HR: 0800h
AN: B31C-1004    [Abstracts]
TI: The Swimming Efficiency of Magnetotactic Bacteria
AU: * Newell, A J
EM: andrew_newell@ncsu.edu
AF: North Carolina State University, Department of Marine, Earth and Atmospheric Sciences, Box 8208, Raleigh, NC 27695-8208 United States
AB: Magnetotactic bacteria are widespread in the oxic-anoxic transition zone (OATZ) of freshwater and marine sediments. They have chains of magnetic particles and exert a high degree of control over their synthesis. Evidently they find it worth the energy cost of synthesizing these particles. The existing model for magnetotaxis compare a one-dimensional search along a magnetic field line with the three-dimensional "run and tumble" behavior of bacteria like E. coli. However, this model is inadequate in more than one respect. First, a search along a field line is only advantageous for relatively steep field lines. Second, most bacterial moments are too small for the one-dimensional approximation to be accurate. Third, it will be shown that tumbling behavior is incompatible with at least one kind of magnetotaxis. Instead, all known magnetotactic bacteria can reverse their direction of swimming. Models are developed for the swimming efficiency of the two kinds of magnetotaxis identified by Frankel et al. (1997). These are polar and axial magneto-aerotaxis (MA), where aerotaxis is an energy-sensing behavior that helps the bacterium find the optimal oxygen concentration. In both kinds of taxis torque on the magnetic chains tends to align the bacteria with the Earth's field. This torque is countered by viscous drag and Brownian rotation. Polar MA has a switch-like response of swimming direction to the oxygen concentration. This type of aerotaxis also uses the direction of the magnetic field to determine which way to swim. In zero field or in a field with the wrong sign this mechanism fails. Axial MA uses a more conventional aerotaxis that responds to gradients in the energy or redox state. This mechanism works reasonably well even in zero field, and also for any field direction as long as the field is not too large. The swimming efficiency of magnetotactic bacteria is determined by two factors: a geometrical factor based on the distribution of bacterial orientations, and the effect of the distribution on the probability of reversing the swimming direction. Non-magnetotactic bacteria generally reverse direction every few seconds because they cannot maintain a consistent direction for longer in the face of Brownian rotation. Magnetic torque counters Brownian rotation, allowing the bacteria to swim in the same direction for much longer. This can provide as much as an order of magnitude improvement in swimming efficiency.
UR: http://www4.ncsu.edu/~ajnewell/Magnetotaxis.html
DE: 0419 Biomineralization
DE: 0448 Geomicrobiology
DE: 0463 Microbe/mineral interactions
DE: 1505 Biogenic magnetic minerals
DE: 1512 Environmental magnetism
SC: Biogeosciences [B]
MN: Fall Meeting 2005