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We rcpmt results on the magnetic properties of magnetites produced by magnetotaetie and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS I. MV J and MV2 and dissimilatory iron-reducing bacterium ,train GS-15. grown in pure cultures, were used in this study, Our results suggest that a comhination of room tempnature 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 magnetiza tion (IRM) using static, pulse and altcrnating fields: (2) acquisition of anhysteretie 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 (Fe) lrom 300 K, Howcver, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference hetween low temperature zero-field and field cooled 51 RMs on warming through the Verwey transition (T z J00 K). Intact chains of unoxidized magnetite magnetoSl)ll1eS have ratios of D,.c/t5ZFC greater than 2, where the parameter D is a measure of the amount of remanenCl' lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosuilles to maghemite reduces the.) FC/OLl-C ratio to around L similar to values observed for some inorganic magnetite. rnaghemitc. greigitc and GS-15 particles. Numerical simulations of t5,.c/Dzvc ratios for simple binary mixtures of magnctosome chains and inorganic magnetic fractions suggest that the 15 njD zFc parameter can he a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomcs and can he a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparati\e case and rapidity with which magnetic measurements can be madc, compared to techniques such as electron III ic roscopy.

Magnetotactic cocci swim persistently along local magnetic field lines in a preferred direction that corresponds to downward migration along geomagnetic field lines. Recently, high cell concentrations of magnetotactic cocci have been found in the water columns of chemically stratified, marine and brackish habitats, and not always in the sediments, as would be expected for persistent, downward-migrating bacteria. Here we report that cells of a pure culture of a marine magnetotactic coccus, designated strain MC-1, formed microaerophilic bands in capillary tubes and used aerotaxis to migrate to a preferred oxygen concentration in an oxygen gradient. Cells were able to swim in either direction along the local magnetic field and used magnetotaxis in conjunction with aerotaxis, i.e., magnetically assisted aerotaxis, or magneto-aerotaxis, to more efficiently migrate to and maintain position at their preferred oxygen concentration. Cells of strain MC-1 had a novel, aerotactic sensory mechanism that appeared to function as a two-way switch, rather than the temporal sensory mechanism used by other bacteria, including Magnetospirillum megnetotacticum, in aerotaxis. The cells also exhibited a response to short-wavelength light (< or = 500 nm), which caused them to swim persistently parallel to the magnetic field during illumination.

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Magnetotactic bacteria contain magnetosomes--intracellular, membrane-bounded, magnetic nanocrystals of magnetite (Fe(3)O(4)) or greigite (Fe(3)S(4))--that cause the bacteria to swim along geomagnetic field lines. We isolated a greigite-producing magnetotactic bacterium from a brackish spring in Death Valley National Park, California, USA, strain BW-1, that is able to biomineralize greigite and magnetite depending on culture conditions. A phylogenetic comparison of BW-1 and similar uncultured greigite- and/or magnetite-producing magnetotactic bacteria from freshwater to hypersaline habitats shows that these organisms represent a previously unknown group of sulfate-reducing bacteria in the Deltaproteobacteria. Genomic analysis of BW-1 reveals the presence of two different magnetosome gene clusters, suggesting that one may be responsible for greigite biomineralization and the other for magnetite.

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