Phototactic orientation mechanism in the ciliate Fabrea salina, as inferred from numerical simulations

The marine ciliate Fabrea salina shows a clear positive phototaxis, but the mechanism by which a single cell is able to detect the direction of light and orient its swimming accordingly is still unknown. A simple model of pho totaxis is that of a biased random walk, where the bias due to light can af fect one or more of the parameters that characterize a random walk, i.e., t he mean speed, the frequency distribution of the angles of directional chan ges and the frequency of directional changes. Since experimental evidence h as shown no effect of light on the mean speed of Fabrea salina, we have exc luded models depending on this parameter. We have, therefore, investigated the phototactic orientation of Fabrea salina by computer simulation of two simple models, the first where light affects the frequency distribution of the angles of directional changes (model MI) and the second where the light bias modifies the frequency of directional changes (model M2). Simulated M 1 cells directly orient their swimming towards the direction of light, rega rdless of their current swimming orientation; simulated M2 cells, on the co ntrary, are unable to actively orient their motion, but remain locked along the light direction once they find it by chance. The simulations show that these two orientation models lead to different macroscopic behaviours of t he simulated cell populations. By comparing the results of the simulations with the experimental ones, we have found that the phototactic behaviour of real cells is mon similar to that of the M2 model. (C) 2000 Elsevier Scien ce S.A. All rights reserved.