Phototaxis in the ciliated protozoan Ophryoglena flava: dose-effect curves and action spectrum determination.

The sensitivity of positive phototactic orientation of cells of the ciliated protozoan Ophryoglena flava has been measured for white light, broad-band blue and red light, and narrow-band monochromatic light, using a laboratory-developed computer aided system. The white-light fluence rate-response curve shows that there is no negative phototaxis in the fluence rate range investigated (0-15 W/m2) and no adaptation phenomena; it is very well fitted by a hyperbolic function; the fluence rate curves under broad band blue and red light (full width at half maximum, FWHM= 100 nm) can be fitted by the same model. The saturation level is, within experimental errors, the same for the three curves, indicating that there are no chromaticity effects and that if there is more than one photoreceptor pigment, they act independently of each other. The fluence rate-response curves determined under narrow band monochromatic light (FWHM = 10 nm) can also be fitted by the same model and show, within experimental errors, the same saturation level. An action spectrum for positive phototaxis at 10-nm intervals has been calculated from fluence rate-response curves: it shows three maxima, at 420, 540 and 590 nm. This action spectrum is significantly different from the ones for photomotile responses in Blepharisma japonicum, Stentor coeruleus and Chlamydodon mnemosyne, whereas it resembles the ones of Paramecium bursaria and Fabrea salina.

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 phototaxis is that of a biased random walk, where the bias due to light can affect one or more of the parameters that characterize a random walk, i.e., the mean speed, the frequency distribution of the angles of directional changes and the frequency of directional changes. Since experimental evidence has shown no effect of light on the mean speed of Fabrea salina, we have excluded 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 M1) and the second where the light bias modifies the frequency of directional changes (model M2). Simulated M1 cells directly orient their swimming towards the direction of light, regardless of their current swimming orientation; simulated M2 cells, on the contrary, 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 the 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 more similar to that of the M2 model.

ELDONET--European Light Dosimeter Network. Structure and functions of the ELDONET server.

The European Light Dosimeter Network (ELDONET) project has been designed with the purpose of establishing an efficient system to monitor solar radiation in Europe, in as many as possible locations. This paper describes the structure of the server that collects and processes the data acquired by the different stations belonging to the network, and makes them freely available on the Internet to the scientific community. The server is able to receive data either via FTP from the Internet or via modem and to process them looking for errors or inconsistencies. Moreover, it automatically generates graphs, Web-pages and FTP archives. The server has been active for some years in testing mode and is now fully operative.

Doppler velocimetry measurements of phototactic response in flagellated algae.

A periodic shading mechanism (about 0.5 s period) is believed to be used by flagellated algae to detect light direction. Time changes in the orientation of a population of Haematococcus pluvialis under positive photoaxis conditions (cell samples were stimulated alternatively by two diametrically opposed beams of actinic light), have been investigated by analysing the Doppler shifts of laser light scattered by the cells by means of heterodyne detection techniques. This technique allows us to measure the mean value of the component of the cells' swimming velocities along the light stimulus axis. Preliminary data indicate that the time taken by a cell population to change orientation is about 1 s.

Effect of pronase treatment on step-down and step-up photophobic responses in Euglena gracilis.

Pronase-treated cells of Euglena gracilis Z show no discernible ultrastructural effects on the photoreceptor apparatus; however, there are physiological effects on swimming speed and on step-up and step-down photophobic responses, especially the latter. Pronase acts differently on the two photophobic responses: the step-down response is completely inhibited after 2 hr., whereas inhibition of the step-up response occurs in only 50% of the cells even after 24 hr. The effects are fully reversible, with step-up recovery quite rapid and step-down recovery considerably slower.