Motion of Magnetotactic Microorganisms

It has been shown that the geomagnetic field could play a role in the orientation, navigation and homing of a great number of organisms (Palmer, 1963; Schmidt-Koenig & Keeton, 1978). Until recently the mechanism of perception of magnetic fields was unknown, although the behavioural effects of weak magnetic fields on different organisms have been studied (Barnothy, 1969). The strength of the geo magnetic field is of the order of 50 μT, varying from 25 μT (in the South Atlantic geomagnetic anomaly) to 60 μT at the poles, and the inclination varies from near 80° at the North magnetic pole to −80° at the South magnetic pole, with null inclination at the geomagnetic equator. The interesting regions are those where the magnitude of the field is very small and/or near the geomagnetic equator and where it is possible to observe how sensible is the response of microorganisms to the local geomagnetic field.

There are at least two different mechanisms by which organisms can detect the geomagnetic field (Frankel, 1984). Certain organisms are capable of detecting weak voltage gradients. They are thus able to detect the field by magnetic induction, as shown by Kalmijn (1978) for elasmobranchs. The other mechanism is based on intracellular, permanently magnetic material as in magnetotactic organisms (Blakemore, 1975, 1982; Frankel, 1984; Kirschvink, 1982). In the particular case of magnetotactic microorganisms, the magnetic material consists of biomineralized single-magnetic-domain magnetite (Fe₃O₄) particles (Frankel, Blakemore & Wolfe, 1979; Towe & Moench, 1981; Mann, Frankel & Blakemore, 1984). The magnetic material is found inside the magnetosome, a specific organelle responsible for the biomineralization of magnetite. The particles are often aligned in a chain which imparts a permanent magnetic dipole moment to the cell. The dipole moment interacts with the external magnetic field, producing a torque which tends to align the moment along the field lines. This response is passive in the sense that killed microorganisms are also oriented in the field.

We report here results of electron and optical microscopy studies of magnetotactic microorganisms from brackish marine and freshwater environments in different regions of the city of Rio de Janeiro. We have previously described a variety of morphological types of magnetotactic microorganisms from these environments (Esquivel et al. 1983; Farina, Lins de Barros, Esquivel & Danon, 1983).

We have also studied the movement of the microorganisms in a laboratory magnetic field, measuring the swimming velocities (v), and times and radii of ‘U-turns’ following field reversals. The results are analysed in terms of a model proposed by Charles P. Bean (Frankel, 1984; see Appendix).

The magnetic moment values for different microorganisms are estimated from U-tum analyses (m_u) and/or from electron micrographs of the magnetite chain (m_em).

Samples with sediments and water were collected in three places on different dates, between 20 and 60 cm deep. Rodrigo de Freitas coastal lagoon is located in Rio de Janeiro city, in the urbanized metropolitan area, where waste disposal has over loaded the normal ecosystem. The organically polluted waters are in the saprobic biological zone, where there is a bacterial population, decomposing the organic matter, and the planktonic blue green algae Anabaena and Oscillatoria. Hydrogen sulphide is common in the bottom of this lagoon and combines with the iron salts and oxides to form sulphides, which colour the anaerobic layer black. Marine waters from Guanabara bay also contain a very high level of sewage and industrial pollutants. Fresh waters, in contrast, are from small rivers located in an ecological reserve in the Rio de Janeiro metropolitan area. Despite the high pollution in Rodrigo de Freitas lagoon and Guanabara bay, we have observed no significant alteration in viscosity. After 4 or 5 days in the laboratory at ambient temperature, the concentrations of magnetotactic microorganisms had increased to up to 10 000 cells cm⁻³.

The magnetotactic microorganisms were further concentrated by a bar magnet or Helmholtz coil. Samples were placed in glass tubes 10cm long and 3 cm in diameter ending as micropipettes. The magnets were positioned so the magnetotactic microorganisms would swim to and concentrate in the micropipette. A few minutes after attachment of the magnet, living magnetotactic microorganisms could be collected without sediment or other microorganisms.

Preparations for TEM (transmission electron microscopy) were made by placing living magnetotactic microorganisms on copper grids (400 mesh) covered with collodion film: it is possible to observe the magnetic response on these grids by using optical microscopy. These cells were fixed in osmium tetroxide vapour without carbon reinforcement and the excess liquid was drawn off the grid surface. The preparations were allowed to dry in air and were observed in a JEOL 100CX electron microscope with maximum voltage filament (100kV).

Optical studies were made using a Leitz-Ortholux microscope with objectives of 10 –l00 × and eyepieces of 10 – 40 × coupled to photographic, cinematographic or video systems. Natural water samples with sediments were placed directly on the slide. The constant magnetic field was obtained by a pair of Helmholtz coils calibrated by a digital magnetometer (DM 2220, Schonstedt Instrument Company). We used clear ground lighting or dark field illumination to observe or record the movement of magnetotactic microorganisms as a function of the applied field. To reduce contributions due to the geomagnetic field we oriented the microscope so that the geomagnetic field was parallel to the field of the Helmholtz coils.

The cellular magnetic moment can be estimated from electron microscope images of the linear chain if we assume that these regions are shaped as hexagonal prisms and are composed of 80% magnetite (Fe₃O₄) (Towe & Moench, 1981; Blakemore, 1982; Matsuda et al. 1983). Magnetic moment = (volume of all particles in the linear chain) × (saturation magnetization per unit volume, 4⁻8xlO⁵Jm⁻³1⁻¹ for magnetite). However, this procedure can only be used when high resolution images of the interior of the cell are obtained and when the chain is linear.

Another method of measuring the total magnetic moment of a microorganism is based on analysis of the U-turn as suggested by Bean (see Appendix). It relies upon the response of the organisms to the reversal of the magnetic field. We have observed that in a constant magnetic field the magnetotactic microorganisms swim in an approximately helical trajectory along the field lines. The stronger the field, the tighter is the helical turn. This trajectory is approximately a straight line and when the field is suddenly reversed, the microorganisms are subjected to a torque which reverses the direction of movement, resulting in an approximate U-trajectory (Fig. 1).

According to the Bean model (see Appendix) the reversal time (τ) and the diameter (L) of the U-turn depend upon the total magnetic moment (m) of the organism and are given by:

where B_o is the magnetic field, R is the radius of the cell, ηis the viscosity (ηHzO ⁼ 10⁻²poise; 1 poise = 10⁻¹ Pa.s), k is the Boltzmann constant and T is the temperature.

Equations 1 and 2 are obtained assuming no influence of flagellar movement. Thus, the U-turn method is a general one for estimating the magnetic moment.

Information about magnetic moment was also obtained by using pure cultures (Rosenblatt, Torres de Araujo & Frankel, 1982 a,b).

Table 1 presents the principal results and some theoretical estimates of the relevant parameters from the analysis of the movement and magnetic properties of the magnetotactic microorganisms. These numerical values are obtained by averaging the results for many samples (more than 20) and for different individual microorganisms of the same type. With this procedure we hope that the estimated values will be representative of each population.

In measurements of the U-turn we used B_o = 930 μT, because this field is sufficiently high for the Bean model to be a good approximation for the movement (see Appendix).

We have measured L, v and R to obtain an estimate of m_u. Then we calculated, using equation 1, the reversal time, τ_u, and compared it with the measured value, τ_exp, in the same field. τ_u and τ_exp are in good agreement, showing that the m_u is a good estimate for the total magnetic moment.

Table 1 also shows that N, the number of particles, and m are increasing functions of R (or, what is more significant, the mean volume αR³). One important characteristic is that the ratio mB_o/kT depends upon the volume of the microorganism. For the smallest observed magnetotactic microorganisms this ratio is about 5, i.e. the average orientation is far from the saturation condition (see Appendix). However, the largest magnetotactic microorganisms have a ratio of mB_o/kT that is much greater than necessary to guarantee a total orientation in the geomagnetic field.

The results presented in this paper permit us to look at magnetotaxis in a new way.

On the other hand, the microorganism that uses the geomagnetic field as an orientation mechanism must have some means of responding efficiently to variations of this field, or to variations of its orientation in this field due to other environmental perturbations.

If we ignore flagellar motion, magnetotactic microorganisms behave like dipole particles with diameters in the range of 1 to a few microns. A particle with a diameter of about 1 μm is randomly disoriented by the medium (Brownian motion). Particles of more than a few microns are not so perturbed by statistical fluctuations of the medium (Purcell, 1977). In this range, other factors can be more important in disorienting these particles. Observations of natural samples analysed by us show very rich populations of different species of microorganisms which swim in different ways. These movements occur simultaneously and make possible random collision between microorganisms.

The movement of a specific microorganism can be perturbed by collisions with other organisms as well as by perturbations like current flow, statistical fluctuations, flagellar motion etc., and we can define a characteristic mean time between two successive perturbations for this microorganism. Hence, if there are other environmental perturbations that make the microorganisms deviate from their normal trajectory, the orientation in the field has to be performed in a time shorter than the mean time between two consecutive perturbations.

The above considerations show that to guarantee magnetic orientation it is necessary that the magnetic interaction energy be greater than the thermal disorder energy, i.e. mB_o/kT> 1. This condition is satisfied for magnetic moments of the order of m_m = 1 ·6 ×10⁻¹⁶ Am² in the local geomagnetic field (B_o = 25 μT). The reversal time (τ) increases with the cube of the radius of the microorganism for a fixed value of m (equation 1). For larger organisms, the rapid increase of rat constant m would make the response to magnetic stimuli inefficient. Curve A (Fig. 2) shows the values of r calculated for m_m and B_o = 25 μT. As shown in Fig. 2, we note that the reversal time (τ₀ in Table 1), calculated with the estimated magnetic moment, also increases with R³, but with a lower rate than the curve A. We think that the reversal time must have an upper limit to guarantee an efficient orientation.

These considerations lead us to suggest that there is an upper limit to the size of organisms for which magnetotaxis by passive orientation would be efficient.

Thus, magnetotaxis seems to be an orientation mechanism that is effective when (a) the magnetic interaction energy is much greater than the thermal energy and (b) the time interval necessary for the torque produced by the geomagnetic field to orientate the microorganism is much smaller than the mean time of perturbations that occur in their habitat.

When the field is suddenly reversed (or, which is completely equivalent, the bacterium rotates in relation to the field), θ₁= πand equation A2 diverges.

We are grateful to Marcos Farina for the electron microscopy. We are also grateful to Dr R. B. Frankel for discussions and critical reading of the manuscript.