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Project Progress

Developing New Biosignatures

One of our tasks right now is the use of DNA as a biosignature. We continue to analyze DNA recovered from multiple horizons of deeply-buried sediment from both the Costa Rica Margin and Hydrothermal Mounds near Okinawa. We have numerous horizons with extracted and sequenced DNA from Okinawa hydrothermal mound, Costa Rican Margin, and the Equatorial Pacific. DNA analysis (genetics and genomics) are proceeding. In general, the analysis have revealed that (1) Archaea make up a substantial proportion of the subsurface biosphere, (2) Chlorofexi make up a significant portion of the bacterial populations, and (3) some notable microbial groups increase as the sediment temperatures increase.

Another significant task is investigating the microorganisms in methane seep environments. Microbial aggregate abundance within the carbonate interior exceeds that of seep sediments, and molecular diversity surveys reveal methanotrophic communities within protolithic nodules and well-lithified carbonate pavements. Aggregations of microbial cells within the carbonate matrix actively oxidize methane as indicated by stable isotope FISH–nanoSIMS experiments and ¹⁴CH₄ radiotracer rate measurements. Carbonate-hosted methanotrophy extends the known ecological niche of these important methane consumers and represents a previously unrecognized methane sink that warrants consideration in global methane budgets. Also, applying compound-specific carbon isotopic analysis to seep sediments, we have found over 100 permit ¹³C range for molecules derived from methane-oxidizing archaea. This is a significant results because it indicates that geochemical signatures of methane seep microbes can preserve signatures including both lipid biomarkers and isotope heterogeneity. Also related to biosignature preservation, our team has been investigating tetragonal microstructures present in the carbonates that resemble seep-endemic Methanosarcinales cell clusters. Despite morphological similarities to the seep-endemic microbes that likely mediated the authigenesis of Eel River Basin carbonates and sulfides, detailed petrographic, SEM, and magnetic microscopic imaging, remnant rock magnetism, laser Raman, and energy dispersive X-ray spectroscopy, suggest that these microstructures are not microfossils, but rather mineral structures that result from the diagenetic alteration of euhedral Fe-sulfide framboids. Electron microscopy shows that during diagenesis, reaction rims composed of Fe oxide form around framboid microcrystalites.

Recently, our work has extended beyond preservation of methane seep microbiota on Earth to the preservation of such a community on Mars. Seven distinct fluids representative of putative martian groundwater were used to calculate Gibbs energy values in the presence of dissolved methane under a range of atmospheric CO₂ partial pressures. In all scenarios, AOM is exergonic, ranging from – 31 to – 135 kJ/mol CH₄. A reaction transport model was constructed to examine how environmentally relevant parameters such as advection velocity, reactant concentrations, and biomass production rate affect the spatial and temporal dependences of AOM reaction rates. Two geologically supported models for ancient martian AOM are presented: a sulfate-rich groundwater with methane produced from serpentinization by-products, and acid-sulfate fluids with methane from basalt alteration. The simulations presented in this study indicate that AOM could have been a feasible metabolism on ancient Mars, and fossil or isotopic evidence of this metabolic pathway may persist beneath the surface and in surface exposures of eroded ancient terrains.

Another portion of our research involves rock weathering to soil. This year, we have made very good progress on our investigations of Fe mobility and isotopes in soil profiles on diabase and basalt on Earth as a way of understanding weathering on Mars.

In looking at potential biosignatures, we have also needed to further understand prebiotic chemistry. If molecules are readily formed and preserved in the absence of life, then they are not reliable biosignatures. In extending known prebiotic chemistry, we found pyridine carboxylic acids in CM2 carbonaceous chondrites. These compounds could be potential precursor molecules for ancient coenzymes. We also have found that lipid vesicles, which form can be formed fairly robustly in prebiotic experiments, protect activated acetic acid. Methyl thioacetate, or activated acetic acid, has been proposed to be central to the origin of life and an important energy currency molecule in early cellular evolution. Our recent work suggests that the hydrophobic regions of prebiotic vesicles and early cell membranes could have offered a refuge for this energetic molecule, increasing its lifetime in close proximity to the reactions for which it would be needed. This model of early energy storage evokes an additional critical function for the earliest cell membranes.