Raman Signatures in Salt Nodules from the Atacama Desert along an Aridity Gradient

Kurzfassung

In one of Earth's driest environments, the Atacama Desert, life has developed remarkable adaptive strategies to decreasing amounts of water. These adaptations include taking refuge within or beneath rocks as endoliths or hypoliths and residing in salt formations in hygroscopic niches [1,2]. In the hyperarid core, life persists within salt crusts by using deliquescence as a water source or by remaining in the subsurface, waiting for brief periods of increased moisture [3]. These adaptive strategies are not only fascinating in their own right but also provide potential analogues for putative Martian life, which might have transitioned from a water-rich past to the harsh conditions seen today [4]. 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, mainly composed of halite, are known to harbour photosynthetic organisms with pigments like carotenoids and scytonemin, identifiable through Raman spectroscopy. While these nodules are associated with polygonal soils, the details of their formation are still unclear. The diverse shapes of salt nodules create different micro-environmental conditions, potentially influencing microbial colonization (habitation of “micro-niches”). To assess the distribution and detectability of life in such habitats via Raman spectroscopy, we sampled nodules from several sites along an aridity gradient to compare active and dormant communities. After cutting and polishing, we performed Raman image scans and fluorescence microscopy along the depth of the nodules from four areas with different humidity levels. Visibly colonized nodules from an area with active fog, displaying dark surface bands, showed a higher count of Raman-detectable signatures (carotenoids, scytonemin, chlorophyll and secondary pigments). Remarkably, even non-visibly colonized nodules from the hyper-arid core, show sporadic remnants of communities with still detectable pigments which might remain dormant for several years. Measurements taken in situ, with a portable Raman system, were also compared to laboratory measurements to better inform us about the optimal use for current and future Raman instruments on Martian rovers. [1] Davila et al., Astrobiology, 16, 159–168 (2016). [2] Schulze-Makuch et al., Microorganisms, 9, 1038 (2021). [3] Schulze-Makuch et al., PNAS, 115, 2670–2675 (2018). [4] Davila et al., Chapter 12 - in Mars Geological Enigmas, Elsevier, (2021), pp. 333–355.