Magnetotactic bacteria are a taxonomically diverse group of bacteria that have chains of ferromagnetic crystals inside. These bacteria mostly live in the oxic-anoxic interface (OAI) of aquatic environments. The magnetic chains orient the bacteria parallel to the Earth's magnetic field and help them to maintain their position near the OAI. These chains show the fingerprint of natural selection acting to optimize the magnetic moment per unit iron. This is achieved in a number of ways: the alignment in chains, a narrow size range, crystallographic perfection and chemical purity.
Because of these distinctive characteristics, the particles can still be identified after the bacteria have died. Such magnetofossils are useful both as records of bacterial evolution and environmental markers. They can most reliably be identified by microscopy, but that is very labor-intensive. A number of magnetic measurements have been developed to identify magnetofossils quickly and non-invasively. However, the only test that can specifically identify the chain structure is ferromagnetic resonance (FMR), which measures the response to a magnetic field oscillating at microwave frequencies.
No theory has been developed to predict the FMR spectra for chains of magnetic particles. The goal of this project is to develop such a theory. The objectives of the proposed work are as follows: 1. To calculate the signature of particle interactions on FMR spectra of magnetosome chains, 2. To determine the effects on FMR spectra of chain disruption and oxidation, 3. To explore possible enhancements of the current FMR methods, and 4. To create resources for analyzing FMR spectra.
This project is supported by the Geophysics and Geobiology & Low Temperature Geochemistry Programs and the Office of Cyberinfrastructure's CI-Reuse Venture Fund.
The goal of this project was to develop a method for calculating ferromagnetic resonance (FMR) spectra for chains of magnetic particles in magnetotactic bacteria. These chains are one of the few kinds of fossil evidence that bacteria leave behind when they die, and a good method for identifying them would provide a useful indicator for the oxygen and sulfur conditions in the water when they were alive. The difficulty in making this calculation has been taking into account magnetostatic interactions between magnets in the chain. Previous calculations had only been made for a single isolated magnet. The PI developed the mathematical framework for calculating FMR absorption by systems of interacting magnets. A front end was created that would allow the user to specify the geometry of the particles (including shapes, orientations and positions). The duration of the grant was devoted to creating this code. The intellectual merit of this work is that it has resulted in the formulation of equations for explicitly calculating the effect of interactions between magnets on FMR spectra. The calculations can be made not only for the usual frequency and field range of cavity resonators, but also outside of this range where the effects of interactions are most likely to show up. The broader impacts of this work include a MATLAB toolbox for calculating FMR spectra that will be published online when it is ready. This code can also be used to model interacting particles in magnetic recording media and in biotechnology applications. The PI has also identified large holes in the Wikipedia coverage of magnetism; he has created or substantially improved several articles on magnetism and the Earth's magnetic field.
