SUMMARY: Zone formation and movement of cells from mixed, natural colonies of magnetococci were studied. We concluded that (i) zoning resulted from hydrodynamic interaction between cells swimming in parallel as a result of magnetotaxis and (ii) magnetic interaction between cells had a role in determining the form and behaviour of zones. Our evidence is as follows. The formation of zones normal to the direction of movement during magnetotaxis requires that a component of the magnetic field be directed into a surface, resulting in cells being concentrated at the surface, facilitating cell interaction. The orientation of the surface with respect to gravity is irrelevant, excluding gravitation as a critical factor. Quasi-stable lateral associations between pairs of cells with similar speeds were observed, confirming a prediction that hydrodynamic interaction between cells could bring about such associations, and have a major role in the production of more extensive lateral associations including zones. Cells travelling much faster than a zone were observed to pass through it, although with some delay, suggesting that cells with speeds near that of the zone would be captured. In populations of slowly moving magnetococci, head-to-tail associations between cells were observed, indicating that at low swimming speeds magnetic instead of hydrodynamic interaction was dominant. When the direction of the magnetic field was reversed, the direction of zone movement was also reversed, and zones became narrower and more sharply defined for a few seconds. This was interpreted as a consequence of a magnetic effect of the mass of cells in a zone on cells at the zone edge. We also observed instability and gravitational fall of a dense layer of cells overlying cell-free fluid (Rayleigh-Taylor instability). This is well known with eukaryotic micro-organisms but has not been previously reported for bacteria.


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