FOURIER ANALYSIS OF ANGULAR DISTRIBUTIONS FOR MOTILE MICROORGANISMS

Orientation of the photosynthetic flagellate, Peridinium gatunense, in hypergravity

The photosynthetic freshwater flagellate, Peridinium gatunense, uses both positive phototaxis and negative gravitaxis to move upwards in the water column. At higher fluence rates approaching those at the surface of their habitat, the cells tend to become unoriented and thus stop their upward movement. Orientation and motility of Peridinium gatunense has been studied in the slow rotating centrifuge microscope (NIZEMI), which allows observation of swimming behavior during centrifugation acceleration between 1 g and 5g. The movement vectors were analyzed by real time image analysis capable of tracking many cells simultaneously. At 1 g the orientation was not very precise, but the degree of orientation increased significantly at higher acceleration forces up to about 3 g. Most cells were capable of swimming even against an acceleration vector of 3.8 g; at higher acceleration forces the cells were not able to cope with the centrifugal force. The linear velocity of cells swimming against 1 g was about 20% lower than that of cells moving in other directions. The velocity decreased even more in cells swimming against higher acceleration forces.

Effects of UV radiation on motility, photo-orientation and pigmentation in a freshwater Cryptomonas

The effects of UV radiation on photo-orientation, motility and pigmentation have been studied in a freshwater Cryptomonas species. The cells show a pronounced diaphototactic orientation which is affected by UV radiation at 50 mW m-2 within about 90 min. Both the average velocity of the swimming cells and the percentage of motile cells within the population decrease within about the same exposure time. UV radiation also bleaches the cellular pigments.

Polarotaxis, gravitaxis and vertical phototaxis in the green flagellate, Euglena gracilis

A fully automatic computer-controlled video analysis system has been used to study the movement of the green unicellular flagellate, Euglena gracilis in a horizontal or vertical cuvette. In darkness, in the absence of gaseous gradients, most cells swim straight upwards. While in a horizontal cuvette the transition between positive and negative phototaxis is found at about 1.5 W m-2, an excess of 30 W m-2 is required to reverse the upward swimming (due to the combined stimulus of negative gravitaxis and positive phototaxis) in a vertical cuvette. By studying the swimming direction in horizontal and vertical cuvettes in polarized light irradiated from above or from the side, respectively, the dichroic orientation of the photoreceptor molecules can be determined in three dimensions with respect to the axes of the cell; In a horizontal cuvette, in a linearly polarized beam from above, the cells orient predominantly at an angle of about 30 degrees clockwise off the electric dipole transition moment as seen from above. The behavior in a vertical cuvette with polarized light entering from above indicates that the photoreceptor pigments are dichroically oriented 60 degrees counterclockwise from the flagellar plane (seen from the front end of the cell). Experiments with horizontal polarized light indicate that the photoreceptor transition moment deviates 25 degrees clockwise off the long axis of the cell.