Abstract
Life can persist under severe osmotic stress and low water activity in hypersaline environments. On Mars, evidence for the past presence of saline bodies of water is prevalent and resulted in the widespread deposition of sulfate and chloride salts. Here we investigate Spotted Lake (British Columbia, Canada), a hypersaline lake with extreme (>3 M) levels of sulfate salts as an exemplar of the conditions thought to be associated with ancient Mars. We provide the first characterization of microbial structure in Spotted Lake sediments through metagenomic sequencing, and report a bacteria-dominated community with abundant Proteobacteria, Firmicutes, and Bacteroidetes, as well as diverse extremophiles. Microbial abundance and functional comparisons reveal similarities to Ace Lake, a meromictic Antarctic lake with anoxic and sulfidic bottom waters. Our analysis suggests that hypersaline-associated species occupy niches characterized foremost by differential abundance of Archaea, uncharacterized Bacteria, and Cyanobacteria. Potential biosignatures in this environment are discussed, specifically the likelihood of a strong sulfur isotopic fractionation record within the sediments due to the presence of sulfate reducing bacteria. With its high sulfate levels and seasonal freeze-thaw cycles, Spotted Lake is an analog for ancient paleolakes on Mars in which sulfate salt deposits may have offered periodically habitable environments, and could have concentrated and preserved organic materials or their biomarkers over geologic time.
Highlights
Hypersaline environments impose severe stresses on microorganisms, such as high osmotic pressures and potentially low water activities (Grant, 2004)
Because salt and other ion levels fluctuate dramatically throughout the year in the water column (Figure S2) with impact on precipitation rates and pore water concentrations in the sediment, we expect that community composition in Spotted Lake sediments may vary seasonally, similar to the archaeal abundances in Lake Tyrrell, Victoria, Australia (Podell et al, 2014) or the relative abundance of Proteobacteria and Cyanobacteria in the waters of nearby Hot Lake, Washington (Crisler et al, 2012; Lindemann et al, 2013)
We have shown that Spotted Lake sediments are inhabited by extremely diverse, mostly anaerobic organisms, with low levels of Archaea and a near absence of detected DNA viruses
Summary
Hypersaline environments impose severe stresses on microorganisms, such as high osmotic pressures and potentially low (aw ∼0.75) water activities (Grant, 2004). A saturated MgSO4 solution has an aw = 0.85 (Ha and Chan, 1999), too low for most bacteria to survive but is habitable to some eukaryotes (Stevenson et al, 2015). At such high salinities, the ionic strength of a solution can become a problem for microorganisms, where a high charge density can perturb cellular activities (Fox-Powell et al, 2016). The habitability of a saline environment relies heavily on water activity, a function of the ionic composition and concentration of the brine
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