Effect of preillumination on photomotile responses of the marine ciliate Fabrea salina

Fabrea salina is a marine ciliate that shows photomotile responses such as positive phototaxis and a step-down photophobic reaction. We found that preilluminated F. salina cells show a phototactic response significantly greater than that of dark-adapted cells when exposed to the same phototactic light stimulus. In particular, positive phototaxis is strongly enhanced by preillumination. This enhancement effect depends on the preillumination light irradiance, on the total preillumination dose, and on the duration of the dark interval between preillumination and the phototaxis measurement. Our results show that the determining factor is the total preillumination dose given to the sample. The enhancement effect shows an asymptotic behavior over a certain range of energy values (10-200 W/m2). Further, the effect is transient; after 120 s in the dark, the cells lose any memory of the preillumination, independent of the preillumination energy received. These results are tentatively discussed in terms of light-driven membrane potential or membrane channel conductances.     

PHOTOMOVEMENT IN AN "EYELESS" MUTANT OF Chlamydomonas

The function of the stigma ("eyespot") in the green flagellate Chlamydomonas reinhardtii was investigated by comparing the photomovement of the wild-type alga with that of an "eyeless" mutant ( ey 627). Movements of individual cells in response to a blue-green stimulus light were recorded using a videomicroscope system and were analyzed using vectorial methods. Cells of the "eyeless" mutant were phototactic; at a high stimulus fluence rate, their swimming paths were directed away from the light source. Although the orientation of the mutant was not as strongly directional as that of the wild type, it was statistically significant. However, the swimming paths of the mutant cells were very erratic in the presence of the stimulus beam, undergoing frequent changes of direction. Despite the differences in their phototactic orientation, cells of mutant and wild type all showed a distinct step-up photophobic response at the onset of stimulation. These results are consistent with the hypothesis that the stigma plays an accessory role in phototaxis, either by shading the photoreceptor or by acting as a quarter-wave reflector.     

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 BEHAVIOR OF INDIVIDUAL CELLS OF CRYPTOMONAS SP. IN RESPONSE TO CONTINUOUS AND INTERMITTENT LIGHT STIMULI

Abstract— The phototactic response of cells of Cryptomonas sp. to stimulation with continuous or intermittent lateral light was determined by an individual cell method using photomicrography and videomicrography. The cells showed positive phototaxis under the conditions studied. The phototactic orientation of individual cells was induced most effectively by irradiation with light of 570 nm; blue light was less effective, and no orientation was found in red light. An intermittent stimulus regime with a long dark interval (250 ms) elicited a weaker phototactic orientation than did a regime with a short dark interval (63 ms) irrespective of the duration of light pulses (16, 250 and 1000 ms). The swimming rate was ca. 240 ums ^-1 and the rotation period ca. 450 ms in the dark, neither of which was greatly affected by stimulation with continuous or intermittent light. Neither step-up nor step-down photophobic responses were observed at the time of onset or removal of the light stimulus under the experimental conditions. The swimming direction of individual cells became gradually oriented toward the light source. Phototactic response was detectable within 4 s after the onset of light stimulation, reaching a saturation level after more than 30 s.     

Behaviors producing photodispersal in Stentor coeruleus

The ciliate Stentor coeruleus exhibits photodispersal, that is, these cells swim away from light sources and collect in dimly lighted areas. We imaged and reconstructed the tracks of 48 Stentor to determine which swimming behaviors produced their photodispersal. We observed that their photodispersal is not due to a change in their swimming speed but rather to a change in the frequency with which they reorient their swimming direction. Therefore, their photodispersal must be due to either (1) a gradual reorientation of the organism's swimming direction determined by the direction of the light beam (phototaxis) or (2) multiple randomly directed reorientations in swimming direction that occur less frequently when the cell is swimming away from the light source (biased random walk). Sixteen (19%) of the 83 observed forward swimming tracks lasting three or more seconds exhibited a gradual bending away from the light source consistent with a phototaxis. However, most tracks were interrupted repeatedly by abrupt reorientations resulting from ciliary reversals and "smooth turns" that caused cells to reorient through 5.4 times as many degrees as were needed to direct them away from the light source. When cells were swimming away from the light source, their probability of reorienting was reduced and photodispersal resulted.     

Tracking phototactic responses and modeling motility of Synechocystis sp. strain PCC6803

The unicellular cyanobacterium Synechocystis sp. Strain PCC 6,803 exhibits phototaxis by moving along a surface towards a light source. This process requires Type IV pili and a phytochrome-like photoreceptor coupled to a complex signal transduction pathway. Cells progress through different phases of interaction before the development of finger-like projections moving in the direction of the light that are characteristic of phototaxis. To probe the interaction between individual cells during the initial phase of phototaxis we tracked and analyzed a large number of cells. We observed that individual cells have limited motility, but when cells divide and/or aggregate to attain a certain minimal group size, enhanced motility and phototaxis is observed. At the later stages of motility, there is noticeable phototactic behavior which results in the appearance of the finger-like projections. Our results indicate that cells prefer to move over areas previously traversed by other cells and confine themselves to these areas and that cells alter local surface characteristics allowing for enhanced motility. Based on cell tracking data we present a preliminary random walk model showing the forces that might interact to create the typical phases of phototaxis and motility. In this model, we can simulate the formation of finger-like projections that are characteristic of phototaxis.     

Photomovement of the Gliding Cyanobacterium Synechocystis sp. PCC 6803

Abstract— Using a computerized videomicroscope motion analysis system, we investigated the photomovements of two Synechocystis sp. (PCC 6803 and ATCC 27184). Synechocystis sp. PCC 6803 displays a relatively slow gliding motion. The phototactic and photokinetic speeds of this cyanobacterium in liquid media were 5μm/min and 15.8 μm/min, respectively, at 3μmol/m²/s of stimulant white light. Synechocystis sp. PCC 6803 senses light direction rather than intensity for phototaxis. Synechocystis sp. ATCC 27184 showed a weak photokinesis but no phototaxis. Analysis of Synechocystis sp. ATCC 27184 suggests that the loss of phototaxis results from spontaneous mutation during several years of subculture. When directional irradiation was applied, the cell population of Synechocystis sp. PCC 6803 began to deviate from random movement and reached maximum orientation at 5 min after the onset of stimulant white light. Synechocystis sp. PCC 6803 showed high sensitivity to the stimulant white light of fluence rates as low as 0.002 |unol/m²/s. Neither 1,3-dichlorophenyldimethyl urea nor cyanide affected phototactic orientation, whereas cyanide inhibited gUding speed. This result suggests that the phototaxis of Synechocystis sp. PCC 6803 is independent of photosynthetic phosphorylation and that its gliding movement is primarily powered by oxidative phosphorylation. In the visible wavelength region, 560 nm, 660 nm and even 760 nm caused positive phototaxis. However, 360 nm light induced strikingly negative phototaxis. Therefore, at least two independent photoreceptors may exist to control phototaxis. The photoreceptor for positive phototaxis appears likely to be a phytochrome-like tetrapyrrole rather than chlorophyll a .     

AN ACTION SPECTRUM FOR PHOTOTAXIS BY PSEUDOPLASMODIA OF Dictyostelium discoideum

Abstract— The sensitivity of phototactic orientation of pseudoplasmodia (slugs) of the cellular slime mold Dictyosrelium discoideum has been measured for white light and monochromatic light using computer aided directional statistics. The zero threshold for white light was found at about 10^-5 Ix. An action spectrum for positive phototaxis has been calculated from fluence rate-response curves; it shows two major maxima at about 420 and 440 nm and secondary peaks at 560 and 610 nm. This action spectrum is significantly different from the one for phototactic orientation in Dictyostelium amoebae.     

CALCIUM CONTROL OF PHOTOTACTIC ORIENTATION IN Chlamydomonas reinhardtii: SIGN AND STRENGTH OF RESPONSE

Abstract— Chlamydomonas reinhardtii responds to a blue light stimulus by an oriented swimming (phototaxis) toward or away from the stimulus source. In this study it is established that the sign and strength of the phototactic response are a complex function of extracellular [Ca²⁺], stimulus fluence rate, time of analysis after onset of stimulation and light pretreatment. At very low extracellular [Ca²⁺] the response is weak and usually negative. At [Ca²⁺] close to the preconditioning level, phototactic response becomes stronger and positive. As [Ca²⁺] is raised further, the initial (2 s) response remains positive but the long term (20 s) becomes negative and very strong. At extremely high [Ca²⁺] the cells become immobile. This bimodal behavior suggests that two different mechanisms determine the direction of the turn. Data cannot be explained in terms of a simple model. The model which accounts for most of the details of the behavior is that of Kamiya and Witman (1984), which proposes that positive response is triggered by a transient increase in intracellular [Ca²⁺] and negative response by a decrease below unstimulated level of Ca²⁺, at least in the range of 10^-9-10^-6 M [Ca²⁺]. The strong negative orientation which follows an initial positive response above this level of [Ca²⁺], in these experiments, is best explained by an adaptation of the cells due to an increased (on average) intracellular [Ca²⁺].     

Flagellar Motions in Phototactic Steering in a Brown Algal Swarmer

Using infrared high-speed video microscopy, we observed light-triggered transitory flagellar motions in flagellate reproductive cells (swarmers) of a brown alga, Scytosiphon lomentaria, under primary helical swimming conditions before and during negative phototactic orientation to unilateral actinic light. The posterior flagellum, which is autofluorescent and thought to be light-sensing, was passively dragged in the dark and exhibited one to several rapid lateral beats during orientation changes for phototactic steering. Notably, a brief cessation of anterior flagellar beating was occasionally observed concomitantly with rapid beats of the posterior flagellum. This behavior caused a pause in helical body rotation, which may contribute to the accuracy of phototactic steering. Thus, coordinated regulation of the movement of the two flagella plays a crucial role in phototactic steering.     

NEGATIVE PHOTOTAXIS OF Dictyostelium discoideum PSEUDOPLASMODIA IN UV RADIATION

Abstract— Pseudoplasmodia of the cellular slime mold Dictyostelium discoideum show negative phototactic orientation in lateral ultraviolet radiation. The action spectrum has a peak in the UV-B band near 280 nm. The absorption spectrum shows a prominent peak in the same wavelength range. Thus, negative phototaxis can be easily explained by the assumption that the lens effect, by which D. discoideum slugs detect the light direction, is defeated by the high internal absorption in this wavelength range.     

PHOTOTACTIC RESPONSE OF CHLAMYDOMONAS TO FLASHES OF LIGHT –II. RESPONSE OF INDIVIDUAL CELLS

Abstract— –Video-microscope studies provide further evidence that Chlamydomonas can become oriented in response to a single short flash of light. Following a flash, 50% of the cells in a negatively phototactic population undergo a transient deflection in swimming path ('turn response'), 10% show a 'stop response', and 40% continue to swim straight ahead. The direction of turning is related to the direction of the stimulus; a majority of cells turn away from the flash source. Repetitive flashing at 60 per s elicits oriented swimming, indistinguishable from that observed with continuous light. Responses at the onset of repetitive flashing resemble single-flash responses, reinforcing the idea that response to a single flash corresponds to the initial stages of orientation to continuous light. A stop response sometimes occurs at the onset of orientation to repetitive flashing, but it is apparently not an essential component of orientation. The fact that only 60% of the cells turn or stop in response to a flash is consistent with the hypothesis that light direction is perceived by comparing light absorbed in one photoreceptive region at two instants in time (before and during the flash). The only cells to turn or to stop would be those in which the photoreceptor organelle is appropriately oriented at the instant of the flash.     

Stentor coeruleus SHOWS POSITIVE PHOTOKINESIS

Abstract— Stentor coeruleus responds to a sudden increase in light intensity with a step-up photophobic response (avoiding reaction), and to collimated light with negative phototaxis. The peaks of the action spectra for the photophobic response and for the phototaxis are in common, 610 nm.
5. coeruleus showed changes in its steady-state swimming velocity induced with varying intensities of light (photokinesis). The cells swam fast in light regions but slowly in dark ones (positive photokinesis); the mean velocity of swimming was about 0.6 mm/s at 100 lx but reached about 1.0 mm/s at 50000 lx. The peak of the action spectrum for this photokinesis was about 680 nm.
The organism is the first protozoan cell reported to show three types of photoresponse: photophobic response, phototaxis and photokinesis.     

PHOTOTACTIC RESPONSE OF CHLAMYDOMON AS TO FLASHES OF LIGHT-I. RESPONSE OF CELL POPULATIONS

Abstract— Chlamydomonas reinhardi responds phototactically to a single, very short flash of blue light (6-4 μs). Net oriented response of a cell population is monitored photometrically, using the "population system" of Feinleib and Curry (1967). A single high-intensity flash elicits a small, but definite net movement away from the stimulus source. Repetitive flashing at low frequency (between 8 and 60 flashes per min) and at the same intensity elicits a prolonged response in the same direction. Net phototactic response to single or repetitive flashes varies with stimulus intensity in the same way as does response to continuous light (Feinleib and Curry, 1971b); response is positive at low intensity and negative at high intensity. These data indicate that at least some cells become oriented in response to a short flash. The occurrence of such a response has implications for the mechanism of phototactic orientation. If almost all the cells responded, one would assume that Chlamydomonas perceives light direction instantaneously by detecting an absorption gradient within the cell. Unequivocal interpretation of the short-flash response requires examination of the behavior of individual cells.     

THE ACTION SPECTRUM OF DAPHNIA MAGNA (CRUSTACEA) PHOTOTAXIS IN A SIMULATED NATURAL ENVIRONMENT

Abstract— The action spectrum of phototaxis in Daphnia magna (Crustacea) was measured in a chamber which simulated a natural angular distribution of underwater light. A 17% step-down in irradiance was used to stimulate the phototactic response at all wavelengths and irradiances tested. Peaks in the spectral response curves depended on the fluence rate to which the zooplankton were acclimated. The wavelength of maximum response (Z_max) shifted from yellow-green at the highest acclimation fluence rate (5.1 × 10⁻² Wm⁻²) to blue-violet at moderate rates. At low acclimation fluence rates, the blue-violet maximum was retained and another maximum developed in the red. At the lowest fluence rate (1.6 × 10⁻⁵ Wm⁻²), the blue-violet and red maxima were lost and another maximum developed in the near ultraviolet. The action spectrum indicates the presence of three, and possibly four, photopigments with Z_max, at ∼405, 440, 570 and 690nm. The 440 and 690nm maxima may belong to the same photopigment; however, this was not tested. Changes in zooplankton swimming speed, caused either by large changes in irradiance or by mechanical stimuli, were accompanied by changes in the strength of the phototactic response to the −17% stimulus at any irradiance level for white and monochromatic light, and indicated the presence of a mechanism connecting swimming speed and photosensitivity.     

Calcium Control of the Sign of Phototaxis in Brown Algal Gametes of Mutimo cylindricus

Brown algal swarmers usually exhibit positive or negative phototaxis. Such behaviors influence the increasing or decreasing dispersal distance or colonization on the new substratum. We confirmed that the sign of phototaxis (negative or positive) in male gametes of Mutimo cylindricus was affected by extracellular Ca²⁺ influx through Ca²⁺ channels. Under the control condition (10^?2 m [Ca²⁺]), male gametes swimming with a helical rotation of their cell body mostly showed positive phototaxis. At 10^?3 m [Ca²⁺], more than half of the male gametes showed positive phototaxis, whereas the others showed negative phototaxis. From 10^?4–10^?5 m [Ca²⁺], the phototactic sign changed to negative. When these negative phototactic gametes were transferred back to the control condition, the phototactic sign reverted to positive. At 10^?6 m [Ca²⁺], some of male gametes showed negative phototaxis, but most showed no phototaxis or flagellar beating. Lanthanum, a Ca²⁺ channel blocker, affected the sign of phototaxis at 10^?4 m [La³⁺] under 10^?2 m [Ca²⁺], and male gametes mostly showed negative phototaxis. A further increase in [La³⁺] inhibited phototaxis and flagellar beating. These results pointed out the involvement of Ca²⁺ channels that were blocked by La³⁺ in phototaxis and flagellar beating.     

BEHAVIORAL RESPONSES IN Paramecium multimicronucleatum TO VISIBLE LIGHT

Specimens of colorless Paramecium multimicronucleatum were found to respond to visible light. They accumulated in the shaded region (photodispersal) of a half-shaded glass tube during 2 min exposure to visible light. The specimens showed avoiding reaction upon both spatial and temporal increase in light intensity (step-up photophobic response). Steady-state swimming velocity (orthokinesis) was higher, while steady-state frequency of spontaneous change in swimming direction (klinokinesis) was lower when the light intensity was kept higher. In a light with wavelength of 440 nm the velocity was highest, while the frequency was lowest. The specimens did not show phototaxis (light direction-oriented locomotion). Spectral sensitivity curves for both the photodispersal and the step-up photophobic response showed a major peak at 520 nm and a minor peak at 680 nm. The photodispersal seems to be caused mainly by the step-up photophobic response exhibited by the specimens at the dark-light border. The photokinetic responses enhance the degree of the photodispersal.     

Mechanism and computer simulation of the phototactic accumulation of Euglena in a beam of light

Abstract— A mechanistic model is proposed which describes the phototactic behavior of Euglena during accumulation in an illuminated region. Measurements of the lag time occurring between illumination of the culture and net accumulation in the lighted zone as a function of culture density indicate that the relative strengths of the negative phototactic response inside and of the positive phototactic response outside this region are the prime factors controlling the lag phenomenon. Further evidence for this is provided by studies of the temperature dependence of the phototactic responses to polarized actinic light. It is shown that negative phototaxis as measured in the 7lsquo;phototaxigraph’ is not directed, but rather a shock-mediated response. A ‘FOCAL’ computer program for simulation of experiments in the phototaxigraph has been written on the basis of our model. It correctly predicts the observed results under a variety of simulated experimental conditions. Measurements of the lag time and of the rate of accumulation in different parts of the actinic zone allow the calculation of motilities of the organisms with illumination and in the dark, the latter value being 0.08 mm/sec. For a 2–3-weekold culture, the rate of negative phototaxis remained constant at light intensities above 40 ergs/cm²sec at 500 nm. At this wavelength, the threshold for the positive photophobic response was 100 ergs/cm² sec.     

PHOTOTAXIS IN DICTYOSTELIUM DISCOIDEUM AMOEBAE

Abstract— An apparatus has been developed to measure phototactic movement in a population of amoebae of Dictyostelium discoideum. Fluence–response curves in white light show a positive phototaxis to light below 100mW/m². Higher intensities cause a negative phototaxis. An action spectrum, based on the zero-crossing points in fluence–response curves for monochromatic light, shows a major peak at about 405nm and secondary maxima at about 450, 520, 580 and 640nm. This action spectrum resembles the action spectra for accumulations of amoebae in and dispersal from light traps and that of inhibition of aggregation by light, but is distinctly different from the action spectrum for phototaxis by D. discoideum pseudoplasmodia.     

CAFFEINE-ENHANCED PHOTOMOVEMENT IN THE CILIATE, STENTOR COERULEUS

The effects of caffeine, ionophores and calcium flux blockers on the step-up photophobic response, phototactic orientation and the intracellularly recorded, light-induced electrical action potential were studied in the ciliate, Stentor coeruleus . Caffeine alters the absorption and CD spectra and enhances the fluorescence of the photoreceptor pigment, stentorin. Independent of its effects on the spectroscopic properties of the photoreceptor pigment, caffeine shortens the photophobic response time by enhancing the Ca²⁺ conductivity of membranes, while Ca²⁺ flux blockers (LaCI₃ or ruthenium red) prolong it; both effects cancel each other. Evidence is presented that phototactic orientation is brought about by repetitive photophobic responses, since a change in the phobic response time results in a decreased accuracy of phototaxis.