[4]

MAGNETISM AND BIOLOGY: THE MAGNETOTACTIC BACTERIA STORY.  Dennis A. Bazylinski, Dept. of Microbiology, Iowa State Univ., Ames, IA.

     The Earth’s geomagnetic field is thought to influence the behavior of a wide range of organisms. However, the best understood example of magnetoreception and magnetonavigation is that observed in the magnetotactic bacteria (MB). Cells of this ubiquitous and diverse group of motile, mainly aquatic microbes synthesize magnetosomes which are intracellular, membrane-bounded, single-magnetic-domain crystals of a magnetic mineral, either magnetite (Fe₃O₄) or greigite (Fe₃S₄). These crystals impart a permanent magnetic dipole moment to the cell causing it to align along magnetic field lines, like a compass needle,  as it swims; a phenomenon called magnetotaxis. In many MB, magnetotaxis, in conjunction with aerotaxis, appears to function as a means for cells to locate and maintain an optimal position (the oxic-anoxic interface) in vertical O₂ and/or redox gradients in natural habitats by reducing a 3-dimensional search problem to a 1-dimensional search problem.

     Although little is known about the biochemistry and chemistry involved in magnetosome synthesis, the narrow size (~35–120 nm) distributions, the species-specific morphologies, and the pure chemical compositions of the mineral crystals in the MB and the fact that most MB organize their magnetosomes in chains within the cell indicate that the MB use a precise biologically-controlled mineralization process in synthesizing the magnetosome mineral phase. Some elongated Fe₃O₄ particle morphologies appear to be unique to the MB and have never been observed in abiotically synthesized Fe₃O₄ particles. Dead cells eventually release their mineral crystals in the surrounding environment (e.g., sediments) as “magnetofossils” that have been used as evidence of the past presence of MB. Such crystals have also been found in the Martian meteorite ALH84001 and their presence has been used as a line of evidence for life on ancient Mars.