Ferromagnetic resonance and rock magnetic identification of magnetotactic bacteria

How can we identify fossil magnetotactic bacteria?

One of the major focuses of my recent work has been the development of FMR spectroscopy as a tool to supplement more traditional rock magnetic techniques. FMR spectroscopy is a form of microwave spectroscopy based on the resonant absorption of microwaves by electrons precessing in a magnetic field. In magnetic materials, FMR provides a way of assessing the internal fields generated by interparticle interactions and by particle anisotropy. It is thus a macroscopic tool for assessing microscopic particle arrangement and structure.

FMR spectroscopy is a rapid technique that generally requires about five minutes to acquire a single spectrum and is thus extremely useful for high-resolution stratigraphic sampling. Although FMR spectra are generally measured using an electron paramagnetic resonance spectrometer, which employs a fixed frequency microwave source and a variable magnetic field, a group at the Jet Propulsion Laboratory is working on developing a zero-field, variable-frequency, field-ready FMR instrument.

Because FMR is sensitive to microscopic particle arrangement and structure, one potential application is screening for samples likely to contain fossil magnetotactic bacteria. Magnetotactic bacteria are a group of bacteria defined by the common trait of generating intracellular chains of magnetic particles, which passively orient them within the Earth's magnetic field. These particles have narrow distributions of size, shape, and arrangement that increase the efficiency with which the bacteria produce a magnetic moment. This optimization suggests that the chains of magnetic particles provide the bacteria with a selective advantage, likely related to navigating within the sharp chemical gradients in which the bacteria commonly live. These distinctive, optimized traits are all in theory detectable with macroscopic physical techniques.

Using laboratory-grown cultures of magnetotactic bacteria, I have demonstrated that these traits generate distinctive FMR spectra (Kopp et al., 2006). Mutants grown by Cody Nash (Caltech) have allowed me to untangle the different contributions made to the FMR spectra of bacterial magnetite by the magnetite crystal structure, particle elongation, and chain arrangement. I have also constructed first-order physical models of ferromagnetic resonance that fit many of the features observed well. There remains, however, much physical modeling work to be done by a student with a strong physics focus.

Although FMR spectroscopy is a 60-year-old technique, its application to natural samples is currently in its infancy. At present, therefore, it is still important to use FMR in conjunction with standard rock magnetic techniques. Rock magnetism provides supporting evidence for certain traits that contribute to FMR spectra, such as sample composition, particle size, and three-dimensional interparticle interactions. My work with the mutant bacteria has demonstrated a correlation between these traits as measured by rock magnetism and as extracting from FMR spectra.

FMR spectroscopy is the only technique, however, capable of measuring the distinctive microscopic anisotropy produced by particle elongation and arrangements in chains. Moreover, rock magnetic experiments are considerably slower than FMR measurements, so I therefore use FMR to select representative samples for rock magnetic experiments. The complementary use of FMR and rock magnetism is therefore ideal for high-resolution stratigraphic measurements of magnetic properties.

Related Publications

• R. E. Kopp and J. L. Kirschvink (2007). The identification and biogeochemical interpretation of fossil magnetotactic bacteria. Earth Sci. Rev. doi:10.1016/j.earscirev.2007.08.001 [Journal link]
• R. E. Kopp, T. D. Raub, D. Schumann, H. Vali, A. V. Smirnov, and J. L. Kirschvink (2007). Magnetofossil Spike During the Paleocene-Eocene Thermal Maximum: Ferromagnetic Resonance, Rock Magnetic, and Electron Microscopy Evidence from Ancora, New Jersey, USA. Paleoceanography 22: PA4103. doi:10.1029/2007PA001473 [Journal link]
• A. C. Maloof, R. E. Kopp, J. P. Grotzinger, D. A. Fike, T. Bosak, H. Vali, P. M. Poussart, B. P. Weiss, and J. L. Kirschvink (2007). Sedimentary iron cycling and the origin and preservation of magnetization in platform carbonate muds, Andros Island, Bahamas. Earth Planet. Sci. Lett. 259: 581-598. doi:10.1016/j.epsl.2007.05.021 [Journal link]
• R. E. Kopp, C. Z. Nash, A. Kobayashi, B. P. Weiss, D. A. Bazylinski, and J. L. Kirschvink (2006). Ferromagnetic resonance spectroscopy for assessment of magnetic anisotropy and magnetostatic interactions: A case study of mutant magnetotactic bacteria. J. Geophys. Res. B 111, B12S25. doi:10.1029/2006JB004529 [Journal link]
• R. E. Kopp, B. P. Weiss, A. C. Maloof, H. Vali, C. Z. Nash, and J. L. Kirschvink (2006). Chains, clumps, and strings: magnetofossil taphonomy with ferromagnetic resonance spectroscopy. Earth Planet. Sci. Lett. 247: 10-25. doi:10.1016/j.epsl.2006.05.001 [Journal link]
• A. Kobayashi, J. L. Kirschvink, C. Z. Nash, R. E. Kopp, D. A. Sauer, L. E. Bertani, W. F. Voorhout, and T. Taguchi (2006). Experimental observation of magnetosome chain collapse in magnetotactic bacteria: Sedimentological, paleontological, and evolutionary implications. Earth Planet. Sci. Lett. 245: 538-550. doi:10.1016/j.epsl.2006.03.041 [Journal link]
• Y. Suzuki, R. E. Kopp, et al (2006). Sclerite formation in the hydrothermal-vent "scaly-foot" gastropod -- possible control of iron sulfide biomineralization by the animal. Earth Planet. Sci. Let. 242: 39-50. doi:10.1016/j.epsl.2005.11.029 [Journal link]
• B.P. Weiss, S.S. Kim, J.L. Kirschvink, R.E. Kopp, M. Sankaran, A. Kobayashi, and A. Komeili (2004). Ferromagnetic resonance and low-temperature magnetic tests for biogenic magnetite. Earth Planet. Sci. Lett. 224: 73-89. doi:10.1016/j.epsl.2004.04.024 [Journal link]
• B.P. Weiss, S.S. Kim, J.L. Kirschvink, R.E. Kopp, M. Sankaran, A. Kobayashi, and A. Komeili (2004). Magnetic tests for magnetosome chains in Martian meteorite ALH84001. Proc. Natl. Acad. Sci. 101: 8281-4. doi:10.1073/pnas.0402292101 [Journal link]