We report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS1, MV1 and MV2 and dissimilatory iron-reducing bacterium strain GS-15, grown in pure cultures, were used in this study. Our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from MTBs. The most useful magnetic property measurements include: (1) acquisition and demagnetization of isothermal remanent magnetization (IRM) using static, pulse and alternating fields; (2) acquisition of anhysteretic remanent magnetization (ARM); and (3) thermal dependence of low temperature (20 K) saturation IRM after cooling in zero field (ZFC) or in a 2.5 T field (FC) from 300 K. However, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero-field and field cooled SIRMs on warming through the Verwey transition (T ≈ 100 K). Intact chains of unoxidized magnetite magnetosomes have ratios of δFC δZFC greater than 2, where the parameter δ is a measure of the amount of remanence lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the δFC δZFC ratio to around 1, similar to values observed for some inorganic magnetite, maghemite, greigite and GS-15 particles. Numerical simulations of δFC δZFC ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the δFC δZFC parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made, compared to techniques such as electron microscopy.
Bibliographical noteFunding Information:
RBF and DAB were supportedb y the Office of Naval Research contract N00014-91-j-1290. DAB was also supportedb y National Science Foundation contract MCB-9117694. BMM was supportedb y National Science Foundationc on-tract EAR-8904380.S upportf or the IRM is provided by grants from the Keck Foundationa nd the National ScienceF oundationT. his is contribution 9305 of the Institutef or Rock Magnetism.