New trends in photobiology

Phototaxis and chemotaxis of brown algal swarmers

Brown algae exhibit three patterns of sexual reproduction: isogamy, anisogamy, and oogamy. Unicellular swarmers including gametes and zoospores bear two heterogenous flagella, an anterior flagellum with mastigonemes (fine tripartite hairs) and a posterior one. In seawater, these flagellates usually receive physico-chemical signals for finding partners and good habitats. It is well known that brown algal swarmers change their swimming direction depending on blue light (phototaxis), and male gametes do so, based on the sex pheromones from female gametes (chemotaxis). In recent years, the comparative analysis of chemotaxis in isogamy, anisogamy, and oogamy has been conducted. In this paper, we focused on the phototaxis and chemotaxis of brown algal gametes comparing the current knowledge with our recent studies.

EFFECTS OF ULTRAVIOLET-A RADIATION AND NUTRIENT AVAILABILITY ON THE CELLULAR COMPOSITION OF PHOTOPROTECTIVE COMPOUNDS IN GLENODINIUM FOLIACEUM (DINOPHYCEAE)(1)

The photoprotective response in the dinoflagellate Glenodinium foliaceum F. Stein exposed to ultraviolet-A (UVA) radiation (320-400 nm; 1.7 W · m(2) ) and the effect of nitrate and phosphate availability on that response have been studied. Parameters measured over a 14 d growth period in control (PAR) and experimental (PAR + UVA) cultures included cellular mycosporine-like amino acids (MAAs), chls, carotenoids, and culture growth rates. Although there were no significant effects of UVA on growth rate, there was significant induction of MAA compounds (28 ± 2 pg · cell(-1) ) and a reduction in chl a (9.6 ± 0.1 pg · cell(-1) ) and fucoxanthin (4.4 ± 0.1 pg · cell(-1) ) compared to the control cultures (3 ± 1 pg · cell(-1) , 13.3 ± 3.2 pg · cell(-1) , and 7.4 ± 0.3 pg · cell(-1) , respectively). In a second investigation, MAA concentrations in UVA-exposed cultures were lower when nitrate was limited (P < 0.05) but were higher when phosphate was limiting. Nitrate limitation led to significant decreases (P < 0.05) in cellular concentration of chls (chl c1 , chl c2 , and chl a), but other pigments were not affected. Phosphate availability had no effect on final pigment concentrations. Results suggest that nutrient availability significantly affects cellular accumulation of photoprotective compounds in G. foliaceum exposed to UVA.

Wavelength dependence of mycosporine-like amino acid synthesis in Gyrodinium dorsum

The synthesis or accumulation of mycosporine-like amino acids (MAAs) is an important UV tolerance mechanism in aquatic organisms. To investigate the wavelength dependence of MAA synthesis in the marine dinoflagellate Gyrodinium dorsum, the organism was exposed to polychromatic radiation (PAR and UV) from a solar simulator for up to 72 h. Different irradiance spectra were produced by inserting various cut-off filters between lamp and samples. A polychromatic action spectrum for the synthesis of MAA synthesis was constructed. PAR and long wavelength UV-A radiation showed almost no effect while the most effective wavelength range was around 310 nm. Shorter wavelengths where less effective in the induction of MAA synthesis. Wavelengths below 300 nm damaged the organisms severely as indicated by a decrease in chlorophyll a absorption.

Life under solar UV radiation in aquatic organisms

Aquatic photosynthetic organisms are exposed to solar ultraviolet (UV) radiation while they harvest longer wavelength radiation for energetic reasons. Solar UV-B radiation (280-315 nm) affects motility and orientation in motile organisms and impairs photosynthesis in cyanobacteria, phytoplankton and macroalgae as measured by monitoring oxygen production or pulse amplitude modulated fluorescence analysis. Upon moderate UV stress most organisms respond by photoinhibition which is an active downregulation of the photosynthetic electron transport in photosystem II by degradation of UV-damaged D1 protein. Photoinhibition is readily reversible during recovery in shaded conditions. Excessive UV stress causes photodamage which is not easily reversible. Another major target is the DNA where UV-B mainly induces thymine dimers. Cyanobacteria, phytoplankton and macroalgae produce scytonemin, mycosporine-like amino acids and other UV-absorbing substances to protect themselves from short wavelength solar radiation.

Effects of light on gravitaxis and velocity in Chlamydomonas reinhardtii

The effects of light on gravitaxis and velocity in the bi-flagellated green alga Chlamydomonas reinhardtii were investigated using a real time automatic tracking system. Three distinct light effects on gravitaxis and velocity with parallel kinetics were found. Photosynthetically active continuous red light reversibly enhances the swimming velocity and increases or decreases the precision of gravitaxis, depending on its initial level. Blue light flashes induce fast transient increases in velocity immediately after the photophobic response, and transiently decrease or even reverse negative gravitaxis. The calcium dependence of this response, its fluence-response curve and its spectral characteristics strongly suggest the participation of chlamy-rhodopsin in this effect. The third response, a prolonged activation of velocity and gravitaxis, is also induced by blue light flashes, which can be observed even in calcium-free medium.

Mycosporine-like amino acids in the marine dinoflagellate Gyrodinium dorsum: induction by ultraviolet irradiation

Cultures of the marine dinoflagellate Gyrodinium dorsum have been exposed to polychromatic radiation (photosynthetically active radiation and UV) from a solar simulator for up to 72 h. Different irradiance spectra in the ultraviolet are produced by inserting cut-off filters between lamp and samples. The mycosporine-like amino acid (MAA) content and composition are investigated by spectroscopic and chromatographic analysis. The study reveals that G. dorsum contains a complex mixture of several aminocyclohexenimine-MAAs and one aminocyclohexenone-MAA. UV irradiation around 320 nm induces an increase in the concentration of all MAAs in the samples. In contrast, exposure to short-wavelength UV-B radiation results in decreased overall MAA production. Furthermore, there is a spectral shift in the absorption of the MAA mixture towards shorter wavelengths, indicating that short-wavelength UV-B induces an altered MAA composition. The amount of MAAs is normalized to the chlorophyll a concentration.

Negative gravitactic behavior of Euglena gracilis can not be described by the mechanism of buoyancy-oriented upward swimming

Gravitactic behavior of microorganisms has been known for more than a hundred years. Euglena gracilis serves as a model system for gravity-triggered behavioral responses. Two basic mechanisms are discussed for gravitaxis: one is based on a physical mechanism where an asymmetric mass distribution pulls the cell passively in the correct orientation and, in contrast, the involvement of an active sensory system. A recently developed high-resolution motion-tracking system allows the analysis of single tracks during reorientation. The results are compared to a model developed by Fukui and Asai (1985) which describes gravitaxis of Paramecium caudatum on the basis of a physical mechanism. Taking into account the different size, different density, different mass distribution as well as the different velocity, results of the adapted model description of Paramecium were applied to measured data of Euglena. General shapes as well as the time scale of the predicted reorientational movement compared to measurements were different. The analysis clearly rules out the possibility that gravitaxis of Euglena gracilis is based on a pure physical phenomenon, and gives further support to the involvement of an active reorientational system. In addition, it could be shown that cell form changes during reorientation, even in an initial period where no angular change was observed.

Gravitaxis in unicellular microorganisms

Orientation of organisms with respect to the gravitational field of the Earth has been studied for more than 100 years in a number of unicellular microorganisms including flagellates and ciliates. Several hypotheses have been developed how the weak stimulus is perceived. Intracellular statoliths have been found to be involved in gravitaxis of Loxodes, while no specialized organelles have been detected in other ciliates, e.g. Paramecium. Also in the slime mold Physarum no specialized gravireceptors have been identified yet. In the flagellate Euglena gracilis the whole cell body, which is denser than the surrounding medium, seems to act as a statolith pressing onto the lower membrane where it activates mechanosensitive ion channels. Similar results were obtained for the ciliate Paramecium. In contrast to the flagellate Euglena, several ciliates have been found to show gravikinesis, which is defined as a dependence of the swimming velocity on the direction of movement in the gravity field.

Graviperception in the flagellate Euglena gracilis during a shuttle space flight

During a recent space flight, gravitaxis of the unicellular photosynthetic flagellate, Euglena gracilis, was studied on board of the American shuttle Columbia. Accelerations were varied between 0 and 1.5 x g using a slow rotating centrifuge microscope (NIZEMI). The cells showed a sigmoidal response curve for the dependence of the precision of gravitaxis on acceleration which is indicative of the involvement of an active, physiological gravireceptor with a threshold at g-values < or = 0.16 x g and a saturation at g-values > or = 1 x g. No adaptation to microgravity was found during the prolonged space mission. After return the cells showed a normal gravitactic behavior at 1 x g. Since the cells are heavier than water, their swimming velocity is affected by sedimentation. The velocity distribution at different accelerations closely follows Stokes' law for sedimentation indicating that, in contrast to the ciliate Paramecium, E. gracilis, does not show any gravikinesis.

In-situ measurement of photosynthetic oxygen production in the water column

A novel hardware device is described to determine photosynthetic and respiratory oxygen uptake and release, respectively, in organisms in their natural habitat even under the rough conditions of their marine environment. Both macroalgae and phytoplankton can be utilized and oxygen exchange can be determined in solar radiation. The chamber can be used at or above the water surface or can be lowered into the water column. The data of oxygen concentration, irradiance and temperature are constantly monitored by a laptop computer and stored in disk files. The experimental data measured in some macroalgae as well as in phytoplankton indicate that the irradiance window for positive net photosynthetic oxygen production is fairly limited under natural conditions; at too low irradiances respiration exceeds photosynthesis and at too high irradiances photosynthesis is shut down by photoinhibition, at least in species not adapted to unattenuated solar radiation.

Analysis of photoreceptor proteins of microorganisms by gradient gel electrophoresis and other biochemical separation methods

Photoreceptor proteins for photoorientation in microorganisms are usually membrane bound and can be isolated by standard biochemical methods. Three examples are shown: the flagellates Euglena gracilis, Peridinum gatunense and the slime mold Dictyostelium discoideum. The photoreceptor of Euglena is attached to the basis of the flagellum and is composed of at least four chromoproteins which can be separated by gradient sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE) fast protein liquid chromatography (FPLC) and isoelectric focusing (IEF); it contains pterins and a flavin as chromophoric groups. The photoreceptor of Peridinium absorbs in the red wavelength band. Though not yet identified in detail, multiple receptors are probably involved, as indicated by fluorescence spectroscopy. Dictyostelium shows positive and negative phototaxis in its amoebal form and exclusively positive phototaxis in its pseudoplasmodial form. It is still open to discussion whether the two stages use separate photoreceptors. From amoebae two photoreceptor pigments have been isolated, showing an absorption which resembles the action spectrum, one membrane bound with a molecular mass of 45 kDa and one cytoplasmic fraction with a molecular mass of 27 kDa.

Effects of heavy metals on motility and gravitactic orientation of the flagellate, Euglena gracilis

The effects of copper, mercury, cadmium and lead on the gravitactic orientation of the photosynthetic flagellate Euglena gracilis were investigated. The first two heavy metals reverse the direction of downward swimming (positive gravitaxis) in young cultures (up to 8 days) to an upward swimming (negative gravitaxis); cadmium produced a less pronounced effect. Higher concentrations of heavy metals decrease the precision of orientation as compared to the control due to frequent deviations of the cells from straight paths. Higher concentrations also decrease the swimming velocity of the populations. When the cells were growing in the presence of the heavy metal, copper was effective at > or = 50 microM, cadmium at > or = 3 microM and mercury at > or = 1 microM. Since lead formed insoluble precipitations with the acetate in the growth medium it was tested after the cells were transferred into Tris buffer. Under these conditions lead did not affect the direction of movement or the precision of orientation up to a concentration of 300 microM in the time up to 24 h after the addition of the heavy metal. However, high concentrations of lead strongly decreased the swimming speed of the cells, which was partially reversed with time.

Inhibition of phototaxis in Volvox aureus by natural and simulated solar ultraviolet light

Exposure to artificial UV wavelengths and the UV component of sunlight delays positive phototaxis in the green alga Volvox aureus. Broad band wavelength filters were used to modify the output from UV-B sources (280-320 nm) and natural sunlight. The delay in phototaxis by artificial UV is increased with exposure to shorter UV-B wavelengths. Natural sunlight experiments were performed with exposure to full sunlight and to its UV component only. The UV component present in summer sunlight produced long periods of inhibition in phototaxis and even lethality, while exposure to the total spectrum of sunlight had no significant effects on movement or survival. The data indicate that although this species of alga is well equipped to deal with present levels of UV exposure, increases in the short UV-B wavelengths in sunlight may force an alteration in patterns of photomovement.

Action spectra again?

Action spectroscopy has a long history and is of central importance to photobiological studies. Action spectra were among the first assays to point to chlorophyll as the molecule most responsible for plant growth and to DNA as the genetic material. It is useful to construct action spectra early in the investigation of new areas of photobiological research in an attempt to determine the wavelength limits of the radiation region causing the studied response. But due to the severe absorption of ultraviolet (UV) radiation by biological samples, UV action spectra were first limited to small cells (bacteria and fungi). Advances in techniques (e.g. single cell culture) and analysis allowed accurate action spectra to be reported even for mammalian cells. But precise analytical action spectra are often difficult to obtain when large, pigmented, or groups of cells are investigated. Here some action spectra are limited in interpretation and merely supply a wavelength vs effect curve. When polychromatic sources are employed, the interpretation of action spectra is even more complex and formidable. But such polychromatic action spectra can be more directly related to ambient responses. Since precise action spectra usually require the completion of a relatively large number of careful experiments using somewhat sophisticated equipment over a range of at least six wavelengths, they are often not pursued. But they remain central to the elucidation of the effect being studied. The worldwide community has agreed that stratospheric ozone is depleting, with the possibility of a consequent rise in the amount of UV-B (290-320 nm) reaching the earth's surface. It is therefore essential that new action spectra be completed for UV-B effects on a large variety of responses of human, animal, and aquatic plant systems. Combining these action spectra with the known amounts of UV-B reaching the biosphere can give rise to solar UV effectiveness spectra that, in turn, can give rise to estimates of effect. Preliminary estimates suggest that ozone layer depletion may seriously impact such important biological end-points as skin cancer, cataracts, the immune system, crop yields, and oceanic phytoplankton. So action spectra continue to play a central role in important photobiological research.

Interactive image analysis system to determine the motility and velocity of cyanobacterial filaments

An interactive image analysis system has been developed to analyse and quantify the percentage of motile filaments and the individual linear velocities of organisms. The technique is based on the "difference" image between two digitized images taken from a time-lapse video recording 80 s apart which is overlaid on the first image. The bright lines in the difference image represent the paths along which the filaments have moved and are measured using a crosshair cursor controlled by the mouse. Even short exposure to solar ultraviolet radiation strongly impairs the motility of the gliding cyanobacterium Phormidium uncinatum, while its velocity is not likewise affected. These effects are not due to either type I (free radical formation) or type II (singlet oxygen production) photodynamic reactions, since specific quenchers and scavengers, indicative of these reactions, failed to be effective.

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.

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.

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.

Photoreceptor pigments for photomovement of microorganisms: some spectroscopic and related studies

Optical spectroscopy of photoreceptor pigments can substantially contribute to our understanding of the molecular processes which are the basis of photoreception and sensory transduction in photomotile microorganisms. The main spectroscopic techniques are briefly illustrated, together with the most significant types of progress that can be achieved. A few "case examples" are discussed in some detail: Halobacterium, with particular attention to the contribution of flash photolysis studies to the identification and characterization of sensory rhodopsins; Euglena, and the role of in vivo microspectrofluorometry in confirming the flavin nature of its photoreceptor pigment; the first suggestions on the rhodopsin-like nature of the Chlamydomonas photosensing system; Stentor and Blepharisma and the contribution of static and time-resolved fluorescence studies to a molecular model of the primary events in their photoreceptor pigments (stentorin and blepharismin) and systems.