HR: 0830h
AN: NS51B-03    [Abstracts]
TI: Magnetotactic bacteria: when does the magnetic field help?
AU: * Newell, A J
EM: Andrew_Newell@ncsu.edu
AF: North Carolina State University, Center for Research in Scientific Computation P.O. Box 8208, Raleigh, NC 27695-8208 United States
AB: Magnetotactic bacteria are a diverse group of organisms that use the Earth's magnetic field to orient themselves. They are widespread in marine and freshwater sediments, particularly near the oxic-anoxic transition zone (OATZ). They are involved in the cycling of iron, magnesium, sulphur and carbon. Because they leave "magnetofossils" in the form of chains of magnetite particles with a narrow size range, they affect the magnetic properties of sediments and their presence has been detected in rocks as old as two billion years. Why do magnetotactic bacteria go to the trouble of synthesizing chains of magnetite particles? They use aerotaxis to stay near the optimal concentration of oxygen (at the OATZ), and under some circumstances the magnetic field helps them to locate this zone. However, the magnetic field does not help if it is parallel to the OATZ. A model is developed that compares the motion of magnetotactic bacteria in a magnetic field with the motion in zero field. The comparison depends on the type of aerotaxis. One type, "polar", uses only the magnitude of the oxygen concentration. This type is useless without an orienting mechanism, so it must have developed after magnetotaxis. The other type, "axial", uses both concentration and gradient to determine the probability per unit time of reversing the swimming direction. When a bacterium is going in the correct direction, it is much more likely to keep swimming in that direction. However, it takes about four seconds to accumulate enough information on the oxygen gradient to make a reliable decision. In this time rotational diffusion perturbs the swimming direction by an average of 60 degrees unless there is a restoring torque. Magnetotaxis counteracts rotational diffusion, allowing much longer runs in the correct direction. This effect is so important that magnetotaxis increases the efficiency of aerotaxis even when the field is nearly parallel to the OATZ.
UR: http://www4.ncsu.edu/~ajnewell/magnetotaxis.html
DE: 0400 Biogeosciences
DE: 1505 Biomagnetism
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly