Lichens as survivors in space and on Mars

Many lichens are able to live and photosynthesize under harsh conditions, characterized by low temperatures, aridity and high UV radiation fluxes. Some lichen species are even able to survive simulated and real space conditions. Many tests after space exposure on the satellite FOTON M3 and on the International Space Station have shown their capacity to maintain physiological and photosynthetic activity, and their capacity to germinate and grow after being exposed to space parameters. Further tests using simulated Martian atmospheres, temperatures, humidity profiles and UV radiation spectra and fluxes have shown maintenance of photosynthetic activity of Xanthoria elegans. Results from space and Mars simulation experiments on lichens such as X. elegans are valuable for determining the habitability of a planet and for the search for possible life-supporting habitats on planets like Mars.     

Exposure of phototrophs to 548 days in low Earth orbit: microbial selection pressures in outer space and on early earth

An epilithic microbial community was launched into low Earth orbit, and exposed to conditions in outer space for 548 days on the European Space Agency EXPOSE-E facility outside the International Space Station. The natural phototroph biofilm was augmented with akinetes of Anabaena cylindrica and vegetative cells of Nostoc commune and Chroococcidiopsis. In space-exposed dark controls, two algae (Chlorella and Rosenvingiella spp.), a cyanobacterium (Gloeocapsa sp.) and two bacteria associated with the natural community survived. Of the augmented organisms, cells of A. cylindrica and Chroococcidiopsis survived, but no cells of N. commune. Only cells of Chroococcidiopsis were cultured from samples exposed to the unattenuated extraterrestrial ultraviolet (UV) spectrum (>110 nm or 200 nm). Raman spectroscopy and bright-field microscopy showed that under these conditions the surface cells were bleached and their carotenoids were destroyed, although cell morphology was preserved. These experiments demonstrate that outer space can act as a selection pressure on the composition of microbial communities. The results obtained from samples exposed to >200 nm UV (simulating the putative worst-case UV exposure on the early Earth) demonstrate the potential for epilithic colonization of land masses during that time, but that UV radiation on anoxic planets can act as a strong selection pressure on surface-dwelling organisms. Finally, these experiments have yielded new phototrophic organisms of potential use in biomass and oxygen production in space exploration.     

Space Microbiology

Summary: The responses of microorganisms (viruses, bacterial cells, bacterial and fungal spores, and lichens) to selected factors of space (microgravity, galactic cosmic radiation, solar UV radiation, and space vacuum) were determined in space and laboratory simulation experiments. In general, microorganisms tend to thrive in the space flight environment in terms of enhanced growth parameters and a demonstrated ability to proliferate in the presence of normally inhibitory levels of antibiotics. The mechanisms responsible for the observed biological responses, however, are not yet fully understood. A hypothesized interaction of microgravity with radiation-induced DNA repair processes was experimentally refuted. The survival of microorganisms in outer space was investigated to tackle questions on the upper boundary of the biosphere and on the likelihood of interplanetary transport of microorganisms. It was found that extraterrestrial solar UV radiation was the most deleterious factor of space. Among all organisms tested, only lichens (Rhizocarpon geographicum and Xanthoria elegans) maintained full viability after 2 weeks in outer space, whereas all other test systems were inactivated by orders of magnitude. Using optical filters and spores of Bacillus subtilis as a biological UV dosimeter, it was found that the current ozone layer reduces the biological effectiveness of solar UV by 3 orders of magnitude. If shielded against solar UV, spores of B. subtilis were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within meteorites, the so-called lithopanspermia hypothesis.     

Delivery of extraterrestrial amino acids to the primitive Earth. Exposure experiments in Earth orbit.

A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50 to 100 micrometers size range, the carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon. They might have brought more carbon to the surface of the primitive Earth than that involved in the present surficial biomass. Amino acids such as "-amino isobutyric acid have been identified in these Antarctic micrometeorites. Enantiomeric excesses of L-amino acids have been detected in the Murchison meteorite. A large fraction of homochiral amino acids might have been delivered to the primitive Earth via meteorites and micrometeorites. Space technology in Earth orbit offers a unique opportunity to study the behaviour of amino acids required for the development of primitive life when they are exposed to space conditions, either free or associated with tiny mineral grains mimicking the micrometeorites. Our objectives are to demonstrate that porous mineral material protects amino acids in space from photolysis and racemization (the conversion of L-amino acids into a mixture of L- and D-molecules) and to test whether photosensitive amino acids derivatives can polymerize in mineral grains under space conditions. The results obtained in BIOPAN-1 and BIOPAN-2 exposure experiments on board unmanned satellite FOTON are presented.     

Extremotolerance and Resistance of Lichens: Comparative Studies on Five Species Used in Astrobiological Research II. Secondary Lichen Compounds

Lichens, which are symbioses of a fungus and one or two photoautotrophs, frequently tolerate extreme environmental conditions. This makes them valuable model systems in astrobiological research to fathom the limits and limitations of eukaryotic symbioses. Various studies demonstrated the high resistance of selected extremotolerant lichens towards extreme, non-terrestrial abiotic factors including space exposure, hypervelocity impact simulations as well as space and Martian parameter simulations. This study focusses on the diverse set of secondary lichen compounds (SLCs) that act as photo- and UVR-protective substances. Five lichen species used in present-day astrobiological research were compared: Buellia frigida, Circinaria gyrosa, Rhizocarpon geographicum, Xanthoria elegans, and Pleopsidium chlorophanum. Detailed investigation of secondary substances including photosynthetic pigments was performed for whole lichen thalli but also for axenically cultivated mycobionts and photobionts by methods of UV/VIS-spectrophotometry and two types of high performance liquid chromatography (HPLC). Additionally, a set of chemical tests is presented to confirm the formation of melanic compounds in lichen and mycobiont samples. All investigated lichens reveal various sets of SLCs, except C. gyrosa where only melanin was putatively identified. Such studies will help to assess the contribution of SLCs on lichen extremotolerance, to understand the adaptation of lichens to prevalent abiotic stressors of the respective habitat, and to form a basis for interpreting recent and future astrobiological experiments. As most of the identified SLCs demonstrated a high capacity in absorbing UVR, they may also explain the high resistance of lichens towards non-terrestrial UVR.     

The solar UV environment and bacterial spore UV resistance: considerations for Earth-to-Mars transport by natural processes and human spaceflight

The environment in space and on planets such as Mars can be lethal to microorganisms because of the high vacuum and high solar radiation flux, in particular UV radiation, in such environments. Spores of various Bacillus species are among the organisms most resistant to the lethal effects of high vacuum and UV radiation, and as a consequence are of major concern for planetary contamination via unmanned spacecraft or even natural processes. This review focuses on the spores of various Bacillus species: (i) their mechanisms of UV resistance; (ii) their survival in unmanned spacecraft, space flight and simulated space flight and Martian conditions; (iii) the UV flux in space and on Mars; (iv) factors affecting spore survival in such high UV flux environments.     

Survival of Akinetes (Resting-State Cells of Cyanobacteria) in Low Earth Orbit and Simulated Extraterrestrial Conditions

Cyanobacteria are photosynthetic organisms that have been considered for space applications, such as oxygen production in bioregenerative life support systems, and can be used as a model organism for understanding microbial survival in space. Akinetes are resting-state cells of cyanobacteria that are produced by certain genera of heterocystous cyanobacteria to survive extreme environmental conditions. Although they are similar in nature to endospores, there have been no investigations into the survival of akinetes in extraterrestrial environments. The aim of this work was to examine the survival of akinetes from Anabaena cylindrica in simulated extraterrestrial conditions and in Low Earth Orbit (LEO). Akinetes were dried onto limestone rocks and sent into LEO for 10 days on the ESA Biopan VI. In ground-based experiments, the rocks were exposed to periods of desiccation, vacuum (0.7 × 10⁻³ kPa), temperature extremes (−80 to 80°C), Mars conditions (−27°C, 0.8 kPa, CO₂) and UV radiation (325–400 nm). A proportion of the akinete population was able to survive a period of 10 days in LEO and 28 days in Mars simulated conditions, when the rocks were not subjected to UV radiation. Furthermore, the akinetes were able to survive 28 days of exposure to desiccation and low temperature with high viability remaining. Yet long periods of vacuum and high temperature were lethal to the akinetes. This work shows that akinetes are extreme-tolerating states of cyanobacteria that have a practical use in space applications and yield new insight into the survival of microbial resting-state cells in space conditions.     

Communities Adjust their Temperature Optima by Shifting Producer-to-Consumer Ratio, Shown in Lichens as Models: I. Hypothesis

An apparent paradox exists in the ecology of Antarctic lichens: their net photosynthetic temperature optimum (around 0°C) lies far below the temperature optima of their constituent algae and fungi (around 20°C). To address this paradox, we consider lichens as microbial communities and propose the “community adaptation” hypothesis, which posits that in each thermal regime there is an equilibrium between photosynthetic primary producers (photobionts), and heterotrophic consumers (mycobiont and parasymbiont fungi). This equilibrium, expressed as the producer/consumer ratio (R_p/c), maximizes the fitness of the community. As respiration increases with temperature, more rapidly than does photosynthesis, R_p/c will shift accordingly in warm habitats, resulting in a high-growth temperature optimum for the community (the lichen). This lends lichens an adaptive flexibility that enables them to function optimally at any thermal regime within the tolerance limits of the constituent organisms. The variable equilibrium of producers and consumers may have a similar role in thermal adaptation of more complex communities and ecosystems.     

Will climate warming exceed lethal photosynthetic temperature thresholds of lichens in a southern African arid region?

Predicted elevated temperatures and a shift from a winter to summer rainfall pattern associated with global warming could result in the exposure of hydrated lichens during summer to more numerous temperature extremes that exceed their thermal thresholds. This hypothesis was tested by measuring lethal temperature thresholds under laboratory and natural conditions for four epilithic lichen species (Xanthoparmelia austro‐africana, X. hyporhytida, Xanthoparmelia sp., Xanthomaculina hottentotta) occurring on quartz gravel substrates at a hot arid inland site two epigeous lichen species (Teloschistes capensis, Ramalina sp.) occurring on gypsum‐rich topsoil at a warm humid coastal site. Extrapolated lethal temperatures for photosynthetic quantum yield under laboratory conditions were up to 4°C higher for lichens from a dry inland site than those from a humid coastal site. Lethal temperatures extrapolated for photosynthetic quantum yield at a saturating photosynthetic photon flux density of ≥11,000 μmol photons m^?2 s^?1 under natural conditions were up to 6°C higher for lichens from the dry inland site than the more humid coastal site. It is concluded that only under atypical conditions of lichen exposure in a hydrated state to temperature extremes at high midday solar irradiances during summer could lethal photosynthetic thresholds in sensitive lichen species be potentially exceeded, but whether the increased frequency of such conditions with climate warming would lead to increased likelihood of lichen mortality is debatable.     

Water- and temperature-dependence of DNA damage and repair in the fruticose lichen Cladonia arbuscula ssp. mitis exposed to UV-B radiation

The induction of cyclobutane pyrimidine dimers (CPDs) by ultraviolet‐B radiation (UV‐B, 280–315 nm) and repair mechanisms were studied in the lichen Cladonia arbuscula ssp. mitis exposed to different temperatures and water status conditions. In addition, the development and repair of CPDs were studied in relation to the different developmental stages of the lichen thallus podetial branches. Air‐dried lichen thalli exposed to UV‐B radiation combined with relatively high visible light (HL, 800 μmol m^?2 s^?1; 400–700 nm) for 7 days showed a progressive increase of CPDs with no substantial repair, although HL was present during and after irradiation with UV‐B. Fully hydrated lichen thalli, that had not been previously exposed to UV‐B radiation for 7 days, were given short‐term UV‐B radiation treatment at 25°C, and accumulated DNA lesions in the form of CPDs, with repair occurring when they were exposed to photoreactivating conditions (2 h of 300 μmol m^?2 s^?1, 400–700 nm). A different pattern was observed when fully hydrated thalli were exposed to short‐term UV‐B radiation at 2°C, in comparison with exposure at 25°C. High levels of CPDs were induced at 2°C under UV‐B irradiation, without significant repair under subsequent photoreactivating light. Likewise, when PAR (300 μmol m^?2 s^?1) and UV‐B radiation were given simultaneously, the CPD levels were not lowered. Throughout all experiments the youngest, less differentiated parts of the lichen thallus – namely ‘tips’, according to our arbitrary subdivision – were the parts showing the highest levels of CPD accumulation and the lowest levels of repair in comparison with the older thallus tissue (‘stems’). Thus the experiments showed that Cladonia arbuscula ssp. mitis is sensitive to UV‐B irradiation in the air‐dried state and is not able to completely repair the damage caused by the radiation. Furthermore, temperature plays a role in the DNA damage repairing capacity of this lichen, since even when fully hydrated, C. arbuscula ssp. mitis did not repair DNA damage at the low temperatures.     

地衣对放射性物质吸收和积累的初步研究

地衣植物对放射性物质具有富集作用,在国内有关这方面的研究工作尚未见有报道。本文作者对采自不同地区的22种地衣进行了低本底α,β射线总放射性的测定和NaI(TL)γ能谱的分析,结果表明:地衣中长寿命放射性核素有明显的积累现象,并呈现出一定的空间分布趋势。因而,了解地衣体中放射性含量水平、核素性质及分布状况,进而为研究环境放射学、地衣污染生态学等学科提供科学依据和基础资料。     

The effects of ultraviolet irradiation on a coccoid blue-green alga: survival, photosynthesis, and photoreactivation

The effects of UV irradiation (254 mμ) on a coccoid blue-green alga Agmenellum quadruplicatum, Strain PR-6, have been examined in terms of the survival curve and measurement of short time photosynthetic rates. From study of survival evidence has been found for a strong photoreactivation centered near 430 mμ. Measurements of photosynthetic rate suggest that there is a correlation between decay of photosynthesis and survival after UV exposure. The UV induced decay in photosynthetic activity is reversed by the identical photoreactivation conditions that increase the survival level. The photosynthetic data are interpreted as demonstrating a photoreactivation of photosynthesis in blue-green algae.     

Morphological and physiological responses of bean plants to supplemental UV radiation in a Mediterranean climate

During the last few decades many experiments have been performed to evaluate the responses of plants to enhanced solar UV-B radiation (280–320 nm) that may occur because of stratospheric ozone depletion; most of them were performed in controlled environment conditions where plants were exposed to low photosynthetically active radiation (PAR) levels and high UV-B irradiance. Since environmental radiative regimes can play a role in the response of plants to UV-B enhancement, it appears doubtful whether it is valid to extrapolate the results from these experiments to plants grown in natural conditions. The objective of this work was to evaluate the effects on physiology and morphology of a bean (Phaseolus vulgaris L.) cultivar Nano Bobis, exposed to supplemental UV radiation in the open-air. UV-B radiation was supplied by fluorescent lamps to simulate a 20% stratospheric ozone reduction. Three groups of plants were grown: control (no supplemental UV), UV-A treatment (supplementation in the UV-A band) and UV-B treatment (supplemental UV-B and UV-A radiation). Each group was replicated three times. After 33 days of treatment plants grown under UV-B treatment had lower biomass, leaf area and reduced leaf elongation compared to UV-A treatment. No significant differences were detected in photosynthetic parameters, photosynthetic pigments and UV-B absorbing compounds among the three groups of plants. However, plants exposed to UV-A treatment showed a sort of 'stimulation' of their growth when compared to the control. The results of this experiment showed that plants may be sensitive to UV-A radiation, thus it is difficult to evaluate the specific effects of UV-B (280–320 nm) radiation from fluorescent lamps and it is important to choose the appropriate control. Environmental conditions strongly affect plant response to UV radiation so further field studies are necessary to assess the interaction between UV-B exposure and meteorological variability.     

Photosynthetic capacity in relation to nitrogen content and its partitioning in lichens with different photobionts

We tested the hypothesis that lichen species with a photosynthetic CO₂-concentrating mechanism (CCM) use nitrogen more efficiently in photosynthesis than species without this mechanism. Total ribulose bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) and chitin (the nitrogenous component of fungal cell walls), were quantified and related to photosynthetic capacity in eight lichens. The species represented three modes of CO₂ acquisition and two modes of nitrogen acquisition, and included one cyanobacterial ( Nostoc ) lichen with a CCM and N₂ fixation, four green algal ( Trebouxia ) lichens with a CCM but without N₂ fixation and three lichens with green algal primary photobionts ( Coccomyxa or Dictyochloropsis ) lacking a CCM. The latter have N₂-fixing Nostoc in cephalodia. When related to thallus dry weight, total thallus nitrogen varied 20-fold, chitin 40-fold, Chl a 5-fold and Rubisco 4-fold among the species. Total nitrogen was lowest in three of the four Trebouxia lichens and highest in the bipartite cyanobacterial lichen. Lichens with the lowest nitrogen invested a larger proportion of this into photosynthetic components, while the species with high nitrogen made relatively more chitin. As a result, the potential photosynthetic nitrogen use efficiency was negatively correlated to total thallus nitrogen for this range of species. The cyanobacterial lichen had a higher photosynthetic capacity in relation to both Chl a and Rubisco compared with the green algal lichens. For the range of green algal lichens both Chl a and Rubisco contents were linearly related to photosynthetic capacity, so the data did not support the hypothesis of an enhanced photosynthetic nitrogen use efficiency in green-algal lichens with a CCM.     

Survival of micro-organisms in space

Dried suspensions ofPenicillium roqueforti Thom, Coliphage T-1,Bacillus subtilis and tobacco mosaic virus were exposed to space on board the Gemini-IX-A and XII earth satellites and the Agena-VIII space rocket. All micro-organisms tested survived the direct exposure during the Gemini-IX-A experiment. In the Gemini-XII experiment only the T-1 phage survived the direct exposure. The survival was influenced by the suspending medium and depended on the species of the microorganism. After four months of space flight on the Agena-VIII space rocket surviving fractions between 2×10^–3 and 1.0 were found in the unopened flight container. However, micro-organisms exposed on the cover of the container during this period were completely inactivated. Shielding against solar ultraviolet radiation during flight resulted in survival of micro-organisms exceeding to that of the transport controls, and the survival was considered complete.Sterile methylcellulose collection surfaces were exposed to space on board the Gemini-IX-A and XII satellites in an attempt to collect viable micro-organisms in space. None of the collection surfaces yielded viable micro-organisms.     

Chlorophyll a Fluorescence Emission,Xanthophyll Cycle Activity,and Net Photosynthetic Rate Responses to Ozone in Some Foliose and Fruticose Lichen Species

The lichens Parmelia quercina, Parmelia sulcata, Evernia prunastri, Hypogymnia physodes, and Anaptychia ciliaris were exposed to ozone (O₃) in controlled environment cuvettes designed to maintain the lichens at optimal physiological activity during exposure. Measurements of gas exchange, modulated chlorophyll (Chl) fluorescence, and pigment analysis were conducted before and after exposure to 300 mm³ (O₃) m^–3, 4 h per d for 14 d. No changes in the efficiency of photosystem 2 (PS2) photochemistry, the reduction state of Q_A, or the electron flow through PS2, measured by Chl fluorescence, were detected in any of the five lichen species studied. Additionally, neither photosynthetic CO₂ assimilation nor xanthophyll cycle activity or photosynthetic pigment concentration were affected by high O₃ concentrations. Thus the studied lichen species have significant capacities to withstand oxidative stresses induced by high concentration of O₃.     

Lichens: a promising source of antibiotic and anticancer drugs

Lichens are symbiotic associations between fungi and a photosynthetic alga and/or cyanobacteria. Lichenized fungi have been found to produce a wide array of secondary metabolites, most of which are unique to the lichenized condition. These secondary metabolites have shown an impressive range of biological activities including antibiotics, antifungal, anti-HIV, anticancer, anti-protozoan, etc. This review focuses primarily on the antibiotic and anticancer properties of lichen secondary chemicals. We have reviewed various publications related to antibiotic and anticancer drug therapies emphasizing results about specific lichens and/or lichen compounds, which microbes or cancer cells were involved and the main findings of each study. We found that crude lichen extracts and various isolated lichen compounds often demonstrate significant inhibitory activity against various pathogenic bacteria and cancer cell lines at very low concentrations. There were no studies examining the specific mechanism of action against pathogenic bacteria; however, we did find a limited number of studies where the mechanism of action against cancer cell lines had been explored. The molecular mechanism of cell death by lichen compounds includes cell cycle arrest, apoptosis, necrosis, and inhibition of angiogenesis. Although lichens are a reservoir for various biologically active compounds, only a limited number have been tested for their biological significance. There is clearly an urgent need for expanding research in this area of study, including in depth studies of those compounds which have shown promising results as well as a strong focus on identifying specific mechanisms of action and extensive clinical trials using the most promising lichen based drug therapies followed by large scale production of the best of those compounds.     

CONTINUOUS MONITORING OF CO2EXCHANGE OF LICHENS IN THE FIELD: SHORT-TERM ENCLOSURE WITH AN AUTOMATICALLY OPERATING CUVETTE

Field measurements of CO₂exchange by lichens are difficult because these poikilohydric organisms rely on direct hydration by rain, dew or fog. Continuous enclosure of thalli in conditioned, measurement cuvettes is, therefore, not experimentally reasonable. Porometric instruments with brief enclosure times, have proved useful for studies of lichen gas exchange but allow only spot measurements and recording is not easily possible for extended time periods. We describe a newly developed, automatically operating cuvette that allows long-term monitoring of lichen photosynthetic and respiratory CO₂exchange in the field. Samples, e.g. flat stone slabs covered with epilithic lichens, are positioned on the base of the cuvette where, when the cuvette is open, they are exposed to the same environmental conditions as naturally growing thalli. At regular intervals (typically 30 min), an upper lid automatically encloses the lichen forc. 3 min in a stirred cuvette; CO₂exchange is measured using IRGA techniques and microclimate data are recorded. The successful operation of the cuvette is illustrated by means of diel time courses of CO₂exchange for selected, very different weather conditions.     

不同水分条件下三种附生地衣的光合作用特性

附生地衣是哀牢山湿性常绿阔叶林生态系统中重要的结构性组分。通过对该区域山地森林中3种典型附生地衣平滑牛皮叶 (Sticta nylanderiana)、网肺衣 (Lobaria retigera) 和橄榄斑叶 (Cetrelia olivetorum)在不同水分条件下的光合光响应及荧光参数的测定分析,结果显示,附生地衣光补偿点 (LCP)、光饱和点 (LSP)较高,对强光适应能力较强。在水分胁迫 (含水量5%~10%) 条件下,3种附生地衣的最大净光合速率 (Pmax) 仅为17~50nmol·g-1·s-1。随着含水量的增加,地衣的最大净光合速率 (Pmax) 与暗呼吸速率 (Rday) 逐渐增大,LCP降低,而LSP随之提高,这表明3种附生地衣具备“阳生植物”的某些特性,从而能够在一定程度上适应野外光照较强的灌丛、向阳林冠等生境。地衣叶绿素光反应中心初始荧光参数 (F0) 和最大光化学效率 (Fv/Fm) 随含水量下降而显著降低,暗示其光反应中心对水分有很强的敏感性。水分条件的改善有助于附生地衣的光反应中心进入到较高的生理活性状态。     

Ultraviolet-absorbing scytonemin and mycosporine-like amino acid derivatives in exposed, rock-inhabiting cyanobacterial lichens

 The occurrence of ultraviolet (UV)-absorbing substances like scytonemin and mycosporine-glycine is reported for the first time from cyanobacterial lichens of the genera Collema, Gonohymenia and Peltula, all coming from high-light-intensity habitats. Except for Collema with the filamentous Nostoc, all other cyanobionts belong to the unicellular genera Chroococcidiopsis, Cyanosarcina, Gloeocapsa or Myxosarcina. From transmission electron microscope studies it is evident that the pigmentation (scytonemin) is located extracellularly in the sheath of the outer thallus parts. Fluorescence microscopy and microprobe measurements clearly show UV radiation into the lichen thallus and hence the relevance of UV sunscreens for the protection of the organism. Received: 4 March 1997 / Accepted: 20 May 1997