Tardigrades inhabiting terrestrial environments exhibit extraordinary resistance to ionizing radiation and UV radiation although little is known about the mechanisms underlying the resistance. We found that the terrestrial tardigrade Ramazzottius varieornatus is able to tolerate massive doses of UVC irradiation by both being protected from forming UVC-induced thymine dimers in DNA in a desiccated, anhydrobiotic state as well as repairing the dimers that do form in the hydrated animals. In R. varieornatus accumulation of thymine dimers in DNA induced by irradiation with 2.5 kJ/m2 of UVC radiation disappeared 18 h after the exposure when the animals were exposed to fluorescent light but not in the dark. Much higher UV radiation tolerance was observed in desiccated anhydrobiotic R. varieornatus compared to hydrated specimens of this species. On the other hand, the freshwater tardigrade species Hypsibius dujardini that was used as control, showed much weaker tolerance to UVC radiation than R. varieornatus, and it did not contain a putative phrA gene sequence. The anhydrobiotes of R. varieornatus accumulated much less UVC-induced thymine dimers in DNA than hydrated one. It suggests that anhydrobiosis efficiently avoids DNA damage accumulation in R. varieornatus and confers better UV radiation tolerance on this species. Thus we propose that UV radiation tolerance in tardigrades is due to the both high capacities of DNA damage repair and DNA protection, a two-pronged survival strategy.

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1Recent orbital and landed missions have provided substantial evidence for 2 ancient liquid water on the Martian surface as well as evidence of more recent 3 sedimentary deposits formed by water and/or ice. These observations raise 4 serious questions regarding an independent origin and evolution of life on Mars. 5Future missions seek to identify signs of extinct Martian biota in the form of 6 biomarkers or morphological characteristics, but the inherent danger of space 7 craft-borne terrestrial life makes the possibility of forward contamination a 8 serious threat not only to the life detection experiments, but also to any extant 9Martian ecosystem. A variety of cold and desiccation-tolerant organisms were 10 exposed to 40 days of simulated Martian surface conditions while embedded 11 within several centimeters of regolith simulant in order to ascertain the 12 plausibility of such organisms' survival as a function of environmental parameters 13 and burial depth. Relevant amino acid biomarkers associated with terrestrial life 14 were also analyzed in order to understand the feasibility of detecting biomarker 15 evidence for previous biological activity. Results indicate that stresses due to 16 desiccation, oxidation, and UV-associated damage were the primary deterrent to 17 organism survival, and that the effect of diurnal temperature variations and 18 reactive atmospheric species were minimal. Organisms with resistance to 19 desiccation and radiation environments showed increased levels of survival after 20the experiment compared to organisms characterized as psychrotolerant. Amino 21 2 acid biomarker analyses indicated the presence of an oxidation mechanism that 22 migrated downward through the samples during the course of the experiment and 23 likely represents the formation of various oxidizing species at mineral surfaces as 24 water vapor diffused through the regolith. Current sterilization protocols may 25 specifically select for organisms best adapted to survival at the Martian surface, 26 namely species that show tolerance to radical-induced oxidative damage and low 27 water activity environments. Additionally, any hypothetical Martian ecosystems 28 may have evolved similar physiological traits that allow sporadic metabolism 29 during periods of increased water activity. 30

We propose that nanophase iron-oxide-bearing materials provided important niches for ancient photosynthetic microbes on the Earth that ultimately led to the oxygenation of the Earth's atmosphere and the formation of iron-oxide deposits. Atmospheric oxygen and ozone attenuate ultraviolet radiation on the Earth today providing substantial protection for photosynthetic organisms. With ultraviolet radiation fluxes likely to have been even higher on the early Earth than today, accessing solar radiation was particularly risky for early organisms. Yet, we know that photosynthesis arose early and played a critical role in subsequent evolution. Of primary importance was protection below 290 nm, where peak nucleic acid (y260 nm) and protein (y280 nm) absorptions occur. Nanophase ferric oxide/oxyhydroxide minerals absorb, and thus block, the lethal ultraviolet radiation, while transmitting light through much of the visible and near-infrared regions of interest to photosynthesis (400 to 1100 nm). Furthermore, they were available in early environments, and are synthesized by many organisms. Based on experiments using nanophase ferric oxide/oxyhydroxide minerals as a sunscreen for photosynthetic microbes, we suggest that iron, an abundant element widely used in biological mechanisms, may have provided the protection that early organisms needed in order to be able to use photosynthetically active radiation while being protected from ultraviolet-induced damage. The results of this study are broadly applicable to astrobiology because of the abundance of iron in other potentially habitable bodies and the evolutionary pressure to utilize solar radiation when available as an energy source. This model could apply to a potential life form on Mars or other bodies where liquid water and ultraviolet radiation could have been present at significant levels. Based on ferric oxide/oxyhydroxide spectral properties, likely geologic processes, and the results of experiments with the photosynthetic organisms, Euglena sp. and Chlamydomonas reinhardtii, we propose a scenario where photosynthesis, and ultimately the oxygenation of the atmosphere, depended on the protection of early microbes by nanophase ferric oxides/oxyhydroxides.

As part of a historic remediation project, approximately 61 km 2 of salt evaporation ponds in the southern portion of San Francisco Bay, CA (USA) are scheduled for restoration to natural tidal marsh habitat over the next several decades. We have investigated the correlation of remotely sensed infrared spectral information with in situ field measurements and sampling, and evaluated the usefulness of a remote sensing approach to monitor salinity and population distributions of microbial communities in the hypersaline ponds. The Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) instrument operated by the Jet Propulsion Laboratory has created a ten-year archive of spectral information concerning these ponds. We utilized spectral signatures of microbial populations that are sensitive to salinity, and trained a supervised classification algorithm to identify physical parameters from an AVIRIS scene based upon microbe spectra gathered in the field using a portable visible to near-infrared (VNIR) spectrometer. Our results indicate that automated analyses of hyperspectral observations are capable of detecting variations in microbial populations and discriminating corresponding salinity levels.