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

The discovery of polygonal terrain on Mars underlain by ice heightens interest in the possibility that this water-bearing habitat may be, or may have been, a suitable habitat for extant life. The possibility is supported by the recurring detection of terrestrial microorganisms in subsurface ice environments, such as ice wedges found beneath tundra polygon features. A characterization of the microbial community of ice wedges from the high Arctic was performed to determine whether this ice environment can sustain actively respiring microorganisms and to assess the ecology of this extreme niche. We found that ice wedge samples contained a relatively abundant number of culturable cells compared to other ice habitats (∼10(5) CFU·mL(-1)). Respiration assays in which radio-labeled acetate and in situ measurement of CO(2) flux were used suggested low levels of microbial activity, though more sensitive techniques are required to confirm these findings. Based on 16S rRNA gene pyrosequencing, bacterial and archaeal ice wedge communities appeared to reflect surrounding soil communities. Two Pseudomonas sp. were the most abundant taxa in the ice wedge bacterial library (∼50%), while taxa related to ammonia-oxidizing Thaumarchaeota occupied 90% of the archaeal library. The tolerance of a variety of isolates to salinity and temperature revealed characteristics of a psychrotolerant, halotolerant community. Our findings support the hypothesis that ice wedges are capable of sustaining a diverse, plausibly active microbial community. As such, ice wedges, compared to other forms of less habitable ground ice, could serve as a reservoir for life on permanently cold, water-scarce, ice-rich extraterrestrial bodies and are therefore of interest to astrobiologists and ecologists alike. .

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