Ref ID : 4031
Barry Bean; Geotactic Behavior of Chlamydomonas. J.Protozool. 24(3):394-401, 1977
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The normal negative geotaxis (motility oriented against gravity) of Chlamydomonas is an energy-dependent response that requires coordinated flagellar activity. It is evident from quantitative assays that the rate of geotaxis is steady, and slow relative to the average swimming speed. Geotaxis is inhibited when the horizontal swimming path is less than 200 µm, suggesting that normal geotactic reorientation maneuvers involve long gradual turns. Videomicrographic tracking of cells confirms that such turns are common. In contrast, contact-reorientations generate random cell orientations. When collision frequencies increase, geotaxis in inhibited. The mechanism of normal geotactis orientation, then, depends on long slow reorientation maneuvers (from net downward to net upward vectors) that require hundreds of micrometers of free swimming space. Mechanisms of geotaxis that would require passive reorientation or sedimentation, or rapid active responding, are excluded. Unusually dense populations sediment with atypical rapidly, probably due to formation of functionally aggregated subpopulations. Sodium azide causes an inhibition of orientation behavior that is selective relative to its effects on general motility. Evidence presented suggests that active physiologic mechanisms for geotaxis should be reconsidered.