The perception of the physico-chemical limits of life has expanded considerably in recent years. Extremophilic microorganisms, which are capable of thriving in the presence of one or more factors once considered inimical to life, where discovered around the globe. An example are extremely halophilic archaebacteria (haloarchaea), which were isolated by our group from 250 million year old Alpine salt deposits. We attempt to elucidate the mechanisms for their desiccation tolerance and apparent longevity.

Haloarchaea are found today in hypersaline surface waters, such as brines in solar salterns. However, from Alpine rock salt of Permo-Triassic age several halophilic strains were isolated, which were identified as novel members of the genus Halococcus. In addition, a wide range of haloarchaeal 16SrRNA genes were detected in Alpine rock salt. Such data support the hypothesis of a long survival of these microorganisms. There is also evidence for halophilic phages in salt sediments (unpublished); thus, a genuine microbial community appears present in rock salt. Archaea are not known to produce spores or similar resting stages; it remains thus enigmatic how they stayed viable over the millennia. We are characterizing the haloarchaeal community in Alpine rock salt, in order to identify those species which are apparently capable of long-term survival.

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We are determining the growth-limiting ranges of several physico-chemical factors, including low water activity, low pressure and temperature extremes, with a concomitant analysis of the haloarchaeal responses. Morphological changes, membrane potential, energy charge and DNA damage of stressed haloarchaea are investigated. Archaea are not known to produce spores, but other types of resting stages, such as nanocells, may be formed under starvation conditions. A promising approach appears the analysis of the haloarchaeal proteome, since pronounced alterations of several proteins were observes, when an unknown factor from rock salt was added to growth media.

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The detection of extraterrestrial halite and water in meteorites and in the subsurface ocean on Europa prompted an inclusion of a specific search for halophilic microorganisms in the search for life in outer space. There is a need to develop procedures for the reliable identification of life forms, if any, in future return samples and to deal with the problem of potential contaminations. We are participating in international efforts regarding these issues.