It shows that the microbial remains are preserved in a mineral matrix composed of laminated Fe-oxysulfates and K- and Na-bearing sulfates alternating with secondary silica. A Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) molecular analysis of a subsurface sample in the Peña de Hierro basement has provided novel data of the ancient underground microbial community. Although the gossan is the microbial activity product, which dates back to the Oligocene (25 Ma), no molecular evidence of such activity in the past has been reported yet. The drilling of the Rio Tinto basement has provided evidence of an underground microbial community primarily sustained by the Fe and S metabolism through the biooxidation of pyrite orebodies. The distribution of the organic and inorganic negative ions, which we suggest, resulted from the preservation of at least three microbial consortia (PO4-, and NO2-/NO3-mineralizers PO4-lipid bearing microstructures), would have resulted from different metabolic and preservation pathways. However, they have a higher density in areas with more significant enrichment in iron oxides that are traced by different Fe-bearing anions like FeO2. The NO2-/NO3- and Cl- ions occur as small microstructure clusters (<20 microns), but their distribution is dissimilar to the mineralized microstructures bearing PO3-, and SO3. While a group of >40-micron sized circular micronodules lacks organic compounds, an ovoidal microstructure is associated with m/z of other lipids.
PO3-rich compounds occur in two different groups of microstructures which size, morphology, and composition are different. SOx- appear to be mineralizing some microstructures larger than 25 microns, which have branched morphologies, and that source SO3-bearing adducts. Their distribution correlates with organic molecules that were tentatively assigned to saturated and monounsaturated fatty acids, polyunsaturated fatty acids, saccharides, phospholipids, sphingolipids, and potential peptide fragments. The molecular analysis at a micron-scale mapped the occurrence of several inorganic complexes bearing PO3-, SOx(2 to 4)-, NOx(2,3)-, FeOx(1,2)-, SiO2-, and Cl. To characterize the molecular composition of the subsurface biosphere, we have analyzed some core samples by time-of-flight secondary ion mass spectrometry (ToF-SIMS) that were collected in the borehole BH8 during the operations of the Mars Analog and Technology Experiment (MARTE) project. The molecular composition of such structures can record and preserve information about the metabolic pathways that have produced them. In the subsurface, the interplay between microbial communities and the surrounding mineral substrate, potentially used as an energy source, results in different mineralized structures. Consequently, low-pH oxidative environments on Mars could also record molecular information about ancient life in the same way as the Noachian clay-rich deposits. This relation implies that the quality of molecular preservation is exceptional and provides information on microbial life formerly operating in the shallow regions of the Rio Tinto subsurface.
Time of flight secondary ion mass spectrometry (ToF–SIMS) analysis shows a direct association between physical-templating biological structures and molecular biosignatures. In this paper, an exhaustive mass spectrometry investigation of ferric subsurface materials in the Rio Tinto gossan deposit (~ 25 Ma) provides evidence of well-preserved molecular biosignatures under oxidative and acidic conditions. They were followed by widespread precipitation of acidic sulfates considered adverse for biomolecule preservation. The preservation of biosignatures on Mars is largely associated with extensive deposits of clays formed under mild early Noachian conditions (> 3.9 Ga).
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