Relationships between sucretolerance and salinotolerance in bacteria from hypersaline environments and their implications for the exploration of Mars and the icy worlds

Abstract

The most extremely osmotolerant microbial isolates are fungi from high-sugar environments that tolerate the lowest water activity (0.61) for growth yet reported. Studies of osmotolerant bacteria have focused on halotolerance rather than sucretolerance (ability to grow in high sugar concentrations). A collection of salinotolerant (≥10% NaCl or ≥50% MgSO4) bacterial isolates from the Great Salt Plains of Oklahoma and Hot Lake in Washington were screened for sucretolerance in medium supplemented with ≥50% fructose, glucose or sucrose. Tolerances significantly differed between solutes, even though water activities for saline media (0.92 and 0.85 for 10 and 20% NaCl Salt Plains media, respectively) were comparable or lower than water activities for high-sugar media (0.93 and 0.90 for 50 and 70% sucrose artificial nectar media, respectively). These specific solute effects were differentially expressed among individual isolates. Extrapolating the results of earlier food science studies with yeasts at high sugar concentrations to bacteria in salty environments with low water activity should be done with caution. Furthermore, the discussion of habitable Special Regions on Mars and the icy worlds should reflect an understanding of specific solute effects.

Prevalence of sucretolerant bacteria in common soils and their isolation and characterization

“…Most of the isolates among the 18 taxa observed were within the Gammaproteobacteria , including Pantoea and Pseudomonas . Nearly all of the isolates grew in media containing 30% sucrose, although it should be noted that this concentration does not change water activity (0.96) enough to be restrictive for many bacteria (Fredsgaard et al 2017). …”

“…Microbial growth tolerances to different solutes vary in a manner that is not easily described by a single physicochemical property such as water activity (Fredsgaard et al 2017). The concept of specific solute effects is valuable in this context, where physiological responses to solutes reflect a more complex interplay of parameters that are specific to individual solutes (Scott 1953; Beuchat 1981; Chirife 1994; Chirife et al 1995).…”

“…The development of artificial nectar media with salts (ANS) has been described previously (Fredsgaard et al 2017). ANS series media contained at least 0.5% (w/v) of each fructose, glucose, and sucrose.…”

“…In addition, responses can differ between isolates that cluster together phylogenetically. While arguments have been made for the central role of water activity or chaotropicity in microbial growth, it appears that cellular responses are more complex than can be described by a single physical parameter (Williams and Hallsworth 2009; Cray et al 2013; Fredsgaard et al 2017). The concept of specific solute effects was proposed by Scott (1953) to explain the observation that the inhibition of microbial growth in highly osmotic media was not due to solute concentration and water activity alone (Scott 1953; Beuchat 1981; Chirife 1994; Chirife et al 1995).…”

“…For instance, responses to high concentrations of sugars are not the same as responses to high concentrations of salts for salinotolerant (and hence osmotolerant) bacteria. For many of our bacterial isolates from hypersaline environments, growth tolerance is greater to sugars than to salts (including Nesterenkonia and Planococcus ), while other isolates are more tolerant to salts than sugars (including Bacillus, Halomonas, and Planococcus ), or equally tolerant (or intolerant) to both (including Marinococcus ) (Fredsgaard et al 2017). …”