Physicochemical Parameters Limiting Growth of Debaryomyces hansenii in Solutions of Hygroscopic Compounds and Their Effects on the Habitability of Martian Brines.

Jacob Heinz,Vita Rambags,Dirk Schulze-Makuch

doi:10.3390/life11111194

Abstract

The availability of liquid water is a prerequisite for all lifeforms on Earth. In hyperarid subzero environments like the Dry Valleys in Antarctica or the near-subsurface of Mars liquid water might be provided temporarily by hygroscopic substances that absorb water from the atmosphere and lower the freezing point of water. To evaluate the potential of hygroscopic compounds to serve as a habitat, it is necessary to explore the microbial tolerances towards these substances and their life-limiting properties. Here we present a study investigating the tolerances of the halotolerant yeast Debaryomyces hansenii to various solutes. Growth experiments were conducted via counting colony forming units (CFUs) after inoculation of a liquid growth medium containing a specific solute concentration. The lowest water activities (aw) enabling growth were determined to be ~0.83 in glycerol and fructose-rich media. For all other solutes the growth-enabling aw was higher, due to additional stress factors such as chaotropicity and ionic strength. Additionally, we found that the solute tolerances of D. hansenii correlate with both the eutectic freezing point depressions and the deliquescence relative humidities of the respective solutes. Our findings strongly impact our understanding of the habitability of solute-rich low aw environments on Earth and beyond.

Highlights

Earth harbors a tremendous variety of microbial biotopes ranging from carbon-rich tropical forest soils with a very high biomass [1] to extreme habitats like hyperarid polar deserts comprising highly specialized microbial communities [2]
In the first sample, containing 4.0 m NH4NO3 Ca(NO3)2 Mg(NO3)2 (NH4)(SO4)2, robust growth was detected without a notable lag phase, while in the second sample, containing 4.5 m NH4(SO4)2 and having an aw of 0.864 (Table 1), a long lag phase of more than 10 days and a reduced growth rate indicated that the salt concentration was close to the actual maximum NH4(SO4)2 concentration tolerated by D. hansenii
We found evidence for three parameters limiting the survival of D. hansenii at high solute concentrations

Summary

Introduction

Earth harbors a tremendous variety of microbial biotopes ranging from carbon-rich tropical forest soils with a very high biomass [1] to extreme habitats like hyperarid polar deserts comprising highly specialized microbial communities [2]. A promising putative habitat might be environments which include hygroscopic compounds that can attract water from the atmosphere and form concentrated liquid solutions via a process called deliquescence [3]. The advantage of these water absorbing substances is that they can provide liquid water essential for life even in places of hyperaridity, as has been described for halophilic cyanobacteria thriving in salt crusts in the Atacama Desert, Chile [4]. The lowest temperature at which a solution can stay liquid is called eutectic temperature. Even gases can act as hygroscopic agents like ammonia (NH3), which has been hypothesized to occur in a subzero H2O-NH3 ocean beneath the surface of Saturn’s moon, Titan [8]

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