Is there a common water-activity limit for the three domains of life?

<p>Archaea and Bacteria constitute a majority of life systems on Earth but have long been considered inferior to Eukarya in terms of solute tolerance. Whereas the most halophilic prokaryotes are known for an ability to multiply at saturated NaCl (water activity (a_w) 0.755) some xerophilic fungi can germinate, usually at high-sugar concentrations, at values as low as 0.650–0.605 a_w. Here, we present evidence that halophilic prokayotes can grow down to water activities of <0.755 for <i>Halanaerobium lacusrosei</i> (0.748), <i>Halobacterium</i> strain 004.1 (0.728), <i>Halobacterium</i> sp. NRC-1 and <i>Halococcus morrhuae</i> (0.717), <i>Haloquadratum walsbyi</i> (0.709), <i>Halococcus salifodinae</i> (0.693), <i>Halobacterium noricense</i> (0.687), <i>Natrinema pallidum</i> (0.681) and haloarchaeal strains GN-2 and GN-5 (0.635 <i>a</i>_w). Furthermore, extrapolation of growth curves (prone to giving conservative estimates) indicated theoretical minima down to 0.611 <i>a</i>_w for extreme, obligately halophilic Archaea and Bacteria. These were compared with minima for the most solute-tolerant Bacteria in high-sugar (or other non-saline) media (<i>Mycobacterium</i> spp., <i>Tetragenococcus halophilus</i>, <i>Saccharibacter floricola</i>, <i>Staphylococcus aureus</i> and so on) and eukaryotic microbes in saline (<i>Wallemia</i> spp., <i>Basipetospora halophila</i>, <i>Dunaliella</i> spp. and so on) and high-sugar substrates (for example, <i>Xeromyces bisporus</i>, <i>Zygosaccharomyces rouxii</i>, <i>Aspergillus</i> and <i>Eurotium</i> spp.). We also manipulated the balance of chaotropic and kosmotropic stressors for the extreme, xerophilic fungi <i>Aspergillus penicilloides</i> and <i>X. bisporus</i> and, via this approach, their established water-activity limits for mycelial growth (~0.65) were reduced to 0.640. Furthermore, extrapolations indicated theoretical limits of 0.632 and 0.636 a_w for <i>A. penicilloides</i> and <i>X. bisporus</i>, respectively. Collectively, these findings suggest that there is a common water-activity limit that is determined by physicochemical constraints for the three domains of life.</p>