Abstract
Evidence indicates that Gale crater on Mars harboured a fluvio-lacustrine environment that was subjected to physio-chemical variations such as changes in redox conditions and evaporation with salinity changes, over time. Microbial communities from terrestrial environmental analogues sites are important for studying such potential habitability environments on early Mars, especially in laboratory-based simulation experiments. Traditionally, such studies have predominantly focused on microorganisms from extreme terrestrial environments. These are applicable to a range of Martian environments; however, they lack relevance to the lacustrine systems. In this study, we characterise an anoxic inter-tidal zone as a terrestrial analogue for the Gale crater lake system according to its chemical and physical properties, and its microbiological community. The sub-surface inter-tidal environment of the River Dee estuary, United Kingdom (53°21′15.40″ N, 3°10′24.95″ W) was selected and compared with available data from Early Hesperian-time Gale crater, and temperature, redox, and pH were similar. Compared to subsurface ‘groundwater’-type fluids invoked for the Gale subsurface, salinity was higher at the River Dee site, which are more comparable to increases in salinity that likely occurred as the Gale crater lake evolved. Similarities in clay abundance indicated similar access to, specifically, the bio-essential elements Mg, Fe and K. The River Dee microbial community consisted of taxa that were known to have members that could utilise chemolithoautotrophic and chemoorganoheterotrophic metabolism and such a mixed metabolic capability would potentially have been feasible on Mars. Microorganisms isolated from the site were able to grow under environment conditions that, based on mineralogical data, were similar to that of the Gale crater’s aqueous environment at Yellowknife Bay. Thus, the results from this study suggest that the microbial community from an anoxic inter-tidal zone is a plausible terrestrial analogue for studying habitability of fluvio-lacustrine systems on early Mars, using laboratory-based simulation experiments.
Highlights
The surface of present day Mars is deemed inhospitable to life
We propose the microbial community from the anoxic inter-tidal zones as analogues for the ancient lake system at Gale crater, and other equivalent locations on Mars, which we chose on the basis of the following expected similarities: an estuary environment has a salinity of between 0.5 and 3.5%, which is comparable to the 1–2% salinity proposed for the ancient Martian lake system [11,18,56]
There was no significant difference between temperature, pH values and Total Organic Carbon (TOC) between each sample site, when examined using parametric analyses (ANOVA) (Table 1)
Summary
The surface of present day Mars is deemed inhospitable to life. The environment is cold, dry, highly oxidised and exposed to ultraviolet (UV) and ionizing radiation. Ancient fluvial systems, of various orders of magnitude, are observed, which may have once been habitable (e.g., [5,6,7,8,9]) Sediments formed by those processes preserve evidence of those ancient conditions (and potential biomarkers within), so they can be studied to decipher the past. Unambiguous, ground-based evidence of a range of past aqueous activity has been collected from Gale crater by the Mars Science Laboratory rover Curiosity since 2012 (e.g., [8,10,11,12,13]). We chose this example to compare—and as necessary contrast—our analogue site. It must be noted that there is ubiquitous evidence for similar systems elsewhere on Mars [14]
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