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First published online February 20, 2004
Journal of Experimental Biology 207, 1203-1216 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00881

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Measurement of cell velocity distributions in populations of motile algae

V. A. Vladimirov¹, M. S. C. Wu², T. J. Pedley^3,*, P. V. Denissenko¹ and S. G. Zakhidova¹

¹ Department of Mathematics, University of Hull, Cottingham Road, Hull HU6 7RX, UK
² Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
³ Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK

^* Author for correspondence (e-mail: tjp3{at}damtp.cam.ac.uk)

Accepted 10 December 2003

The self-propulsion of unicellular algae in still ambient fluid is studied using a previously reported laser-based tracking method, supplemented by new tracking software. A few hundred swimming cells are observed simultaneously and the average parameters of the cells' motility are calculated. The time-dependent, two-dimensional distribution of swimming velocities is measured and the three-dimensional distribution is recovered by assuming horizontal isotropy. The mean and variance of the cell turning angle are quantified, to estimate the reorientation time and rotational diffusivity of the bottom-heavy cell. The cells' phototactic and photokinetic responses to the laser light are evaluated. The results are generally consistent both with earlier assumptions about the nature of cell swimming and quantitative measurements, appropriately adjusted. The laser-based tracking method, which makes it possible to average over a large number of motile objects, is shown to be a powerful tool for the study of microorganism motility.

Key words: motility, unicellular algae, Chlamydomonas nivalis, laser-based tracking, velocity distribution, swimming direction

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