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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.\n\nIn 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\nsecond example is magnetite as identi\xafed in tissue of pigeon's heads.\n\nIn 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\noffer 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.\nThe 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).\nThe 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.\n\nDifferent 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\nfor the presence of chains of stable single-domain particles, which form only under strict biological control.\nHowever, 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\nlimited, 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\nproof for bacterial magnetite is the optical identification -although time consuming and\ntedious- by Transmission Electron Microscopy.\n\nThe 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.\n\nThe response of these clusters to magnetic fields has been simulated by spherules of ferrofluid. Depending on their geometrical arrangement these spherules show peculiar\nmagnetic properties. Based on these properties, three models have been developed experimentally and theoretically with respect to a possible application as biological sensory\nelements. 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.