Recovery of Lipid Biomarkers in Hot Spring Digitate Silica Sinter as Analogs for Potential Biosignatures on Mars: Results from Laboratory and Flight-Like Experiments

Digitate siliceous sinter deposits are commonly found in geothermal environments and are thought to form via precipitation from silica-rich fluids, induced by evaporative cooling and microbial activity. Interest in these microstomatolitic sinters has grown over the past decade because of their morphological and mineralogical resemblance to opaline silica-rich rocks discovered on Mars by NASA’s Spirit rover at the Columbia Hills landing site in Gusev crater. However, these deposits remain understudied, specifically in terms of their biosignature content and long-term preservation potential. Opaline silica (opal-A) is found in many geothermal fields on Earth, including the Taupo Volcanic Zone (TVZ) in New Zealand. In this study, six digitate sinter deposits were collected from five TVZ geothermal fields, and their lipid biosignatures were investigated as Mars analogs using current flight techniques. Samples were collected in pools and discharge channels of varied temperatures, pH, and water chemistries. The digitate sinters collected exhibited various morphologies, from spicular to nodular, and all grew at the air-water interface of the studied geothermal sites. Lipids were extracted and quantified, revealing a predominance of biomarkers from microbial lipids and algae from extant and extinct communities. The presence of wax ester lipids in one of the samples (Parariki Stream, Rotokawa geothermal field) revealed the presence of green non-sulfur filamentous anoxygenic phototroph (FAP) cyanobacteria, Chloroflexus and Roseiflexus, typical thermophiles found in polyextreme hydrothermal environments. Temperature, pH, and water chemistry did not seem to affect microbial diversity nor lipid recovery. However, the morphology of the sinters does appear to play a role in the lipid yield, which was higher in the finest spicules in comparison to the knobby digitate sinters studied. The capability of pyrolysis-GC-MS to detect lipid biomarkers was also evaluated in a pilot study of flight conditions simulated in the laboratory on a subset of samples. Lipid detection capability was well correlated to the amount of carbon present in the samples, but pyrolysis can bias interpretation regarding the origin of the lipid biomarkers. Pyrolysis revealed the macromolecular component of some sinters and may have induced chemical reactions between the organic molecules and sulfur-inorganic species present in the samples, therefore obscuring the targeted lipid biomarkers and inducing changes in their carbon number assemblages.