Physiological characterization of gravitaxis in Euglena gracilis

The motile, unicellular freshwater flagellate Euglena gracilis uses external stimuli, like gravity, light or oxygen pressure in order to orient itself in its natural habitat. In the darkness the cells normally show a negative gravitactic behavior, that means they swim upward in the water column, Many ground and space experiment revealed that gravitaxis is most likely based on active physiological mechanisms (involvement of calcium, cAMP, membrane potential and other parameters).     

Inhibition of Motility in the Cyanobacterium, Phormidium uncinatum, by Solar and Monochromatic UV Irradiation

The effect of solar radiation and monochromatic UV radiationon the motility of the filamentous cyanobacterium Phormidiumuncinatum was determined. Solar radiation (mid-day, in midsummerat a location near Lisboa, Portugal) was found to impair motilitywithin about 30 min. This effect is neither a result of a temperatureincrease nor of visible light. The spectral sensitivity determinedusing the Okazaki Largé Spectrograph shows the maximaleffectiveness of radiation of 300 nm. The short time requirementfor the response and the lack of any photoreactivation of motilityargues against DNA being the UV target. Investigations usingreagents diagnostic of superoxide free radicals and singletoxygen failed to confirm the involvement of photodynamic effectsas the molecular mechanism causing UV inhibition of motility. ³Present address: Biology Department, Faculty of Science, Universityof Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received March 15, 1986; Accepted April 25, 1986)     

Phototactic Orientation in Plasmodia of the Acellular Slime Mold, Physarum polycephalum

Phototactic orientation to lateral light was studied in plasmodiaof the yellow-pigmented strain (CL) and a white mutant of theslime mold Physarum polycephalum. The orientation changes wereperformed by either a turn or a reversal of the internal polarityof the plasmodium. Fluence rate-response curves for white andmonochromatic light showed positive phototaxis with low fluencerates and negative phototaxis with higher ones. The wavelengthdependences for both strains were similar and indicated thepredominant role of a UV/blue light-absorbing photoreceptor,which was not the same as one of the yellow pigments found inthe wild-type strains. Wavelengths >500 nm induced only aninsignificant effect. The control of the motor apparatus andthe possible linkage of the photoresponse with the other sensoryprocesses known in Physarum are discussed. (Received June 3, 1983; Accepted October 11, 1983)     

High resolution scanning of absorbing and fluorescent electrophoresis gels using video image analysis

A low cost, microcomputer-controlled image analysis system isdescribed which scans electrophoresis gels or photographic negativeswith high resolution. Absorbing gels (e.g. with stained proteins)are analyzed using broadband or monochromatic visible light.The gel is scanned by a video camera with a macro objective;the gray level of each pixel is digitized sequentially and thevalues are stored in the computer memory. Repetitive scanningis used to average the absorption values before plotting ona digital plotter. Photographic negatives of gels and autoradiogramsare scanned using the same technique. Fluorescent, non-absorbinggels (e.g. nucleic acids stained with ethidium bromide) areanalyzed on a transilluminator with u. v. excitation radiation.The fluorescence intensity of each pixel is digitized and processedas described above. Received on June 4, 1987; accepted on July 21, 1987     

Effects of UV-B on motility and photoorientation in the cyanobacterium,Phormidium uncinatum

UV-B irradiation has a detrimental effect on the survival of populations of the filamentous cyanobacterium, Phormidium uncinatum, at levels slightly higher than those currently measured at the surface of the earth. The organisms are not damaged or killed by UV-B radiation at 300 nm of 200 Wm^-2 for up to 20 h; but slightly increased levels of UV-B irradiation (2 h of 200 Wm^-2 at 300 nm) drastically impair motility, phototactic orientation and photophobic responses. These photosynthetic organisms require a narrow light intensity range for growth so that any decrease in their ability to actively search for and move into areas of favorable light conditions is bound to affect the survival of a population. The fluorescence yield of both phycobilins and chlorophyll is not altered even after 20 h of UV-B irradiation (200 Wm^-2 at 270 nm) indicating that UV-B at that dose does not affect the photosynthetic apparatus. The organisms are killed either by too bright intensities which bleach the photosynthetic pigments or by the lack of energy when they are unable to avoid moving into dark areas.     

Ecological consequences of photomovement and photobleaching in the marine flagellate Cryptomonas maculata

Abstract The marine flagellate Cryptomonas maculata is bleached and eventually killed by exposure to even moderate white-light fluence rates. Bleaching affects all of its photosynthetic pigments and the kinetics depend on the fluence rate of the radiation the organisms are exposed to. Nitrogen-deficient cells which show a reduced pigment concentration and impaired photosynthetic efficiency tolerate bleaching white-light exposure far better than the normally colored cells. In their natural environment the organisms escape this situation by a pronounced negative phototaxis at fluence rates above 3.6 klx (= 15 W.m⁻²), while they show positive phototaxis at lower fluence rates. In nitrogen-deficient cells, however, though being less prone to photobleaching, negative phototaxis commences even at a fluence rate of about 830 lx (= 3.5 W.m⁻²). The ecological consequences of the remarkable light sensitivity and the phototactic orientation are being discussed.     

Motility and gravitactic orientation of the flagellate,Euglena gracilis,impaired by artificial and solar UV-B radiation

The effects of ultraviolet radiation on the gravitactic orientation of the freshwater flagellate,Euglena gracilis, were determined by a real time image analysis system. Both artificial UV radiation and solar radiation in a temperature-controlled growth chamber were employed. Histograms of gravitaxis showed that the degree of orientation decreased with increasing exposure time; this can be quantified using the Rayleigh test and upper quadrant summation. The effects of artificial UV radiation on the orientation are considerably stronger than those of solar radiation, probably because the radiation source emits higher fluence rates below 300 nm than found in solar radiation. The effects of monochromatic ultraviolet radiation on motility have been determined, and an action spectrum has been calculated.