Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life

Abstract

SummarySince a key requirement of known life forms is available water (water activity; a w), recent searches for signatures of past life in terrestrial and extraterrestrial environments have targeted places known to have contained significant quantities of biologically available water. However, early life on Earth inhabited high-salt environments, suggesting an ability to withstand low water-activity. The lower limit of water activity that enables cell division appears to be ∼ 0.605 which, until now, was only known to be exhibited by a single eukaryote, the sugar-tolerant, fungal xerophile Xeromyces bisporus. The first forms of life on Earth were, though, prokaryotic. Recent evidence now indicates that some halophilic Archaea and Bacteria have water-activity limits more or less equal to those of X. bisporus. We discuss water activity in relation to the limits of Earth's present-day biosphere; the possibility of microbial multiplication by utilizing water from thin, aqueous films or non-liquid sources; whether prokaryotes were the first organisms able to multiply close to the 0.605-a w limit; and whether extraterrestrial aqueous milieux of ≥ 0.605 aw can resemble fertile microbial habitats found on Earth.

The Effect of Cell Immobilization by Calcium Alginate on Bacterially Induced Calcium Carbonate Precipitation

“…It is known that different genera of bacteria have different water activity limits and solute tolerance [17]. Each bacterium has a very specific and narrow water activity for optimum metabolism and growth [18]. More recent studies show that Bacilli and closely related bacteria are moderately xerotolerant [19,20].…”

Reduction of the Temperature Sensitivity of Halomonas hydrothermalis by Iron Starvation Combined with Microaerobic Conditions

“…The physicochemical boundaries beyond which multiplication of all microorganisms is prevented are imposed by low water activity, extremes of temperature (approximately Ϫ20 to 120°C), and other situations, including high pH (Ͼ12), chaotropicity, and oxidative damage (e.g., due to UV radiation) (8)(9)(10). The stress mechanism for a number of these parameters involves changes in the entropic status of lipid bilayers and other macromolecular systems (see reference 9 and citations therein).…”

“…Detailed investigations of this topic are required to determine the limits of the biosphere under conditions that account for the physicochemical complexity of extreme environments on Earth, not only with reference to the minimal and maximal boundaries for life but also within the entire range for cell division (8). Moreover, we know relatively little about the impacts of small fluctuations in environmental conditions on rates of microbial multiplication and on the level of cellular sensitivity to water activity and other fundamental parameters (10).…”

A Halophilic Bacterium Inhabiting the Warm, CaCl 2 -Rich Brine of the Perennially Ice-Covered Lake Vanda, McMurdo Dry Valleys, Antarctica

“…Water activity, per se, does not seem to be a serious problem in the Lake Vanda brine. Our calculated water activity for the Lake Vanda brine indicates that it is well above the water activities of other Ca-rich brines (Table 1) and the minimum water activity known to support microbial growth (44)(45)(46). However, the Lake Vanda brine confers more than just ionic effects because its major saltsCaCl 2 and MgCl 2 -are highly chaotropic (47).…”

“…However, the deep Mediterranean brines are nearly devoid of CaCl 2 , and most have high levels (3 to 4 M) of NaCl; hence, the chaotropic effects of MgCl 2 are offset somewhat by the kosmotropic effects of NaCl (47,50). Notably, however, the Discovery brine is an exception in this regard, as it contains low levels of NaCl; in terms of chaotropicity and water potential, the characteristics of this highly unusual brine are probably at or very near the limits of those that can support life (45)(46)(47)(48)(49)(50).…”