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crystal lattice (Bradley et al. 1998; Barber and Scott 2002). This could be evidence that
these magnetites formed abiogenically
in situ
. Golden et al. (2001, 2002, 2003)
demonstrated that thermal decomposition of pure siderite (FeCO
3
) above 450°C results in
the formation of magnetite crystals with a size-range and projected shapes similar to
those found in ALH84001. Some of these magnetite crystals are elongated along [111],
as are the magnetite crystals in a number of magnetotactic bacteria, although there are
some differences in the relative sizes of facets of the low index forms {100}, {110} and
{111} (Golden et al. 2002, 2003). However, in MV-1 there are variations in the relative
sizes of the corner faces from crystal to crystal, even in the same chain.
Although statistical analyses of the sizes and shapes of of fine-grained magnetite and
greigite crystals (Devouard et al. 1998; Pósfai et al. 2001) as well as the characteristics
listed above might prove to be useful criteria for distinguishing between biogenic and
non-biogenic magnetite and greigite, it seems additional criteria are needed to distinguish
between biogenic and non-biogenic nanophase magnetic iron minerals. This is
particularly important in the case of magnetite produced via BIM by dissimilatory iron-
reducing bacteria since these crystals morphologically resemble those produced
abiogenically (Sparks et al. 1990). In addition, Zhang et al. (1998) showed that magnetite
crystals formed via BIM by thermophilic iron-reducing bacteria had a size distribution
that peaked in the single-magnetic-domain size range as does the BCM magnetite crystals
produced by the magnetotactic bacteria. Mandernack et al. (1999) found a temperature
dependent fractionation of oxygen isotopes in magnetite produced by
Magnetospirillum
magnetotacticum
and strain MV-1 that closely matched that for extracellular magnetite
produced by a bacterial consortium containing thermophilic iron-reducing bacteria
(Zhang et al. 1997). No detectable fractionation of iron isotopes was observed in the
magnetite. In contrast, Beard et al. (1999) found enrichment of Fe
54
compared to Fe
56
in
the soluble Fe
2+
produced by a dissimilatory iron-reducing bacterium,
Shewanella algae
,
growing with the iron oxide mineral ferrihydrite. How this isotopic fractionation is
reflected in magnetite formed by this organism and other iron-reducing bacteria remains
to be seen, but could ultimately provide a means for distinguishing BIM magnetite from
abiotically-produced magnetite.
An important question that arises from the discussion presented above is whether the
magnetotactic bacteria are or can be considered as an ancient group of prokaryotes. We
presently do not know how long magnetotactic bacteria have been on Earth or even how
widely the magnetotactic phenotype is currently distributed among contemporary
prokaryotes. Phylogenetic analysis (discussed earlier in this chapter), based on 16S rRNA
gene sequences of numerous magnetite-producing magnetotactic bacteria, show that these
organisms are associated with the
-subgroups of the Proteobacteria with one
exception that belongs in Nitrospira group in the domain Bacteria. Only one greigite-
producing magnetotactic bacterium has been analyzed to date, and this organism is also
associated with the Proteobacteria, in the
α
- and
δ
-subgroup (DeLong et al. 1993). Neither the
Proteobacteria nor the Nitrospira group are deeply branching lineages in the Domain
Bacteria (Woese 1987) and thus these groups are not generally considered to be ancient
groups of procaryotes. Estimating the age at which different bacterial groups evolved is
currently impossible or at best incredibly difficult. Moreover, in addition, phylogenetic
analyses suggest that magnetotaxis may have evolved several times in the past (DeLong
et al. 1993; Spring and Bazylinski 2000).
Magnetic sensitivity in other organisms
Magnetic sensitivity has been reported in other microbes including single-celled
algae (Torres de Araujo et al. 1986) and various types of protists (Bazylinski et al. 2000)
and a relatively large number of higher organisms (Wiltschko and Wiltschko 1995). In
δ
