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.

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The effects of ultraviolet-B irrad~ation on the cyanobacteria Anabaena vanabilis, Osclllat o m tenujs. Nodulana baltica, h ' harveyana and Phormid~um unc~natum (Baikal a n d Tubingen s t r a~n )Indicated that pigmentation and energy transfel to the photosynthetic reaction centers were impaired Absorption and fluorescence spectra showed photobleaching of photosynthetic pigments especially in the accessory antenna plgments wlth prolonged irradiat~on Fluorescence emission and excltation spectra indicated an initial d~sturbance of energy transfer and subsequent photobleachlng of these pigments Exposure to solar and artificial UV-B irradiation lesulted In a very rapid lnhibltion of photosynthetic oxygen exchange as compared to photobleaching, including structural changes In the photosynthetic apparatus

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