Phototactic Sign Research Articles

Characterization of Chlamydomonas reinhardtii Mutants That Exhibit Strong Positive Phototaxis.

The most motile phototrophic organisms exhibit photo-induced behavioral responses (photobehavior) to inhabit better light conditions for photosynthesis. The unicellular green alga Chlamydomonas reinhardtii is an excellent model organism to study photobehavior. Several years ago, we found that C. reinhardtii cells reverse their phototactic signs (i.e., positive and negative phototaxis) depending on the amount of reactive oxygen species (ROS) accumulated in the cell. However, its molecular mechanism is unclear. In this study, we isolated seven mutants showing positive phototaxis, even after the induction of negative phototaxis (ap1~7: always positive) to understand the ROS-dependent regulatory mechanism for the phototactic sign. We found no common feature in the mutants regarding their growth, high-light tolerance, and photosynthetic phenotypes. Interestingly, five of them grew faster than the wild type. These data suggest that the ROS-dependent regulation of the phototactic sign is not a single pathway and is affected by various cellular factors. Additionally, the isolation and analyses of mutants with defects in phototactic-sign regulation may provide clues for their application to the efficient cultivation of algae.

Flagellar motility during sperm chemotaxis and phototaxis in fucalean algae

ABSTRACT The swimming of male gametes of brown algae is controlled by two heterokont flagella, regardless of reproductive system, but the patterns of flagellar movement can vary. We investigated sperm chemotaxis and phototaxis in four oogamous Australasian fucalean species. Sperm morphology was similar to that of male gametes of isogamous and anisogamous species but their chemotactic behaviours were much closer to those of other oogamous species than those of isogamous and anisogamous species. Moreover, unlike anisogamous species, sperm chemotaxis of the oogamous species, Hormosira banksii, was not inhibited by a phosphodiesterase inhibitor, theophylline, suggesting variability in regulatory mechanisms of sperm chemotaxis amongst brown algae. In our previous work, lower extracellular Ca2+, supernatant of female gametes and theophylline induced the reversal of phototactic signs in anisogamous male gametes. In contrast, lower extracellular Ca2+ and theophylline did not significantly affect phototaxis in sperm of the oogamous H. banksii. Moreover, high extracellular Ca2+ and supernatant of eggs disrupted the negative phototaxis but did not reverse their phototactic sign. These results suggest that the regulation of phototaxis also varies amongst brown algal gametes. Further investigation of brown algal species is warranted to assess the generality of these patterns of chemotaxis and phototaxis among anisogamous and oogamous gametes.

Channelrhodopsin-1 Phosphorylation Changes with Phototactic Behavior and Responds to Physiological Stimuli in Chlamydomonas.

The unicellular alga Chlamydomonas (Chlamydomonas reinhardtii) exhibits oriented movement responses (phototaxis) to light over more than three log units of intensity. Phototaxis thus depends on the cell's ability to adjust the sensitivity of its photoreceptors to ambient light conditions. In Chlamydomonas, the photoreceptors for phototaxis are the channelrhodopsins (ChR)1 and ChR2; these light-gated cation channels are located in the plasma membrane. Although ChRs are widely used in optogenetic studies, little is known about ChR signaling in algae. We characterized the in vivo phosphorylation of ChR1. Its reversible phosphorylation occurred within seconds as a graded response to changes in the light intensity and ionic composition of the medium and depended on an elevated cytosolic Ca2+ concentration. Changes in the phototactic sign were accompanied by alterations in the phosphorylation status of ChR1. Furthermore, compared with the wild type, a permanently negative phototactic mutant required higher light intensities to evoke ChR1 phosphorylation. C-terminal truncation of ChR1 disturbed its reversible phosphorylation, whereas it was normal in ChR2-knockout and eyespot-assembly mutants. The identification of phosphosites in regions important for ChR1 function points to their potential regulatory role(s). We propose that multiple ChR1 phosphorylation, regulated via a Ca2+-based feedback loop, is an important component in the adaptation of phototactic sensitivity in Chlamydomonas.

A brown algal sex pheromone reverses the sign of phototaxis by cAMP/Ca2+-dependent signaling in the male gametes of Mutimo cylindricus (Cutleriaceae)

Male gametes of the brown alga Mutimo cylindricus show positive phototaxis soon after spawning in seawater but gradually change the sign of phototaxis with time. This conversion appears to need the decrease of intracellular Ca2+ concentration. In this study, we revealed that the conversion of male gamete phototactic sign, positive to negative, was accelerated by mixing with female gametes. The supernatant after the centrifugation of female gamete suspension showed the same activity to change the phototactic sign, suggesting that a factor released from female gametes was responsible for the reaction. A known brown algal sex pheromone, ectocarpene, induced chemotaxis of male gametes of M. cylindricus. The addition of this compound induced the change of phototactic sign, clearly indicating that a sex pheromone is essential for the reversal. An inhibitor of phosphodiesterase, theophylline, inhibited the chemotaxis and phototactic sign reversion by a factor released from female gametes and ectocarpene. Measurements of cyclic nucleotides showed that the increase in intracellular concentration of cAMP, not cGMP, was parallel to the change of phototactic sign. The inhibition of phototactic sign by theophylline was not observed in low Ca2+ sea water. These results suggest that a signaling pathway mediated by cAMP and Ca2+ concentrations drives the interconversion between two important behaviors of male gametes, phototaxis and chemotaxis.

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.

Eyespot-dependent determination of the phototactic sign in Chlamydomonas reinhardtii

The biflagellate green alga Chlamydomonas reinhardtii exhibits both positive and negative phototaxis to inhabit areas with proper light conditions. It has been shown that treatment of cells with reactive oxygen species (ROS) reagents biases the phototactic sign to positive, whereas that with ROS scavengers biases it to negative. Taking advantage of this property, we isolated a mutant, lts1-211, which displays a reduction-oxidation (redox) dependent phototactic sign opposite to that of the wild type. This mutant has a single amino acid substitution in phytoene synthase, an enzyme that functions in the carotenoid-biosynthesis pathway. The eyespot contains large amounts of carotenoids and is crucial for phototaxis. Most lts1-211 cells have no detectable eyespot and reduced carotenoid levels. Interestingly, the reversed phototactic-sign phenotype of lts1-211 is shared by other eyespot-less mutants. In addition, we directly showed that the cell body acts as a convex lens. The lens effect of the cell body condenses the light coming from the rear onto the photoreceptor in the absence of carotenoid layers, which can account for the reversed-phototactic-sign phenotype of the mutants. These results suggest that light-shielding property of the eyespot is essential for determination of phototactic sign.

Redox regulation of phototactic migration in the green alga Chlamydomonas reinhardtii and its possible application

Most flagellated green algae swim toward or away from the light source, i.e., display positive or negative phototaxis. Usually, the sign of phototaxis, positive or negative, is variable depending on various factors, but the mechanism that controls it has been unclear. Using Chlamydomonas reinhardtii, an excellent model organism for studying phototaxis, we have recently shown that cellular redox poise plays a key role; cells show positive phototaxis when treated with reactive oxygen species (ROS), whereas they show negative phototaxis when treated with ROS quenching reagents. Here we discuss the possible mechanism of the redox regulation of phototactic sign, questions to be clarified in the future and its possible application.

2PS050 クラミドモナスにおける走光性符号のレドックス調節(日本生物物理学会第50回年会(2012年度))

2PS050 クラミドモナスにおける走光性符号のレドックス調節(日本生物物理学会第50回年会(2012年度))

第一白卵（ｗ‐１）のカイコ幼虫の明暗順応による走光性の符号転換

第一白卵（ｗ‐１）のカイコ幼虫の明暗順応による走光性の符号転換

Phototaxis of a sand-beach amphipod: Physiology and tidal rhythms

1. The response spectrum for phototaxis of the sand-beach amphipodSynchelidium sp. shows relatively uniform responsiveness from 460 to 600 nm (Fig. 1). 2. Animals on rising tides are more negatively phototactic and less sensitive to light than on falling tides (Fig. 2). 3. When suddenly stimulated with light on rising tides, they show an initial positive phototaxis, which quickly reverses to negative. Similar stimulation on falling tides evokes either continuous positive phototaxis or a positive response which reverses to negative after a much longer time than on rising tides. 4. Freshly collected animals have tidal rhythms in the general sign of phototaxis (more positive on falling tides) (Fig. 4) and the reversal in phototactic sign from positive to negative upon sudden stimulation with light (Fig. 3). These rhythms are endogenous and tidal, since they persist under constant conditions, and the timing is dependent upon natural tidal times. In addition, a circadian rhythm in sensitivity is suggested, in which they have a lower intensity threshold for phototaxis at night than during the day (Fig. 6). 5. It is proposed that these photoresponses are functionally significant during migration up the beach in the swash zone on rising tides and down the beach on falling tides.

POSITIVE AND HOST-INDUCED NEGATIVE PHOTOTAXIS OF THE SYMBIOTIC WATER MITEUNIONICOLA FORMOSA

1. The symbiotic mite Unionicola formosa is positively phototactic when free of any chemical influence of its molluscan host, Anodonta imbecilis, and negatively phototactic in the presence of host tissue homogenate.2. Light-adapted mites exhibit maximal positive and negative phototaxis at 500 nm, while dark-adapted animals display maximal positive phototaxis between 500 and 540 nm and greatest negative responsiveness between 460 and 500 nm.3. Dark-adaptation increases the sensitivity of U. formosa, resulting in a 1 to 2 log unit shift to lower intensities for maximal phototaxis and of the thresholds for both positive and negative responses.4. Unionicola formosa tested in host homogenate exhibits negative phototaxis at high intensities and postive phototaxis at low intensities.5. An apparent wavelength-dependent change in phototactic sign is more likely a function of differential sensitivity to intensity.6. Although presumably a means by which host-symbiont contact is enhanced, the functional significance ...

The effect of temperature on phototaxis and geotaxis by larvae of the crab Rhithropanopeus harrisi (Gould)

Investigations of the effect of sudden temperature change on the phototaxis of Stage I and IV zoeae upon stimulation from horizontal and vertical directions with 500-nm light indicate a temperature-induced geotactic response in larvae of the crab Rhithropanopeus harrisi (Gould). For the horizontal tests both zoea stages were reared at 20 °C. Stage I showed positive phototaxis at temperatures between 15 ° and 35 °C, while Stage IV responded over the range of 10–30 °C. For the vertical tests, larvae, reared at 25 °C, were stimulated with overhead lights. Stage I zoeae ascended at 15 °, 20 ° and 25 °C and descended at 5 °, 10 °, 30 ° and 35 °C. Stage IV zoeae ascended at 20 ° and 25 °C and descended at 5 °, 10 °, 15 °, 30 ° and 35 °C. Although the descent at high temperatures could result from a negative phototaxis, a reversal in phototactic sign at high temperatures was not found in the horizontal experiments and the same vertical movement pattern is observed in total darkness. Upon exposure to high temperatures near the water surface, larvae would descend by means of a positive geotaxis rather than a negative phototaxis. This response involves active swimming by Stage IV larvae and passive sinking by Stage I.