| Ferromagnetic resonance spectroscopy (FMR) is increasingly being used as a diagnostic tool
for identifying bacterial magnetite in sediments [e.g., Kopp et al. 2007; Kind et al. 2011,
Roberts et al. 2011 ], the reason being that magnetic bacteria have a characteristic FMR
fingerprint which is not known from inorganic geological samples [Kopp & Kirschvink,
2008]. The diagnostic FMR features of single-stranded magnetite chains are a g-value < 2
and a markedly asymmetric FMR absorption spectrum, which produces several low-field
peaks and a deep high-field minimum in the first-derivative spectrum. These key features can
be reproduced not only with a chain-of-spheroids model, but - somewhat astonishingly - also
with a single-particle model (Stoner-Wohlfarth-type), provided the easy cubic axis ( )
coincides with the long particle axis [Charilaou et al. 2011]. This agreement weakens the
diagnostic strength of the FMR screen, which would render false positive results for the
admittedly exotic case of an assemblage of elongated magnetite particles of inorganic
origin. Likewise, it will render false negatives by not recognizing bacterial magnetite
in other than single-stranded configurations. For example, the FMR absorption
spectrum of two-stranded magnetosome chains, which represent the preferred chain
arrangement in a number of uncultured but otherwise widespread coccoid bacteria, lacks
asymmetry and has a g-value > 2, quite opposite to what we know from single-stranded
chains. Therefore, in order to better understand possible biogenic FMR fingerprints
and to refine the screen, there is a clear need to acquire FMR spectra of magnetic
bacteria with different chain configurations and, in particular, of greigite producing
bacteria.
References:
Charilaou M, Winklhofer M, Gehring AU (2011) J. Appl. Phys. 109, 093903
Kind J, Gehring AU, Winklhofer M, Hirt AM (2011) Geochem. Geophys. Geosyst. 12,
Q08008
Kopp RE et al (2007) Paleoceanogr. 22, PA4103
Kopp RE & Kirschvink JL (2008) Earth Sci. Rev. 86, 42-61.
Roberts AP et al. (2011) Earth Planet. Sci. Lett. 310, 441-452. |