Michael Lebert

Ground-based facilities for simulation of microgravity: organism-specific recommendations for their use, and recommended terminology.

Research in microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature.

REAL TIME COMPUTER-CONTROLLED TRACKING OF MOTILE MICROORGANISMS

Abstract— A hardware and software system is described which is capable of automatically following the movement of microorganisms and analyzing the deviation of individuals in a population from a predefined direction. The image of the moving organisms is viewed by a video camera, digitized in real time and stored in a dedicated video memory holding 512 times 256 pixels with 64 grey levels. A microcomputer has access to this video memory via four parallel ports. The software analyzes the position of an organism selected randomly and attempts to follow its track for a predefined period of time. If successful, it calculates the direction of movement and stores the data in a file for statistical analysis.

ELDONET – European Light Dosimeter Network. Hardware and Software

Abstract A three-channel dosimeter has been developed to measure solar radiation in the UV-B (280–315 nm), UV-A (315–400 nm) and PAR (400–700 nm) wavelength bands. A total of 31 instruments have been installed in Europe from Abisko in Northern Sweden to Gran Canaria, covering most light-climate zones. In addition, instruments are installed in India, Africa, New Zealand and South America. Seven of the instruments have been installed under water (each in conjunction with a terrestrial instrument), and two instruments are located in high mountain locations (Zugspitze, Germany and Sierra Nevada, Spain). The instruments use an integrating Ulbricht sphere and silicon photodiodes in combination with custom-made filters. All instruments are carefully calibrated to ensure a high standard of quality control and documentation. The software records all data (three light channels, external and internal temperature and depth for the underwater instruments) at 1 min intervals, displays them on a PC, stores the data on disk and transmits the data to the central server in Pisa (http://power.ib.pi.cnr.it:80/eldonet), where all available data can be seen in graphical form and downloaded as ASCII files.

Photoactivated Adenylyl Cyclase Controls Phototaxis in the Flagellate Euglena gracilis

Euglena gracilis, a unicellular freshwater protist exhibits different photomovement responses, such as phototaxis (oriented movement toward or away from the light source) and photophobic (abrupt turn in response to a rapid increase [step-up] or decrease [step-down] in the light fluence rate) responses. Photoactivated adenylyl cyclase (PAC) has been isolated from whole-cell preparations and identified by RNA interference (RNAi) to be the photoreceptor for step-up photophobic responses but not for step-down photophobic responses (M. Iseki, S. Matsunaga, A. Murakami, K. Ohno, K. Shiga, C. Yoshida, M. Sugai, T. Takahashi, T. Hori, M. Watanabe [2002] Nature 415: 1047-1051). The present study shows that knockdown of PAC by RNAi also effectively suppresses both positive and negative phototaxis, indicating for the first time that PAC or a PAC homolog is also the photoreceptor for photoorientation of wild-type E. gracilis. Recovery from RNAi occurred earlier for step-up photophobic responses than for positive and negative phototaxis. In addition, we investigated several phototaxis mutant strains of E. gracilis with different cytological features regarding the stigma and paraxonemal body (PAB; believed to be the location for the phototaxis photoreceptor) as well as Astasia longa, a close relative of E. gracilis. All of the E. gracilis mutant strains had PAC mRNAs, whereas in A. longa, a different but similar mRNA was found and designated AlPAC. Consistently, all of these strains showed no phototaxis but performed step-up photophobic responses, which were suppressed by RNAi of the PAC mRNA. The fact that some of these strains possess a cytologically altered or no PAB demonstrates that at least in these strains, the PAC photoreceptor responsible for the step-up photophobic responses is not located in the PAB.

ELDONET—A Decade of Monitoring Solar Radiation on Five Continents

The European light dosimeter network (ELDONET) comprises more than 40 stations in 24 countries on 5 continents. The present report compares solar radiation data in the photosynthetic active radiation, UV‐A (315–400 nm) and UV‐B (280–315 nm) wavelength ranges for 17 stations at different latitudes on the northern and southern hemispheres for up to 10 years of monitoring. While the maximal irradiances on clear days follow a latitudinal gradient due to the cosine dependence on the solar angle, the total doses strongly depend on the local climate and atmospheric conditions as well as the day‐length distribution over the year. UV‐B irradiances and doses are strongly influenced by the total column ozone, which is recorded for all covered stations.

Signal perception and transduction of gravitaxis in the flagellate Euglena gracilis

Summary The motile flagellate Euglena gracilis orients itself in its natural environment by means of gravitaxis and phototaxis. Until recently the mechanism of gravitaxis was not understood. Current results indicate that the density difference between cell body and surrounding medium causes sedimentation of the cell body, resulting in a stretching of the lower membrane. This activates stretch-sensitive ion channels, which leads to a modulation of the membrane potential. The modulation triggers a flagellar reorientation. In order to test this model we performed a series of experiments regarding the role of different ions in signal transduction. Increasing the external calcium concentration (but not decreasing), addition of a calcium ionophore and changes in the calcium fluxes had a drastic effect on gravitaxis. Changes in the potassium concentration resulted in an even higher inhibition. In addition, direct changes in the membrane potential affected gravitaxis. The obtained results indicate that both potassium and calcium play an important role in gravitaxis.

Effect of ultraviolet radiation on thallus absorption and photosynthetic pigments in the red alga Porphyra umbilicalis

Abstract The effect of ultraviolet (UV) radiation on thallus absorption, package effect, the concentration of photosynthetic pigments, photosynthetic oxygen production and effective quantum yield has been studied in the intertidal red macroalga Porphyra umbilicalis in the laboratory. High doses of UV-A and UV-B radiation result in a rapid decrease of thallus absorption and, after a 6 h exposure, total absorption is reduced to 25% of the initial value. Moreover, significant differences in the absorption peaks of the main pigments are found: while chlorophyll a (Chl a ) and phycocyanin absorption peaks decrease by 65–67%, carotenoids and phycoerythrin (PE) peaks decrease by 75–82%. Uncoupling of the transfer of energy between PE and Chl a by UV is revealed by a gradual increase of fluorescence of PE up to 11 h of exposure, followed by a subsequent decrease of fluorescence of the PE, in parallel with the photobleaching of the pigments. Thalli with higher pigment concentration present a greater sensitivity to UV radiation, as revealed by a more pronounced decrease in total thallus absorption, oxygen production and effective quantum yield, and a less effective recovery under low irradiation. Exposure of the thalli to artificial UV radiation in an experimental chamber with spectra and doses more similar to those of the natural environment reveals that PAR + UV-A radiation promotes a gradual increase of the total absorption over 24 h; in contrast, PAR + UV-A + UV-B induces a significant decrease of the thallus absorption. However, the concentration in vitro of Chl a , carotenoids and biliproteins does not change in any of these light treatments. The spectrally averaged in vivo absorption cross section normalized to Chl a ( a * ) increases after 24 h in PAR + UV-A, but it does not change in PAR and PAR + UV- A + UV-B, indicating that the degree of packing of the pigments in the membranes of the thylakoids (package effect) is decreased by PAR + UV-A, but that the reverse is induced by PAR + UV-A + UV-B. It is proposed that UV-A radiation induces an enhancement of the efficiency of light capture mediated by a relaxation of pigment packing in the light-harvesting antennae of this intertidal macroalga, while the reverse is promoted by UV-B radiation.

How and why does the proteome respond to microgravity

For medical and biotechnological reasons, it is important to study mammalian cells, animals, bacteria and plants exposed to simulated and real microgravity. It is necessary to detect the cellular changes that cause the medical problems often observed in astronauts, cosmonauts or animals returning from prolonged space missions. In order for in vitro tissue engineering under microgravity conditions to succeed, the features of the cell that change need to be known. In this article, we summarize current knowledge about the effects of microgravity on the proteome in different cell types. Many studies suggest that the effects of microgravity on major cell functions depend on the responding cell type. Here, we discuss and speculate how and why the proteome responds to microgravity, focusing on proteomic discoveries and their future potential.

Role of protective and repair mechanisms in the inhibition of photosynthesis in marine macroalgae

The mechanism of photoinhibition was investigated in three representative macroalgal species growing on the coast of Patagonia: the chlorophyte Ulva rigida C. Agardh, the rhodophyte Porphyra columbina Montagne and the phaeophyte Dictyota dichotoma (Huds.) Lamour. Dark adapted specimens were exposed to 15 min unfiltered solar radiation to induce photoinhibition, and subsequently the recovery of the photosynthetic quantum yield was followed for up to 6 h. Photoinhibition is believed to be due to the damage and proteolysis of the D1 protein in the reaction center of Photosystem II. During recovery this protein is resynthesized. In order to prove this hypothesis, inhibitors of the chloroplast protein synthesis, streptomycin and chloramphenicol were applied. Both retarded the repair process indicating an inhibition of the D1 protein resynthesis during recovery after the damage they experienced during light exposure. Some algal groups use the xanthophyll cycle to ameliorate the inhibition by excessive light. Dithiothreitol, an inhibitor of violaxanthin de-epoxidase, was administered, to impair the xanthophyll cycle. It strongly affected both photoinhibition and recovery even in the red algal species, which do not have the xanthophyll cycle, indicating that this drug has significant side effects and should be used with caution for the study of the involvement of this protective cycle in algae. Pigmentation was followed in the three species using absorption spectroscopy of thallus extracts at 665 nm during continuous exposure to natural solar radiation or radiation deprived of the UV component during two days. The results indicated a pronounced variation in pigmentation over time due to bleaching and resynthesis. Solar radiation was monitored during the experiments in three channels (UV-B, UV-A and PAR) using an ELDONET instrument on site.

EFFECTS OF SOLAR RADIATION ON PHOTOSYNTHESIS AND PHOTOINHIBITION IN RED MACROPHYTES FROM AN INTERTIDAL SYSTEM OF SOUTHERN SPAIN

The effects of solar radiation on photosynthesis, chlorophyll content and photoinhibition of the red macrophytes Asparagopsis armata, Gelidium sesquipedale, Plocamium cartilagineum and Feldmannophycus rayssiae from an intertidal system of southern Spain were estimated by means of pulse amplitude modulated fluorometer (PAM), by measurement of the O2 exchange, and by quantification of the chlorophyll content of the thalli. The effective quantum yield ( / ^) decreased in all the experimental organisms after 60 min of exposure to solar radiation; moreover, photoinhibition seemed to be more pronounced and long-lasting in shade-type plants. Secondly, all macrophytes from this study suffered more or less pronounced photoinhibition during some hours of the day at their natural living site. Photoinhibition was maximal around noon and the early afternoon, but almost complete recovery of photosynthesis was achieved by dusk. Similarly, oxygen production and chlorophyll a content were minimal around noon. The ratio (l-qP)/qN was lower in shade than in sun algae indicating a lower electron flow rate, and a lower rate of protective energy dissipation in shade than in sun-grown algae. This ratio is suggested as a good indicator of light stress, to compare different algal species with different pigment content, i. e. sunand shade-type algae.