Understanding Brines and Habitability, on Earth and Beyond
The majority of hypersaline systems on Earth are derived from seawater (thalassohaline) and thus much of our knowledge, as well as our instrumentation, is based around NaCl systems. A body of water is considered to be hypersaline when it exceeds 35 ppt (g/L). The saltiest surficial body of water on Earth is Don Juan Pond in Antarctica - a CaCl2 brine located in the McMurdo Dry Valleys with salinities reaching 400 g/L. Brines that are not derived from seawater (athalassohaline), are all endorheic (closed basin) systems, and are a product of dissolution of the regional geology. These systems are generally alkaline soda or sulfate lake systems, but can also be highly acidic, especially in volcanic regions. Finally, there is another class of hypersaline system on Earth - so called deep hypersaline anoxic brines (DHABs). These subsurface brine systems result from the dissolution of exposed evaporite lithologies to seawater, causing the creation of a lens of saturated brine on the ocean floor. These DHABs are some of the most extreme environments on Earth, with some which are currently thought to be uninhabitable, e.g. Discovery Brine, Mediterranean (see Fisher et al. 2021).
In the solar system, brines represent some of the best locations for potential life detection, representing the only know instances of liquid water beyond Earth. The presence of extensive evaporite deposits on Mars point towards the presence of saline lakes in the planet's past, many of which were magnesium sulfate. Today it is hypothesized that the Martian near-surface environment likely hosts brines, possibly dominated by perchlorate. In the outer solar system, Jupiter's and Saturn's moons, Europa and Enceladus, host liquid saline oceans and provide an exciting avenue towards the discovery of putative non-Earth standard life.
Enceladus. Credit: NASA/JPL
When discussing habitability in brine systems, there are a few terms that should be understood:
Ionic Strength: The ability of a solution to shield two charges from each other, and is equal to the sum of the molar concentrations of all ions in solution, plus the square of their charge. This shielding can result in a serious disruption of microbial membrane processes, and pose a limit to life (e.g. Fox-Powell et al. 2016).
Water Activity: The thermodynamic availability of water in a given system, where pure water has a value of 1, and this value decreases as solute concentration increases. The majority of microorganisms exist at a water activity < 0.9. Below this value the microorganisms that are capable of persisting are generally specialists in both salt and desiccation tolerance. The most resistant organisms are fungi, capable of surviving at water activities of 0.605 (Stevenson et al. 2015). Water activity is a complex metric, resulting from ion-ion interactions, both with each other and with water, which is especially prevalent at concentrations >100 mM. Water activity is frequently used as the primary metric for discussions of habitability in aqueous systems, however, some caution should be used as not all water activities are equally habitable - where the ionic composition can result in uninhabitable conditions at higher water activities. For example, an NaCl brine with a water activity of 0.67 is known to be habitable, however, an MgCl2 solution at the same water activity is not (Hallsworth et al. 2007).
Chaotropicity: The measure of the destabilizing effect which an ion has on the physical structure of both water and biological molecules. Anions tend to have the greatest stabilizing (kosmoptropic) or destabilizing (chaotropic) effects in solution, and are ordered according to the Hoffmeister series (Hoffmeister, 1888). Cations are a more complicated topic: below they are ordered based on their destabilizing effects on water, but to understand their effects on biological molecules, they must be flipped. Moreover, due to the difference in the size of ionic radii, some ions are capable of interacting more closely with the backbone of various biological molecules, and so the effects are not necessarily consistent. In multi-ion brine systems, at higher salinities, ion-ion interactions become complex and it becomes more difficult to predict the chaotropic effects of the environment. For example, in MgSO4 systems like the Basque Lakes (see Biosignatures), sulfate is a stabilizing anion for both water and biology, Mg at high concentrations, however, can be highly disruptive for microorganisms (Oren 2013). Work is underway both in the field, and the lab, to understand the ability of microbes to persist in these environments, as well as the role that these ions have in stabilizing various biological molecules. Chaotropicity is measured empirically, through the measurement of gelation temperature of a given salt concentration. However, at high salt concentrations, the gelation temperature becomes so low that it is not possible to empirically derive the chaotropic value. For more information on water activity and chaotropicity, see suggested reading below.
Project: Biosignature Preservation in MgSO4 Environments
Overview: This research focuses on developing a comprehensive understanding of a hypersaline, Mars analog environment, addressing the central question: “What types of biosignatures form and are preserved over time?” We are addressing this question specifically in a magnesium sulfate dominated system as sulfate salts have been widely documented on Mars, but are not the dominant type here on Earth. As such, an understanding of the preservation potential of these types of salts over traditional sodium chloride environments is lacking in the literature. Furthermore, the community composition will differ from traditional hypersaline environments, leading to a different set of biosignatures being present. Here we focus on understanding four different biosignatures, each with varying residency times in the geologic record: DNA, amino acids, lipids and sulfur isotopic fractionation signatures. Salt has a documented ability to preserve biological molecules over longer timescales than non-saline environments, and whole cells have been preserved in a viable state within fluid inclusions on the order of thousands of years. The field site is located in and around Clinton, British Columbia, Canada. Currently our team is working on the Basque Lakes, Last Chance Lake and Salt Lake.
Team: Alexandra Pontefract (PI), Magdalena Osburn (Co-I), Chris Carr (Co-I), Mitchell Barklage (Collaborator), Jacob Buffo (Collaborator), Shuhei Ono (Collaborator), Virginia Walker (Collaborator), Jack Szostak (Collaborator)
Funding: NASA ROSES Exobiology
Project: Oceans Across Space and Time (OAST)
Overview: Oceans Across Space and Time (OAST) is an Ocean Worlds mission to Earth, designed to address gaps in our understanding of habitability in extreme terrestrial environments while simultaneously leveraging these unique analogs to guide upcoming ocean world missions in our solar system. OAST aims to investigate the central question “How do ocean worlds and their biospheres co-evolve to produce detectable signals of a past or present living world?” to address the span of potentially habitable environments planetary missions have encountered already or are likely to in the future.
Our team is split across three areas of investigation: Habitability, Microbial Activity, and Microbial Adaptation. Through these lenses, we explore a range of field analogs representative of contemporary, remnant, and relict ocean environments as they progress through stages of wetting, drying, or freezing over geologic time. These field investigations will be characterized with a unifying intellectual focus: a suite of instruments and analytical techniques currently available to the astrobiology and planetary science community will be employed to characterize the physical and chemical challenges and metabolic opportunities in each environment that together determine habitability.
Team: PI - Britney Schmidt, Deputy PI - Jeff Bowman, Theme 1 Lead (Habitability) - Peter Doran, Theme 2 Lead (Activity) - Jen Glass, Theme 3 Lead (Adaptation) - Alexandra Pontefract.
Funding: NASA ICAR
References and Suggested Reading
Fisher, L., Pontefract, A., Bowman, J., Schmidt, B.E., and Bartlett, D. (2020) Review: Looking for Life in Deep Hypersaline Anoxic Brines. Environmental Microbiology, 2021: 1-10.
Fox-Powell, M.G., et al., Ionic Strength Is a Barrier to the Habitability of Mars. Astrobiology, 2016. 16(6): p. 427-42.
Hallsworth, J.E., et al., Limits of life in MgCl2-containing environments: chaotropicity defines the window. Environ Microbiol, 2007. 9(3): p. 801-13.
Hofmeister, F., Zur lehre von der wirkung der salze. Naunyn-Schmiedeberg's Archives of Pharmacology, 1888. 25(1): p. 1-30.
Oren, A., Life in Magnesium- and Calcium-Rich Hypersaline Environments: Salt Stress by Chaotropic Ions. 2013. 27: p. 215-232.
Pegram, L.M., et al., Why Hofmeister effects of many salts favor protein folding but not DNA helix formation. Proc Natl Acad Sci U S A, 2010. 107(17): p. 7716-21.
Stevenson, A., et al., Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life. Environ Microbiol, 2015. 17(2): p. 257-77.
Tosca, N.J., A.H. Knoll, and S.M. McLennan, Water activity and the challenge for life on early Mars. Science, 2008. 320(5880): p. 1204-1207.
Tuovila, B.J., et al., Preservation of ATP in Hypersaline Environments. Applied and Environmental Microbiology, 1987. 53(12): p. 2749-2753.