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The existence of ammonia on Titan and Enceladus and potentially other outer Solar System bodies raises questions about the habitability of these worlds. Of particular concern for planetary protection is the possibility for Earth microbes inadvertently transported on spacecraft, to survive or even reproduce in these 'hostile' ammonia/ammonia-water environments. Our current understanding of microbial tolerances to ammonia is incomplete and the limits for microbial survival, growth or reproduction have not been established. While some fungi can not only survive but also grow in atmospheres containing 95% N H 3 (1) elsewhere the growth of microorganisms may be inhibited at concentrations of < 1%. The conversion of N H 3 to nitrite by the ubiquitous terrestrial and aquatic ammonia-oxidizing bacteria is a key step in the global nitrogen cycle, and such organisms may potentially possess some of the highest tolerances to ammonia among the bacteria. One of the key factors influencing microbial survival at high ammonia concentrations is pH, determining the ionization of N H 3 to N H + 4 , with N H 3 the more toxic and more abundant form at high pH. Our initial laboratory experiments with Actinobacteria and Proteobacteria strains have demonstrated the lethality of NH3 at concentrations of up to 17.5% at room temperature. However, our findings suggest loss of viability was most likely an effect of high pH rather than a direct N H 3 -specific effect. However, exposure of soil samples (which contain many spore-forming organisms typically found on spacecraft surfaces) showed the survival and subsequent prolific growth of soil bacteria belonging to the Bacillus genus following NH3 exposure over more prolonged periods, also at room temperature at 17.5% N H 3 . The effect of a challenge of low temperature extremes in concert with ammonia is also pertinent to determining the survival and reproductive possibilities in extraterrestrial settings. Soil exposed for six days to concentrations of up to 35% N H 3 at −80 • C resulted in the isolation of viable bacteria at numbers lower than obtained at room temperature exposure. At both temperatures, viable numbers were reduced compared to the 0% NH3 controls at neutral pH and at pH 13.5 (the pH of a 35% N H 3 solution), with viability at −80 • C lower than that at
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