Detection and function of biogenic magnetite

Magnetite is a widespread accessory mineral in rocks and soils. As
was first shown by Lowenstam (1962), magnetite frequently forms
also by biochemical processes, with varying degrees of control of
the organisms over the mineralization process. Lowenstam
distinguishes between biologically induced (BIM) and biologically
controlled mineralization(BCM). The former refers to processes with
no biological control, and the later to processes with strict
metabolic and genetic control. In this thesis, two examples of
biogenic magnetite with eminently different magnetic properties are
studied. One is magnetite as found in so-called magnetotactic
bacteria; the second example is magnetite as identi¯ed in tissue of
pigeon's heads. In the first part of this work, the results of a
series of rock magnetic measurements on concentrated samples of
pure magnetotactic bacteria will be presented. These bacteria offer
a unique opportunity to study the process of biologically
controlled mineralization, since these organisms synthesize
intracellular particles of magnetite or greigite arranged in
chains, that give the bacterium the characteristic property of a
swimming compass needle. The magnetic crystals, so-called
magnetosomes, are magnetically speaking stable single-domain
particles, characterized by a size such that the particles have
minimum induced magnetization and maximum permanent magnetic moment
(i.e. particle size between 30 and 130 nm). The bacteria studied
here have been harvested in sediments from lake Chiemsee. They were
extracted from the sediments and concentrated to an extent that
enabled a detailed characterization by macroscopic magnetic
measurements. The so-called Bacteriodrome was used to concentrate
samples of approximately 10E7 cells. Different magnetic criteria,
aiming to specifically identify bacterial magnetite in sediments,
have been tested, including: (1) acquisition and demagnetization of
isothermal remanent magnetization (IRM); (2) acquisition of
anhysteretic remanent magnetization and (3) thermal dependence of
low temperature saturation IRM, after cooling in zero field (ZFC)
or in a 2.5 T field (FC) from 300 to 5 K. The best method turns out
to be the so-called delta-delta test (dFC/dZFC), first proposed by
Moskowitz et al. (1993), and based on the low temperature variation
of the SIRM, measured both in a strong field (FC) and in zero field
(ZFC). At the Verwey transition (ca. 120 K) the d-value for each
curve is determined and the d-ratio (dFC/dZFC) calculated. Values
exceeding 2, are indicators for the presence of chains of stable
single-domain particles, which form only under strict biological
control. However, it is shown that the suitability of rock magnetic
techniques to detect and characterize biogenic magnetite in bulk,
natural samples such as lake sediments is still limited, due to
diagenetic processes and the occurrence of other non-biogenic
magnetic minerals, which blur the distinct magnetic properties of
the former. The only certain proof for bacterial magnetite is the
optical identification -although time consuming and tedious- by
Transmission Electron Microscopy. The magnetite inclusions found in
pigeon tissue are very different in their magnetic properties with
respect to bacterial magnetite. With their small grain size
(between 2 and 10 nm), these particles fall within the
superparamagnetic size range and are characterized by a large
induced magnetization and no permanent magnetic moment. The pigeon
magnetite is concentrated in spherical clusters of approximately
1-3 micrometers in diameter. The response of these clusters to
magnetic fields has been simulated by spherules of ferrofluid.
Depending on their geometrical arrangement these spherules show
peculiar magnetic properties. Based on these properties, three
models have been developed experimentally and theoretically with
respect to a possible application as biological sensory elements.
The magnetic properties of the sensory models are tested in the
light of behavioral experiments conducted in the past on homing
pigeons and migratory birds. In these experiments, the birds were
exposed to defined magnetic fields to specifically affect a
magnetite-based magnetoreceptor. As will be seen, most of the
responses of the birds observed in the behavioral experiments can
be explained by simulating the effects of these magnetic treatment
on ferrofluid spherules.