A New Record for Microbial Perchlorate Tolerance: Fungal Growth in NaClO4 Brines and its Implications for Putative Life on Mars

Abstract

The habitability of Mars is strongly dependent on the availability of liquid water, which is essential for life as we know it. One of the few places where liquid water might be found on Mars is in liquid perchlorate brines that could form via deliquescence. As these concentrated perchlorate salt solutions do not occur on Earth as natural environments, it is necessary to investigate in lab experiments the potential of these brines to serve as a microbial habitat. Here, we report on the sodium perchlorate (NaClO4) tolerances for the halotolerant yeast Debaryomyces hansenii and the filamentous fungus Purpureocillium lilacinum. Microbial growth was determined visually, microscopically and via counting colony forming units (CFU). With the observed growth of D. hansenii in liquid growth medium containing 2.4 M NaClO4, we found by far the highest microbial perchlorate tolerance reported to date, more than twice as high as the record reported prior (for the bacterium Planococcus halocryophilus). It is plausible to assume that putative Martian microbes could adapt to even higher perchlorate concentrations due to their long exposure to these environments occurring naturally on Mars, which also increases the likelihood of microbial life thriving in the Martian brines.

Exploiting a perchlorate-tolerant desert cyanobacterium to support bacterial growth for in situ resource utilization on Mars

“…For example, halophilic Archaea of the family Halobacteriaceae and Halomonas elongata showed a weak growth in 600 mM Na-perchlorate (Oren et al ., 2014), the growth of the archaeon Halorubrum lacusprofundi was still measurable in 800 mM perchlorate ions (Laye and DasSarma, 2018), while Hydrogenothermus marinus did not replicate in 400 mM Na-perchlorate (Beblo-Vranesevic et al ., 2017). So far the highest perchlorate tolerance was reported for the yeast Debaryomyces hansenii growing in 2.4 M Na-perchlorate (Heinz et al ., 2020), a value twice that of the bacterium Planococcus halocryophilus (Heinz et al ., 2019).…”

Physicochemical Salt Solution Parameters Limit the Survival of Planococcus halocryophilus in Martian Cryobrines

“…Not much is known about the microbial toxicity of dissolved perchlorate ions. In fact, dissimilatory perchlorate reduction is a common metabolic pathway in bacteria (Bardiya and Bae, 2011) and archaea (Liebensteiner et al, 2013), and perchlorate can be tolerated in high concentrations by eukaryotes, e.g., the halotolerant yeast Debaryomyces hansenii can tolerate 2.4 M NaClO 4 (Heinz et al, 2020). Furthermore, dissolved perchlorate ions are relatively inert and non-oxidizing due to kinetic barriers (Urbansky, 1998).…”

“…Furthermore, it has the highest bacterial perchlorate tolerance (13.6 wt/vol% NaClO 4 at +25 • C) reported to date (Heinz et al, 2019). Only some fungi are known to tolerate higher perchlorate concentrations (Heinz et al, 2020). Hence, this halo-and cryotolerant organism is highly suitable as a model organism for studying the habitability of Martian cryobrines and was therefore used in this study.…”

Survival and growth of soil microbial communities under influence of sodium perchlorates

“…It has been revealed that halophilic bacteria, isolated from various saline natural environments, have the ability to reproduce on culture media containing 1% perchlorate mixture simulating the composition of Mars regolith (Al Soudi et al ., 2017). The growth of halotolerant bacteria Halomonas venusta and Planococcus halocryophilus has been shown on liquid media containing 12.2 and 13.6% sodium perchlorate, respectively (Al Soudi et al ., 2017; Heinz et al ., 2019, 2020). It has been demonstrated that bacteria could develop on media simulating the salt composition of Martian soil containing perchlorates at a concentration of 0.5% (Nicholson et al ., 2012), in brines containing a mixture of sulphates and perchlorates with a concentration of the latter up to 3.7% (Beblo-Vranesevic et al ., 2017 b ), as well as maintain viability after short-term (15 min) exposure to sodium perchlorate with concentration up to 43% (Beblo-Vranesevic et al ., 2017 a , 2020).…”

“…To date, the maximum concentration of sodium perchlorate which allows microbial cells proliferation in vitro is considered to be 9.8% for Archaea ( Halorubrum lacusprofundi ), 13.6% for Bacteria ( P. halocryophilus ) and 29.9% for micromycetes ( Debaryomyces hansenii ) (Heinz et al ., 2020).…”