Survival of microorganisms under the extreme conditions of the Atacama Desert.

Spores of Bacillus subtilis, conidia of Aspergillus niger, versicolor and ochraceus and cells of Deinococcus radiodurans have been exposed in the dark at two locations (at about 23 degrees S and 24 degrees S) in the Atacama Desert for up to 15 months. B. subtilis spores (survival approximately 15%) and A. niger conidia (survival approximately 30%) outlived the other species. The survival of the conidia and spores species was only slightly poorer than that of the corresponding laboratory controls. However, the Deinococcus radiodurans cells did not survive the desert exposure, because they are readily inactivated at relative humidities between 40 and 80% which typically occur during desert nights. Cellular monolayers of the dry spores and conidia have in addition been exposed to the full sun light for up to several hours. The solar fluences causing 63% loss in viability (F37-values) have been determined. These F37-values are compared with those determined at other global locations such as Punta Arenas (53 degrees S), Key Largo (25 degrees N) or Mainz (50 degrees N) during the same season. The solar UVB radiation kills even the most resistant microorganisms within a few hours due to DNA damages. The data are also discussed with respect to possible similarities between the climatic conditions of the recent Atacama Desert and the deserts of early Mars.

Biochemical constraints for survival under Martian conditions.

A wide variety of terrestrial organisms, the so-called "anhydrobiotes," has learned to survive in a state of extreme dehydration in dry environments. Strategies for survival include the accumulation of certain polyols and nonreducing saccharides, which help to prevent damage to membranes and proteins, but at low water partial pressure DNA is also progressively damaged by various lesions, including strand breaks and cross-linking to proteins. These lesions, if they are not too numerous, can be repaired before the first replication step after rehydration, but long-term exposure to dry conditions finally diminishes the chances of survival as these lesions accumulate. If an organism has no chance to repair the accumulated DNA damage during intermittent periods of active life, survival will not exceed a few decades. The restriction of survival by dryness-induced DNA lesions is corroborated by new data on conidia of Aspergillus and the free plasmid pBR 322. Our results will be discussed with respect to the chance of finding dormant life or biochemical fossils on the surface of Mars.

ERA-experiment "Space Biochemistry".

The general goal of the experiment was to study the response of anhydrobiotic (metabolically dormant) microorganisms (spores of Bacillus subtilis, cells of Deinococcus radiodurans, conidia of Aspergillus species) and cellular constituents (plasmid DNA, proteins, purple membranes, amino acids, urea) to the extremely dehydrating conditions of open space, in some cases in combination with irradiation by solar UV-light. Methods of investigation included viability tests, analysis of DNA damages (strand breaks, DNA-protein cross-links) and analysis of chemical effects by spectroscopic, electrophoretic and chromatographic methods. The decrease in viability of the microorganisms was as expected from simulation experiments in the laboratory. Accordingly, it could be correlated with the increase in DNA damages. The purple membranes, amino acids and urea were not measurably effected by the dehydrating condition of open space (in the dark). Plasmid DNA, however, suffered a significant amount of strand breaks under these conditions. The response of these biomolecules to high fluences of short wavelength solar UV-light is very complex. Only a brief survey can be given in this paper. The data on the relatively good survival of some of the microorganisms call for strict observance of COSPAR Planetary Protection Regulations during interplanetary space missions.

Survival in extreme dryness and DNA-single-strand breaks.

A wide variety of organisms (the so-called "anhydrobiotes') is able to survive long periods of time in a state of utmost dehydration and can thus survive in extremely dry environments including artificially imposed or space vacuum. Known strategies of survival include the accumulation of certain polyols, especially disaccharides, which help prevent damage to membranes and proteins. Here we report that DNA in vacuum-dried spores is damaged to a very substantial degree by processes leading to DNA strand breaks. Most of these lesions are obviously repaired during germination, but extensive damage to DNA and enzymes after long exposure times (months to years) finally diminish the chances of survival.

Extreme dryness and DNA-protein cross-links.

Exposure of fungal conidia (Aspergillus ochraceus) or spores of Bacillus subtilis to extreme dryness or vacuum induces DNA lesions, including strand breaks and the formation of DNA-protein cross-links. In wet cells only a small amount of protein is bound to DNA, but exposure to conditions of lowered water activity results in an increasing number of cross-links between DNA and proteins. In fungal conidia these cross-links are detected after selective iodination (125 J) of the DNA-bound proteins followed by gel electrophoresis and subsequent autoradiography. Another approach is the labelling of DNA with 32P by means of nick translation and the detection of differences in the electrophoretic mobility of DNA before and after digestion with proteinase K of proteins bound to DNA.

DNA-strand breaks limit survival in extreme dryness.

The inactivation of the anhydrobiotic organisms Bacillus subtilis (spores) and Deinococcus radiodurans during long-term exposure (up to several weeks) to extreme dryness (especially vacuum) is correlated with an increase in the number of DNA-strand breaks and other DNA lesions. Survival finally depends on the repair of DNA damages. Exposure of anhydrobiotic organisms to extreme dryness (e.g. on Mars or in space) for geological times will lead to so extended DNA lesions that recovery is extremely unlikely.