Chemistry Reference
In-Depth Information
bacteria are universal and will certainly be utilized again in other examples of BCM in
bacteria discovered in the future.
THE MAGNETOTACTIC BACTERIA
Classification and general features
The magnetotactic bacteria are a heterogeneous group of prokaryotes that passively
align and actively swim along the Earth's geomagnetic field lines (Blakemore 1975,
1982). This group is morphologically, metabolically, and phylogenetically diverse and
cellular morphotypes that have been observed include coccoid (roughly spherical or
ovoid), rod-shaped, vibrioid (curved), spirilloid (helical) and even multicellular forms
(Blakemore et al. 1982; Farina et al. 1983; Rogers et al. 1990; Bazylinski 1995). The
term “magnetotactic bacteria” therefore has no taxonomic significance and should be
interpreted as a collection of diverse bacteria that possess the apparently widely
distributed trait of magnetotaxis (Bazylinski 1995).
Despite the diversity of these magnetotactic bacteria, they have several important
features in common: 1) all that have been described are gram-negative members of the
Domain Bacteria (it is possible that some members of the Domain Archaea produce
magnetosomes but none have been reported); 2) they are all motile, generally by flagella
(although this does not preclude the possibility of the existence of non-motile bacteria
that synthesize magnetosomes which, by definition, would be magnetic but not
magnetotactic); 3) all exhibit a negative tactile and/or growth response to atmospheric
concentrations of oxygen; 4) all strains in pure culture have a respiratory form of
metabolism (i.e., none are known to ferment substrates); and 5) they all possess a number
of magnetosomes, the signature feature of the group (Bazylinski 1995; Bazylinski and
Moskowitz 1997). The bacterial magnetosome is defined as an intracellular single-
magnetic-domain crystal of a magnetic iron mineral enveloped by a membrane (Balkwill
et al. 1980). The membrane is intracellular and may be connected to the cytoplasmic
membrane. There is some evidence that the magnetosome membrane comprises a vesicle
in which the magnetosome subsequently nucleates and grows (Gorby et al. 1988).
There are two general types of magnetotactic bacteria based upon the minerals they
biomineralize: there are the iron-oxide types that mineralize crystals of magnetite (Fe
3
O
4
)
and the iron-sulfide types that mineralize crystals of greigite (Fe
3
S
4
) (Bazylinski and
Frankel 2000b). The iron-oxide type are obligate microaerophiles, facultative anaerobes
(that are microaerophilic when growing with O
2
), or obligate anaerobes while the iron-
sulfide type appear to be obligate anaerobes.
Ecology of magnetotactic bacteria
Magnetotactic bacteria are ubiquitous in aquatic environments containing water with
pH values close to neutrality and are not thermal, strongly polluted, or well-oxygenated
(Moench and Konetzka 1978; Blakemore 1982). They are cosmopolitan in distribution
(Blakemore 1982) and because magnetotactic bacterial cells are easy to observe and
separate from mud and water by exploiting their magnetic behavior using simple
laboratory magnets (Moench and Konetzka 1978), there are frequent reports of their
occurrence in various freshwater and marine locations (e.g., Matitashvili et al. 1992; Iida
and Akai 1996).
On a local basis, magnetotactic bacteria are generally present in the highest numbers
at the microaerobic oxic-anoxic transition zone (OATZ) and just below it in the anaerobic
zone (Bazylinski 1995). In many freshwater habitats, the OATZ is located at the
sediment-water interface or just below it. However, in some brackish-to-marine systems,
