The isolation of viable extremely halophilic archaea from 250-million-year-old rock salt suggests the possibility of their long-term survival under desiccation. Since halite has been found on Mars and in meteorites, haloarchaeal survival of martian surface conditions is being explored. Halococcus dombrowskii H4 DSM 14522(T) was exposed to UV doses over a wavelength range of 200-400 nm to simulate martian UV flux. Cells embedded in a thin layer of laboratory-grown halite were found to accumulate preferentially within fluid inclusions. Survival was assessed by staining with the LIVE/DEAD kit dyes, determining colony-forming units, and using growth tests. Halite-embedded cells showed no loss of viability after exposure to about 21 kJ/m(2), and they resumed growth in liquid medium with lag phases of 12 days or more after exposure up to 148 kJ/m(2). The estimated D(37) (dose of 37 % survival) for Hcc. dombrowskii was > or = 400 kJ/m(2). However, exposure of cells to UV flux while in liquid culture reduced D(37) by 2 orders of magnitude (to about 1 kJ/m(2)); similar results were obtained with Halobacterium salinarum NRC-1 and Haloarcula japonica. The absorption of incoming light of shorter wavelength by color centers resulting from defects in the halite crystal structure likely contributed to these results. Under natural conditions, haloarchaeal cells become embedded in salt upon evaporation; therefore, dispersal of potential microscopic life within small crystals, perhaps in dust, on the surface of Mars could resist damage by UV radiation.

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A simple method has been worked out for measuring the biologically effective dose (BED) of solar radiation. The method uses phage T7 as a biosensor and it includes field measurements of global and direct UV radiation from the sun in the air; it has been applied to underwater measurements as well. Results of field measurements are presented, with discussion of the angle-dependent sensitivity of the biosensor. A model of spectral irradiance based on the measured values is presented. Relevance of the HT7 unit-derived earlier by us from T7 phage inactivation upon UV radiation-as a measure of the BED is also discussed.