Halophiles on Mars: Insights into the biophysical limits for terrestrial life

Halophiles on Mars

John E. Hallsworth
Institute for Global Food Security, School of Biological Sciences, MBC, Queen’s University Belfast, Belfast, BT9 7BL, Northern Ireland.

The main environmental parameters which define habitability for halophiles relate to the thermodynamic availability of water and conditions which facilitate biologically permissive stability and flexibility of macromolecular systems. Our recent studies have focused on (i) water activity and the spontaneous formation of aqueous thin films; (ii) temperature and chao-/kosmotropicity; and (iii) ionic strength in relation to the ecophysiology of halophiles and comparator microbes. Even the most halophilic bacteria and Archaea have been considered to have a water-activity limit equivalent to that of saturated NaCl; i.e. 0.755. However, by using combinations of MgCl2 plus other salts, which enable lower water-activity values, we found that some halophiles grow in the range 0.693-0.635 water activity (e.g. Halococcus salifodinae, Halobacterium noricense, Natrinema pallidum, and haloarchaeal strains GN-2 and GN-5). Extrapolations suggest theoretical minima close to the long-accepted water-activity limit for fungal xerophile, i.e. 0.605 [1]. We also studied the xerophile/halophile Aspergillus penicilliodes, however, and have demonstrated differentiation and cell division down to 0.585 water activity (under high-glycerol conditions) [3]. An ancient fossilized biofilm of Archaean microbes, discovered in South Africa [1;2], reveals that halophiles were growing in the presence of magnesium (and other minerals) around 3.33 billion years ago, suggesting that early life may have been exceptionally xerophilic [1]. Other recent evidence suggests that microbial cells are sensitive to changes in water activity of as little as +/-0.001 [1;4]. Salts can, however, also deliquesce and can thereby create thin films, invisible to the naked eye but able to supply water to microbial communities; thin-film formation was visualized using images obtained by environmental scanning electron microscopy to create a movie [1]. The robust stress biology of halophiles such as Haloquadratum walsbyi, Salinibacter ruber and Dunaliella salina is associated with their ability to evade protozoan grazers and dominate the microbial communities found in solar salterns; we have described the biology of such species in relation to their ecology as microbial weeds in diverse terrestrial habitats [5;6]. Halophiles have also been found in extraterrestrial locations; on spacecraft ([7] and refs therein) and presumably, therefore, may contaminate extraterrestrial environments. Brines and/or are known to exist on Europa, Enceladus, Mars and in other parts of the Solar System. Our studies of eukaryotic extremophiles revealed that chaotropic substances, including some salts, which can enhance the flexibility of macromolecular systems during cold conditions, reducing the temperature minima for growth by as much as 5 to 10°C [8;9]. Chaotropic substances are those which entropically disorder biomacromolecules, and should not be defined according to their (presumed) interactions with solvent water [10;11]; MgCl2 and some other salts can simultaneous impose chaotrope-induced and osmotic stress on microbial cells [12]. Many chaotropic salts, including MgCl2 and perchlorates, are present in the regolith Mars; evidence also suggests that brines and aqueous thin films are relatively commonplace [1;7;8]. Whereas martian temperatures are frequently in the range -40 to -60°C, considerably higher temperatures have also been recorded. The lower limit for microbial cell division on Earth is around -18°C, but there are indications of metabolic activity of various species in the range -20 to -40°C (for references, see [7]). Furthermore, such studies were not carried out in the presence of chaotropes, so it has yet to be established whether some extremophiles or polyextremophiles can colonize potential habitats on Mars [1;7]. Our studies of terrestrial systems show that chaotropicity interacts with temperature, water activity and kosmotropicity as a determinant for microbial life. These include studies of high-MgCl2, deep-sea basins [13;14], and microbes in industrial and laboratory systems [8;9;12;15]. Chaotropicity also plays a role in pathogenic processes and other trophic interactions [5;16;17]. Further studies were carried out, in collaboration with the Charles S. Cockell group, which indicate interactions between iron- or oxygen availability and NaCl- or temperature tolerance in Halomonas hydrothermalis [18] and recreated the chemically diverse brines that would have existed on early Mars [19]; these brines were then inoculated with halophile communities. This eliminated water activity, chaotropicity or kosmotropicity as the primary determinant(s) for habitability of these brines. The determinant was rather, ionic strength; a hitherto unidentified life-limiting parameter. The implications of such findings are discussed for terrestrial microbial systems.
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