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Distribution of Raman biosignatures in salt nodules from the hyperarid core of the Atacama Desert

Abstract

Even in one of the driest places on Earth, the Atacama Desert, life has found adaptive strategies to decreasing amounts of water: from refuges inside or below rocks as endoliths or hypoliths to inside salts in hygroscopic niches (Davila & Schulze-Makuch, 2016). In the hyperarid core though, one of the last refuges for life are inside salt crusts using deliquescence as a water source or in the subsurface waiting for transitory episodes of increased moisture (Schulze-Makuch et al., 2018). These adaptive strategies might also apply to a putative Martian life which endured a transition from a water rich past to the very harsh surface conditions of the present and giving us clues on where to best look for traces of life on the Red Planet. Salt crusts and salt nodules are particularly interesting targets in this regard because they reside on or very near the surface and are thus easily accessible to future robotic missions. In the Atacama, salt nodules have been shown to host photosynthetic organisms containing easily identifiable pigments by Raman spectroscopy such as carotenoids or scytonemin. Mostly composed of halite, they are associated with polygonated soils, but their formation processes are still not fully understood. Salt nodules occur in varying morphologies which can control micro-environmental conditions and possibly microbial colonization (habitation of “micro-niches”). One of the most damaging factors for life and its remains, both in the Atacama and on Mars, is solar radiation. To investigate the distribution of potential Raman signatures in micro-niches we mapped/reconstructed the sampling areas using photogrammetry techniques and plotted the dose received according to the nodules’ orientation. We then analysed salt nodules sections using Raman mapping to infer any relations between the amount of light received and the presence of detectable signal. Raman instruments are indeed part of the next two rover missions to Mars: ESA/Roscosmos’s ExoMars2020 and NASA’s Mars2020. To support and interpret future spectroscopic data correctly, a better understanding of potential habitable environments and putative biosignatures, using analogue environments such as the Atacama Desert, is of paramount importance.
Distribution of Raman biosignatures in salt nodules from the hyperarid core of the
Atacama Desert
Mickaël Baqué1, Christof Sager2, Alessandro Airo2, Dirk Schulze-Makuch2, 3 and Jean-Pierre de Vera1
(1) German Aerospace Center (DLR), Institute of Planetary Research, Management and Infrastructure,
Research Group Astrobiological Laboratories, Berlin, Germany
(2) Center of Astronomy & Astrophysics, Technical University Berlin, Germany
(3) School of the Environment, Washington State University, Pullman, USA
Presentation: Poster
Session: Planetary analog research and extreme conditions on Earth
Abstract:
Even in one of the driest places on Earth, the Atacama Desert, life has found adaptive strategies to
decreasing amounts of water: from refuges inside or below rocks as endoliths or hypoliths to inside salts in
hygroscopic niches (Davila & Schulze-Makuch, 2016). In the hyperarid core though, one of the last refuges
for life are inside salt crusts using deliquescence as a water source or in the subsurface waiting for
transitory episodes of increased moisture (Schulze-Makuch et al., 2018). These adaptive strategies might
also apply to a putative Martian life which endured a transition from a water rich past to the very harsh
surface conditions of the present and giving us clues on where to best look for traces of life on the Red
Planet. Salt crusts and salt nodules are particularly interesting targets in this regard because they reside on
or very near the surface and are thus easily accessible to future robotic missions. In the Atacama, salt
nodules have been shown to host photosynthetic organisms containing easily identifiable pigments by
Raman spectroscopy such as carotenoids or scytonemin. Mostly composed of halite, they are associated
with polygonated soils, but their formation processes are still not fully understood. Salt nodules occur in
varying morphologies which can control micro-environmental conditions and possibly microbial
colonization (habitation of “micro-niches”). One of the most damaging factors for life and its remains, both
in the Atacama and on Mars, is solar radiation. To investigate the distribution of potential Raman
signatures in micro-niches we mapped/reconstructed the sampling areas using photogrammetry
techniques and plotted the dose received according to the nodules’ orientation. We then analysed salt
nodules sections using Raman mapping to infer any relations between the amount of light received and the
presence of detectable signal. Raman instruments are indeed part of the next two rover missions to Mars:
ESA/Roscosmos’s ExoMars2020 and NASA’s Mars2020. To support and interpret future spectroscopic data
correctly, a better understanding of potential habitable environments and putative biosignatures, using
analogue environments such as the Atacama Desert, is of paramount importance.
References
Davila, A.F. and Schulze-Makuch, D. (2016) The last possible outposts of life on Mars. Astrobiology 16: 159-168.
Schulze-Makuch, D., Wagner, D, Kounaves, S.P., Mangelsdorf, K., Devine, K.D., de Vera, J.P., et al. (2018) Transitory
habitat for microorganisms in the hyperarid Atacama Desert. Proc. Natl. Acad. Sci. (USA) 115: 2670-2675, doi:
10.1073/pnas.1714341115.
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Article
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Traces of life are nearly ubiquitous on Earth. However, a central unresolved question is whether these traces always indicate an active microbial community or whether, in extreme environments, such as hyperarid deserts, they instead reflect just dormant or dead cells. Although microbial biomass and diversity decrease with increasing aridity in the Atacama Desert, we provide multiple lines of evidence for the presence of an at times metabolically active, microbial community in one of the driest places on Earth. We base this observation on four major lines of evidence: (i) a physico-chemical characterization of the soil habitability after an exceptional rain event, (ii) identified biomolecules indicative of potentially active cells [e.g., presence of ATP, phospholipid fatty acids (PLFAs), metabolites, and enzymatic activity], (iii) measurements of in situ replication rates of genomes of uncultivated bacteria reconstructed from selected samples, and (iv) microbial community patterns specific to soil parameters and depths. We infer that the microbial populations have undergone selection and adaptation in response to their specific soil microenvironment and in particular to the degree of aridity. Collectively, our results highlight that even the hyperarid Atacama Desert can provide a habitable environment for microorganisms that allows them to become metabolically active following an episodic increase in moisture and that once it decreases, so does the activity of the microbiota. These results have implications for the prospect of life on other planets such as Mars, which has transitioned from an earlier wetter environment to today’s extreme hyperaridity.
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The evolution of habitable conditions on Mars is often tied to the existence of aquatic habitats and largely constrained to the first billion years of the planet. Here, we propose an alternate, lasting evolutionary trajectory that assumes the colonization of land habitats before the end of the Hesperian period (ca. 3 billion years ago) at a pace similar to life on Earth. Based on the ecological adaptations to increasing dryness observed in dryland ecosystems on Earth, we reconstruct the most likely sequence of events leading to a late extinction of land communities on Mars. We propose a trend of ecological change with increasing dryness from widespread edaphic communities to localized lithic communities and finally to communities exclusively found in hygroscopic substrates, reflecting the need for organisms to maximize access to atmospheric sources of water. If our thought process is correct, it implies the possibility of life on Mars until relatively recent times, perhaps even the present. Key Words: Life-Mars-Evolution-Desert-Land ecosystems-Deliquescence. Astrobiology 16, xxx-xxx.