Chemistry Reference
In-Depth Information
SIGNIFICANCE OF BCM
Magnetofossils and biomarkers
A major problem for understanding the origin of life and the evolutionary origin and
phylogeny of prokaryotes is the general lack of microbial fossils. Moreover, much of the
material thought to be fossilized microbes is subject to alternate interpretations. For
example, microbial fossils supposedly representing cyanobacterial species from some of
the oldest rocks on Earth, 3.5 billion year old cherts from western Australia (Schopf and
Packer 1987; Schopf 1993), have recently been under intense scrutiny (Dalton 2002).
Years after the original microbial fossil interpretation of the structures in these rocks had
become “textbook orthodoxy” (Dalton 2002), Brasier et al. (2002) reexamined the
structures and offered an alternative explanation for their formation, i.e., the structures
are secondary artifacts formed from amorphous graphite within multiple generations of
metalliferous hydrothermal vein chert and volcanic glass, or as Dalton (2002) puts it
more simply, they represent “carbonaceous blobs, probably formed by the action of
scalding water on minerals.” The point here illustrates that there is a significant need for
unequivocal evidence for the past presence of microbes. If BCM-produced minerals are
unusual enough (i.e., cannot be formed by geological or chemical processes alone) and
persist over long periods of geologic time, they might prove be excellent evidence of the
past presence of certain microbes. This is thought by some to be the case with certain
types of magnetite crystals.
Based on a number of studies (e.g., Sparks et al. 1990; Bazylinski et al. 1994;
Devouard et al. 1998) magnetite crystals produced via BCM by the magnetotactic
bacteria have the following characteristics: a) high chemical and structural purity; b) high
structural perfection; c) consistent habits within a given species or strain, most commonly
equidimensional cubooctahedra or non-equidimensional, pseudo-hexagonal prisms with
(110) side faces and truncated (111) end caps, elongated along the [111] axis
perpendicular to the endcaps; d) a certain fraction (~10%) of twined crystals
characterized by rotations of 180 degrees around [111] axis with a common (111) contact
plane; e) a consistent width to length ratio; and f) an asymmetric crystal-size distribution
with a sharp cutoff for larger sizes within the single magnetic domain size range
(Devouard et al. 1998). In addition, they have novel magnetic properties (Moskowitz et
al. 1989, 1993).
After magnetotactic cells die and lyse, their magnetosome crystals are released into the
surrounding environment where they may settle into sediments and persist for some time.
Nanometer-sized magnetite grains have been recovered from a number of soils (Maher and
Taylor 1988; Maher 1990), freshwater sediments (Peck and King 1996), and modern and
ancient deep sea sediments (Stolz et al. 1986, 1990; Petersen et al. 1986; Chang et al. 1989;
Chang and Kirschvink 1989; Akai et al. 1991). In some cases, they were identified as
biogenic by their shape and size similarity to crystals in magnetotactic bacteria and were
thus referred to as "magnetofossils" (Kirschvink and Chang 1984; Chang et al. 1989;
Chang and Kirschvink 1989). Regarding ancient sediments, putative fossil magnetotactic
bacterial magnetite crystals have been found in stromatolitic and black cherts from the
Precambrian some dating as far back as 2 billion years (Chang and Kirschvink 1989;
Chang et al. 1989). If these crystals are actually magnetofossils from magnetotactic
bacteria, they represent some of the oldest bacterial fossils on Earth. Many of the crystals
found in these rocks were partially degraded and showed indistinct edges (compared to
freshly-isolated magnetite crystals from magnetotactic bacteria) indicating that the particles
underwent partial oxidation (Chang et al. 1987, 1989). The crystals were used as
supporting evidence that free O
2
had begun to accumulate in the atmosphere before 2000
Ma ago, that the present level of the earth's magnetic field strength had appeared by 2000
