Fingerprinting molecular and isotopic biosignatures on different hydrothermal scenarios of Iceland, an acidic and sulfur-rich Mars analog

Abstract

Detecting signs of potential extant/extinct life on Mars is challenging because the presence of organics on that planet is expected to be very low and most likely linked to radiation-protected refugia and/or preservative strategies (e.g., organo-mineral complexes). With scarcity of organics, accounting for biomineralization and potential relationships between biomarkers, mineralogy, and geochemistry is key in the search for extraterrestrial life. Here we explored microbial fingerprints and their associated mineralogy in Icelandic hydrothermal systems analog to Mars (i.e., high sulfur content, or amorphous silica), to identify potentially habitable locations on that planet. The mineralogical assemblage of four hydrothermal substrates (hot springs biofilms, mud pots, and steaming and inactive fumaroles) was analyzed concerning the distribution of biomarkers. Molecular and isotopic composition of lipids revealed quantitative and compositional differences apparently impacted by surface geothermal alteration and environmental factors. pH and water showed an influence (i.e., greatest biomass in circumneutral settings with highest supply and turnover of water), whereas temperature conditioned the mineralogy that supported specific microbial metabolisms related with sulfur. Raman spectra suggested the possible coexistence of abiotic and biomediated sources of minerals (i.e., sulfur or hematite). These findings may help to interpret future Raman or GC–MS signals in forthcoming Martian missions.

Raman spectroscopic peculiarities of Icelandic poorly crystalline minerals and their implications for Mars exploration

“…The equivalence with the nomenclature of the samples previously analyzed in Sánchez-García et al . 26 , where we focused on the fingerprinting molecular and isotopic biosignatures, can be seen in Table S2 of the Supplementary Online Information.…”

“…Both minerals have intense Raman signatures in the wavelength of the laser used (i.e., 532 nm) that even can obstruct the detection of other mineral phases when located at their side in the same sample (see Fig. 5 of 26 ). As mentioned, we focused our attention on five mineral types: (a) native sulfur, (b) hematite, (c) anatase, (d) sulfates, and (e) clay minerals, to study the relationship between structural disorder (represented in Raman shift and peak broadening) and biological activity and/or hydrothermal processes.…”

“…On the other hand, organic processes are those that involve biological activity on mineral surfaces, altering the crystalline structure through the use of chemical elements of the mineral 36 .

Figure 1 Index of crystallinity determined by XRD of selected samples found in a diversity of substrates, where previously the molecular distribution and stable-carbon isotopic composition of lipid biomarkers were determined 26 . IF: inactive fumarole, AF: active fumarole, MP: mud pot, MAT: microbial mat.

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“… Index of crystallinity determined by XRD of selected samples found in a diversity of substrates, where previously the molecular distribution and stable-carbon isotopic composition of lipid biomarkers were determined 26 . IF: inactive fumarole, AF: active fumarole, MP: mud pot, MAT: microbial mat.…”

“…Mud and steam springs are abundant on the surface, with a wide pH range. This diversity provides that all the minerals mentioned in the first paragraph are found in Iceland, where we selected three hydrothermal areas to be explored in situ and to analyze in the laboratory by Raman and IR spectroscopies, XRD, and for the molecular distribution and stable-carbon isotopic composition of lipid biomarkers 26 . These three areas were selected because of their abundance of Fe-rich basalts common in the region, the oxidation conditions of their hydrothermal fluids, and the presence of extensive sulfate and phyllosilicates-rich deposits from basalt weathering.…”