Petra Rettberg

Tardigrades survive exposure to space in low Earth orbit

Summary Vacuum (imposing extreme dehydration) and solar/galactic cosmic radiation prevent survival of most organisms in space [1]. Only anhydrobiotic organisms, which have evolved adaptations to survive more or less complete desiccation, have a potential to survive space vacuum, and few organisms can stand the unfiltered solar radiation in space. Tardigrades, commonly known as water-bears, are among the most desiccation and radiation-tolerant animals and have been shown to survive extreme levels of ionizing radiation [2–4]. Here, we show that tardigrades are also able to survive space vacuum without loss in survival, and that some specimens even recovered after combined exposure to space vacuum and solar radiation. These results add the first animal to the exclusive and short list of organisms that have survived such exposure.

Role of DNA Repair by Nonhomologous-End Joining in Bacillus subtilis Spore Resistance to Extreme Dryness, Mono- and Polychromatic UV, and Ionizing Radiation

The role of DNA repair by nonhomologous-end joining (NHEJ) in spore resistance to UV, ionizing radiation, and ultrahigh vacuum was studied in wild-type and DNA repair mutants (recA, splB, ykoU, ykoV, and ykoU ykoV mutants) of Bacillus subtilis. NHEJ-defective spores with mutations in ykoU, ykoV, and ykoU ykoV were significantly more sensitive to UV, ionizing radiation, and ultrahigh vacuum than wild-type spores, indicating that NHEJ provides an important pathway during spore germination for repair of DNA double-strand breaks.

Biological dosimetry of solar radiation for different simulated ozone column thicknesses.

During the Spacelab mission D-2, in the experiment RD-UVRAD, precalibrated biofilms consisting of dry monolayers of immobilised spores of Bacillus subtilis (strain Marburg) were exposed, for defined intervals, to extraterrestrial solar radiation filtered through an optical filtering system, to simulate different ozone column thicknesses. After the mission, the biofilms were processed and optical densities indicative of any biological activity were determined for each exposure condition by image analysis. For the different simulated ozone column thicknesses, biologically effective irradiances were experimentally determined from the biofilm data and compared with calculated data using a radiative transfer model and the known biofilm action spectrum. The data show a strong increase in biologically effective solar UV irradiance with decreasing (simulated) ozone concentrations. The full spectrum of extraterrestrial solar radiation leads to an increment of the biologically effective irradiance by nearly three orders of magnitude compared with the solar spectrum at the surface of the Earth for average total ozone columns.

EXPOSE-E: an ESA astrobiology mission 1.5 years in space.

The multi-user facility EXPOSE-E was designed by the European Space Agency to enable astrobiology research in space (low-Earth orbit). On 7 February 2008, EXPOSE-E was carried to the International Space Station (ISS) on the European Technology Exposure Facility (EuTEF) platform in the cargo bay of Space Shuttle STS-122 Atlantis. The facility was installed at the starboard cone of the Columbus module by extravehicular activity, where it remained in space for 1.5 years. EXPOSE-E was returned to Earth with STS-128 Discovery on 12 September 2009 for subsequent sample analysis. EXPOSE-E provided accommodation in three exposure trays for a variety of astrobiological test samples that were exposed to selected space conditions: either to space vacuum, solar electromagnetic radiation at >110 nm and cosmic radiation (trays 1 and 3) or to simulated martian surface conditions (tray 2). Data on UV radiation, cosmic radiation, and temperature were measured every 10 s and downlinked by telemetry. A parallel mission ground reference (MGR) experiment was performed on ground with a parallel set of hardware and samples under simulated space conditions. EXPOSE-E performed a successful 1.5-year mission in space.

The potential of the lichen symbiosis to cope with extreme conditions of outer space – I. Influence of UV radiation and space vacuum on the vitality of lichen symbiosis and germination capacity

The lichen symbiotic organisms Fulgensia bracteata and Xanthoria elegans as well as their isolated photobionts and mycobionts were exposed to conditions simulating the extreme parameters of outer space in order to assess their limits of survival, e.g. during a potential interplanetary transfer or on the surface of an extraterrestrial body. Using the space simulation facilities at DLR, the test parameters under investigation were vacuum (10 −3 Pa) and ultraviolet (UV) radiation at wavelength ranges from vacuum-UV to UV-A, applied separately or in combination. An analysis of vitality was made using confocal laser scanning microscopy and LIVE/DEAD staining using FUN I or SYTOX green. Intact lichens were extremely resistant to vacuum exposure as well as to UV radiation up to doses of about 160 kJ m −2 (200 −2 reduced the vitality rate to nearly 50%; however, the dose effect curves levelled off and continued treatment did not reduce the vitality further. Similar survival curves levelling off at higher UV doses were observed for the isolated photobionts; however, in the latter cases, the saturation occurred at five times lower doses (full UV spectrum). Also spores of the mycobionts showed a remarkable UV (254 nm) resistance up to doses of about 3 kJ m −2 . The data suggest that the symbiotic features peculiar to lichens allow them to cope with the extreme conditions of outer space or even with Martian surface conditions provided suitable niche habitats are available to serve as refuges and complementary endogenous or exogenous protection mechanisms are established.

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.

Microgravity inhibits intestinal calcium absorption as shown by a stable strontium test.

Little is known about the onset and degree of biochemical and functional alterations in calcium metabolism during microgravity.

The Influence of Microgravity on Repair of Radiation Induced DNA Damage in Bacteria and Human Fibroblasts

The influence of the space flight environment, above all microgravity, on the repair of radiation-induced DNA damage was examined during the Spacelab mission IML-2 as (1) rejoining of DNA strand breaks induced by X irradiation in cells of Escherichia coli B/r (120 Gy) and (2) in human fibroblasts (5 and 10 Gy); (3) induction of the SOS response after gamma irradiation (300 Gy) of cells of Escherichia coli PQ37; and (4) survival of spores of Bacillus subtilis HA 101 after UV irradiation (up to 340 J m(-2)). Cells were irradiated prior to the space mission and were kept frozen (E. coli and fibroblasts) until incubation for defined periods (up to 4.5 h) in orbit; thereafter they were frozen again for laboratory analysis. Germination and growth of spores of B. subtilis on membrane filters was initiated by humidification in orbit. Controls were performed in-flight (1g reference centrifuge) and on the ground (1g and 1.4g). We found no significant differences between the microgravity samples and the corresponding controls in the kinetics of DNA strand break rejoining and of the induction of the SOS response as well as in the survival curves (as proven by Students t test, P < or = 0.1). These observations provide evidence that in the microgravity environment cells are able to repair radiation-induced DNA damage almost normally. The results suggest that a disturbance of cellular repair processes in the microgravity environment might not be the explanation for the reported synergism of radiation and microgravity.

Survival of microorganisms in space protected by meteorite material: Results of the experiment ‘EXOBIOLOGIE’ of the PERSEUS mission

During the early evolution of life on Earth, before the formation of a protective ozone layer in the atmosphere, high intensities of solar UV radiation of short wavelengths could reach the surface of the Earth. Today the full spectrum of solar UV radiation is only experienced in space, where other important space parameters influence survival and genetic stability additionally, like vacuum, cosmic radiation, temperature extremes, microgravity. To reach a better understanding of the processes leading to the origin, evolution and distribution of life we have performed space experiments with microorganisms. The ability of resistant life forms like bacterial spores to survive high doses of extraterrestrial solar UV alone or in combination with other space parameters, e.g. vacuum, was investigated. Extraterrestrial solar UV was found to have a thousand times higher biological effectiveness than UV radiation filtered by stratospheric ozone concentrations found today on Earth. The protective effects of anorganic substances like artificial or real meteorites were determined on the MIR station. In the experiment EXOBIOLOGIE of the French PERSEUS mission (1999) it was found that very thin layers of anorganic material did not protect spores against the deleterious effects of energy-rich UV radiation in space to the expected amount, but that layers of UV radiation inactivated spores serve as a UV-shield by themselves, so that a hypothetical interplanetary transfer of life by the transport of microorganisms inside rocks through the solar system cannot be excluded, but requires the shielding of a substantial mass of anorganic substances.

Roles of the Major, Small, Acid-Soluble Spore Proteins and Spore-Specific and Universal DNA Repair Mechanisms in Resistance of Bacillus subtilis Spores to Ionizing Radiation from X Rays and High-Energy Charged-Particle Bombardment

The role of DNA repair by nonhomologous end joining (NHEJ), homologous recombination, spore photoproduct lyase, and DNA polymerase I and genome protection via alpha/beta-type small, acid-soluble spore proteins (SASP) in Bacillus subtilis spore resistance to accelerated heavy ions (high-energy charged [HZE] particles) and X rays has been studied. Spores deficient in NHEJ and alpha/beta-type SASP were significantly more sensitive to HZE particle bombardment and X-ray irradiation than were the recA, polA, and splB mutant and wild-type spores, indicating that NHEJ provides an efficient DNA double-strand break repair pathway during spore germination and that the loss of the alpha/beta-type SASP leads to a significant radiosensitivity to ionizing radiation, suggesting the essential function of these spore proteins as protectants of spore DNA against ionizing radiation.