Gilson Michael

No Thumbnail Available
Last Name
Gilson
First Name
Michael
ORCID

Filters

1980 - 1981 2
2
2
Reset filters

Settings

Search Results

Now showing 1 - 2 of 2
  • Technical Report
    Statistical mechanics of geomagnetic orientation in sediment bacteria
    (Woods Hole Oceanographic Institution, 1981-04) Gilson, Michael ; Kalmijn, Adrianus J.
    Last year we reported on time-of-transit experiments in which magnetically orienting bacteria crossed a 1-mm stretch in the direction of a uniform magnetic field. The bacteria were found to behave as tiny self-propelled compass needles subject both to magnetic field alignment and to the randomizing effect of thermal agitation. In strong fields, magnetic bacteria are held in tight aligment; in weaker fields, their swimming paths meander more and transit times are greater. Paul Langevin derived an expression for the distribution of orientation in an ensemble of free-moving dipole particles as a function of ambient field strength. His theory becomes applicable to our experiments when bacterial migration is analyzed as a sequence of short steps during each of which the cell swims in a direction randomly selected from the Langevin distribution . The duration of each step, Δt, is actually a time constant of the cell's loss of directionality due to thermal agitation. By thus treating the migration as a process of random walk with drift, we are able to predict the mean and variance of the time of transit across a 1-mm stretch.
  • Technical Report
    Migration rate of mud bacteria as a function of magnetic field strength
    (Woods Hole Oceanographic Institution, 1980-11) Teague, Barbara D. ; Gilson, Michael ; Kalmijn, Adrianus J.
    Certain marine and freshwater mud bacteria are endowed with a permanent magnetic dipole moment. This moment is attributed to an endogenous chain of tightly coupled, single-domain magnetite crystals. When separated from the mud, these magnetic bacteria swim north, following the earth's magnetic field lines. As at Woods Hole, Massachusetts, the field lines are steeply vertically inclined, the bacteria rapidly return to the bottom substrate where they seem to thrive best. To quantify this migration, we measure the time to traverse the distance between two lines, 1 mm apart, as a function of the ambient magnetic field strength. Using dark-field illumination, we observe single organisms as they migrate in a low-oxygen hemocytometer chamber. We control the ambient magnetic field by regulating the current through a Helmholtz-coil system. At high magnetic field strengths, the bacteria follow a virtually straight path, swimming at rates around 150 µm/sec. At lower field strengths, they take a more random path which reduces their migration rate. Although they swerve moderately at the earth's magnetic field strength (0.5 gauss) , the bacteria still achieve about 80% of their maximum migration rate observed at higher-gauss fields. This suggests that the bacterial dipole moments are well adapted to orientation in the earth's magnetic field. Since the strength of their magnet determines the degree to which the organisms overcome random motion, we can estimate the magnitude of their dipole moment.