Swinging magnetic bacteria.

Magnetic bacteria contain chains of magnetic material by which they orient themselves in the earth's magnetic field (SN: 8/11/84, p. 87). Presumably these orientations help them find food. A group of Brazilian scientists has been working with the south-seeking magnetic bacteria found in Brazilian waters to determine their magnetic moment and the efficiency with which they reorient themselves in a suddenly changed magnetic field.

Magnetic moment is a measure of an object's intrinsic magnetism and also of the strength of its response to a magnetic field imposed from outside. Magnetic bacteria have not yet been successfully cultured, so the researchers used wild ones collected from waters 50 centimeters deep in the region of Rio de Janeiro. They froze a sample containing 100 million cells per cubic centimeter and measured the magnetic moment with a magnetometer. They then calculated what the magnetic moment ought to be from the amount of magnetic material in a bacterium, assuming the material to be 80 percent magnetite (Fe3O4). The average magnetic moment per cell came to 1.8 x 10.sup.-12 electromagnetic units with the magnetometer and 1.3 x 10.sup-12 electromagnetic units by calculation -- a good agreement, they say.

Magnetic moments of individual cells tend to vary according to the size of the cell from 3 x 10.sup.-13 electromagnetic units for the smallest to 54 x 10.sup.-12 electromagnetic units for the largest. Studies of the time they took to turn about after sudden reversals of the magnetic field indicate that the reversal time (which varies by size from one-half second to 50 seconds) is smaller than the mean time of other perturbations that take place in the organisms' environment, and the energy of the magnetic interaction is greater than the thermal disorder energy. This indicates that the magnetic mechanism is efficient for putting and holding the cells in a particular orientation. The researchers are Darcy Motta Esquivel, Henrique Luis de Barros, E. Wajnberg and L.H. Souza of the Brazilian Center for Physical Research in Rio de Janeiro.