivre
MTB have been ubiquitously found in aquatic environments such as fresh- and
marine water columns or the upper layers of the respective sediments. Their
habitats are usually chemically stratified with respect to oxygen, which diffuses
from the water–air surface downward, and sulfide generated by sulfate-reducing
bacteria, which diffuses upward from the anaerobic zone. This stratification
establishes a respective double gradient within the water column. MTB preferably
move to and inhabit the so-called oxic–anoxic transition zone (OATZ), where they
find optimal conditions and can reach cell densities of 10
5 –10
6 per mL (Blakemore
1982). It is thought that magnetotaxis aids them in finding the OATZ by simplifying
their search problem to one dimension, as the magnetic field lines of Earth are
inclined in the northern and southern hemispheres and align bacteria mostly
vertically in the water column.
All MTB discovered thus far are microaerophiles and/or anaerobes, and the
biomineralization of intracellular magnetite crystals is favored under the low
oxygen concentrations found in their optimal habitats (Heyen and Sch€ uler 2003;
Flies et al. 2005). Typical iron concentrations in such environments are around
0.01–1 mg/L, i.e., 0.2–20 mM, and higher abundance in lab experiments does not
lead to increasing numbers of bacteria but rather has been shown to be toxic
(Sch€ uler and Baeuerlein 1996). MTB have also been detected in highly alkaline
and saline environments where available iron is even scarcer due to its low
solubility at high pH (Nash 2004). Thus, they must have developed means to
accumulate iron against a large concentration gradient. Studied strains have been
isolated by taking sediment samples from aquatic habitats, storage in microcosms,
and subsequent cell harvest with a magnet (Schleifer et al. 1991). Only a few strains
have been obtained in pure culture so far, which is probably due to difficulties in
providing similarly complex environmental conditions in the lab as MTB inhabit in
nature.
1.2.2 Diversity
MTB are polyphyletic, meaning they represent a heterogeneous group of species as
determined by 16 S rRNA analysis. Known strains have very different
morphologies including rods, vibrios, spirilla, cocci and more complex forms
such as giant, barbell-shaped, and multicellular bacteria (see Fig. 1.1). They have
been attributed to the Gram-negative Alpha-, Gamma-, Deltaproteobacteria, and
the Nitrospira phylum (Amann et al. 2007). Best studied is the Magnetospirillum
genus as several species can be cultured in the laboratory, namely, M. magnetotacticum (Blakemore et al. 1979), M. magneticum (Matsunaga et al. 1991), and
M. gryphiswaldense (Schleifer et al. 1991). The vibrio strain MV-1 from the
Alphaproteobacteria, Desulfovibrio magneticus (Sakaguchi et al. 2002) from the
6
J. Baumgartner and D. Faivre