Introducing the concept of “astrobiological time-analogs”: Ecological successions throughout the desiccation of hypersaline lagoons on Earth and the wet-to-dry transition on early Mars

Early Mars most likely had a diversity of environments in terms of pH, redox conditions, temperature, geochemistry, and mineralogy. Field research in terrestrial analog environments contribute to understand the habitability of this diversity of environments on Mars in the past, because terrestrial analogues are places on Earth characterized by environmental, mineralogical, geomorphological, or geochemical conditions similar to those observed on present or past Mars. So far, analogs have been referred to terrestrial locations closely similar to any of the geochemical environments that have been inferred on Mars, i.e., they are “place-analogs” that represent snapshots in time: one specific environmental condition at a very specific place and a very specific time. Because of this, each individual field analog site cannot be considered an adequate representation of the changing martian environmental conditions through time. Here we introduce the concept of “astrobiological time-analog”, referred to terrestrial analogs that may help understand environmental transitions and the related possible ecological successions on early Mars. As Mars lost most of its surface water at the end of the Hesperian, this wet-to-dry global transition can be considered the major environmental perturbation in the geological history of Mars, and therefore merits to be the first one to be assigned a “time-analog” for its better understanding and characterization. 
At the end of the Hesperian, several paleolakes on Mars were characterized by episodic inundation by shallow surface waters with varying salinity, evaporation, and full desiccation repeatedly over time, until the final disappearance of most surface water after the wet-to-dry transition. We show here that similar conditions can be tested through time in the terrestrial analog Tirez lagoon. Tirez was a small and seasonal endorheic athalassohaline lagoon that was located in central Spain. In recent years, the lagoon has totally dried out, offering for the first time the opportunity to analyze its desiccation process as a “time-analog” to similar events occurred during the wet-to-dry transition on early Mars. To do so, here we describe (i) the microbial ecology of Tirez when the lagoon was still active 20 years ago, with prokaryotes adapted to extreme saline conditions; (ii) the composition of the microbial community in the dried lake sediments today, in many case groups that thrive in sediments of extreme environments; and (iii) the molecular and isotopic analysis of the lipid biomarkers that can be recovered from the sediments today. We conclude that Tirez was habitable for a wide range of prokaryotes before and after its complete desiccation, in spite of the repeated seasonal dryness; and our results may inform about research strategies to search for possible biomarkers in Mars after all the water was lost. Our 25 yearlong analyses of the ecological transitions in the Tirez lagoon represent the first terrestrial astrobiological “time-analog” for desiccating saline lakes on early Mars