Influence of Geology and Geochemistry on Microbial Dynamics and Associated Lipid Biosignatures in a Subsurface, Serpentinite-hosted Ecosystem

Hydrogen and reduced carbon compounds generated during the hydration of iron-rich ultramafic rock (&ldquo;serpentinization&rdquo;) are commonly considered as sustained sources of reductants for microbial energy metabolism on early Earth, and possibly on other rocky extraterrestrial bodies. However, the availability of oxidants and nutrients for life in serpentinizing settings is likely limiting, particularly in the subsurface. In collaboration with the Oman Drilling Project and the &ldquo;Rock-Powered Life&rdquo; team, I have had the unique opportunity to collect deep biomass from fluid and rock in the subsurface of a serpentinite aquifer in Oman to (1) study how life could function near its energetic limit and (2) what robust biosignatures may be produced due to biogeochemical activity. Microbial community composition in serpentinite-hosted fluids, as inferred by 16S amplicon gene sequencing, was strongly correlated with fluid geochemistry, likely driven by differential energy availability that results from fundamental differences in both host rock lithology and the extent of water-rock reaction. Microbial richness corresponded with greater concentrations of nitrate in shallow fluids; this nitrate is attributed to primarily an atmospheric origin due to its relict meteoric oxygen isotopic signature (&delta;18O ~ 22&permil;, &Delta;17O ~ 6&permil;). Groundwater nitrate was subsequently reduced to ammonium in deeper fluids, retaining essential fixed nitrogen in the subsurface. I conducted an intact polar lipid (IPL) analysis of the microbial biomass extracted from rocks and fluids in order to identify organic signatures that may be representative of microbial communities active under the diverse environmental conditions. To identify IPLs produced by microorganisms functioning under extreme conditions, I established a theoretical database of &gt;106 IPL compounds which enabled me to identify a predominance of unusual glyco- and amino-lipids in subsurface fluids that I interpret as representing a physiological adaptation to phosphate limitation. Despite contamination introduced during drilling, robust (sample/contamination ratio of &gt;100) signatures of non-isoprenoidal ether-linked glycolipids likely produced by endemic sulfate-reducing bacteria were detected in intervals of serpentinite rock core characterized by sulfide mineralization, thus providing a promising target for future efforts to detect organic biosignatures preserved in serpentinite-hosted systems, such as those potentially left on early Earth, Mars, or similar planetary systems.</p

Intact polar lipidome and membrane adaptations of microbial communities inhabiting serpentinite-hosted fluids

The generation of hydrogen and reduced carbon compounds during serpentinization provides sustained energy for microorganisms on Earth, and possibly on other extraterrestrial bodies (e.g., Mars, icy satellites). However, the geochemical conditions that arise from water-rock reaction also challenge the known limits of microbial physiology, such as hyperalkaline pH, limited electron acceptors and inorganic carbon. Because cell membranes act as a primary barrier between a cell and its environment, lipids are a vital component in microbial acclimation to challenging physicochemical conditions. To probe the diversity of cell membrane lipids produced in serpentinizing settings and identify membrane adaptations to this environment, we conducted the first comprehensive intact polar lipid (IPL) biomarker survey of microbial communities inhabiting the subsurface at a terrestrial site of serpentinization. We used an expansive, custom environmental lipid database that expands the application of targeted and untargeted lipodomics in the study of microbial and biogeochemical processes. IPLs extracted from serpentinite-hosted fluid communities were comprised of &gt;90% isoprenoidal and non-isoprenoidal diether glycolipids likely produced by archaeal methanogens and sulfate-reducing bacteria. Phospholipids only constituted ~1% of the intact polar lipidome. In addition to abundant diether glycolipids, betaine and trimethylated-ornithine aminolipids and glycosphingolipids were also detected, indicating pervasive membrane modifications in response to phosphate limitation. The carbon oxidation state of IPL backbones was positively correlated with the reduction potential of fluids, which may signify an energy conservation strategy for lipid synthesis. Together, these data suggest microorganisms inhabiting serpentinites possess a unique combination of membrane adaptations that allow for their survival in polyextreme environments. The persistence of IPLs in fluids beyond the presence of their source organisms, as indicated by 16S rRNA genes and transcripts, is promising for the detection of extinct life in serpentinizing settings through lipid biomarker signatures. These data contribute new insights into the complexity of lipid structures generated in actively serpentinizing environments and provide valuable context to aid in the reconstruction of past microbial activity from fossil lipid records of terrestrial serpentinites and the search for biosignatures elsewhere in our solar system

Geological and Geochemical Controls on Subsurface Microbial Life in the Samail Ophiolite, Oman.

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite ("gabbro," "alkaline peridotite," "hyperalkaline peridotite," and "gabbro/peridotite contact") that vary strongly in pH and the concentrations of

Subsurface biogeochemical cycling of nitrogen in the actively serpentinizing Samail Ophiolite, Oman

Nitrogen (N) is an essential element for life. N compounds such as ammonium (NH+4) may act as electron donors, while nitrate (NO&minus;3) and nitrite (NO&minus;2) may serve as electron acceptors to support energy metabolism. However, little is known regarding the availability and forms of N in subsurface ecosystems, particularly in serpentinite-hosted settings where hydrogen (H2) generated through water&ndash;rock reactions promotes habitable conditions for microbial life. Here, we analyzed N and oxygen (O) isotope composition to investigate the source, abundance, and cycling of N species within the Samail Ophiolite of Oman. The dominant dissolved N species was dependent on the fluid type, with Mg2+-HCO&minus;3 type fluids comprised mostly of NO&minus;3, and Ca2+-OH&minus; fluids comprised primarily of ammonia (NH3). We infer that fixed N is introduced to the serpentinite aquifer as NO&minus;3. High concentrations of NO&minus;3 (&gt;100 &mu;M) with a relict meteoric oxygen isotopic composition (&delta;18O ~ 22&permil;, &Delta;17O ~ 6&permil;) were observed in shallow aquifer fluids, indicative of NO&minus;3 sourced from atmospheric deposition (rainwater NO&minus;3: &delta;18O of 53.7&permil;, &Delta;17O of 16.8&permil;) mixed with NO&minus;3 produced in situ through nitrification (estimated endmember &delta;18O and &Delta;17O of ~0&permil;). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH3 (&gt;100 &mu;M) with little NO&minus;3 detectable. We interpret that NH3 in hyperalkaline fluids is a product of NO&minus;3 reduction. The proportionality of the O and N isotope fractionation (18&epsilon; / 15&epsilon;) measured in Samail Ophiolite NO&minus;3 was close to unity (18&epsilon; / 15&epsilon; ~ 1), which is consistent with dissimilatory <span class="mjx-char MJ

Deciphering Biosignatures in Planetary Contexts

Microbial life permeates Earth's critical zone and has likely inhabited nearly all our planet's surface and near subsurface since before the beginning of the sedimentary rock record. Given the vast time that Earth has been teeming with life, do astrobiologists truly understand what geological features untouched by biological processes would look like? In the search for extraterrestrial life in the Universe, it is critical to determine what constitutes a biosignature across multiple scales, and how this compares with “abiosignatures” formed by nonliving processes. Developing standards for abiotic and biotic characteristics would provide quantitative metrics for comparison across different data types and observational time frames. The evidence for life detection falls into three categories of biosignatures: (1) substances, such as elemental abundances, isotopes, molecules, allotropes, enantiomers, minerals, and their associated properties; (2) objects that are physical features such as mats, fossils including trace-fossils and microbialites (stromatolites), and concretions; and (3) patterns, such as physical three-dimensional or conceptual n-dimensional relationships of physical or chemical phenomena, including patterns of intermolecular abundances of organic homologues, and patterns of stable isotopic abundances between and within compounds. Five key challenges that warrant future exploration by the astrobiology community include the following: (1) examining phenomena at the “right” spatial scales because biosignatures may elude us if not examined with the appropriate instrumentation or modeling approach at that specific scale; (2) identifying the precise context across multiple spatial and temporal scales to understand how tangible biosignatures may or may not be preserved; (3) increasing capability to mine big data sets to reveal relationships, for example, how Earth's mineral diversity may have evolved in conjunction with life; (4) leveraging cyberinfrastructure for data management of biosignature types, characteristics, and classifications; and (5) using three-dimensional to n-D representations of biotic and abiotic models overlain on multiple overlapping spatial and temporal relationships to provide new insights

The potential science and engineering value of samples delivered to Earth by Mars sample return

© The Meteoritical Society, 2019. Executive Summary: Return of samples from the surface of Mars has been a goal of the international Mars science community for many years. Affirmation by NASA and ESA of the importance of Mars exploration led the agencies to establish the international MSR Objectives and Samples Team (iMOST). The purpose of the team is to re-evaluate and update the sample-related science and engineering objectives of a Mars Sample Return (MSR) campaign. The iMOST team has also undertaken to define the measurements and the types of samples that can best address the objectives. Seven objectives have been defined for MSR, traceable through two decades of previously published international priorities. The first two objectives are further divided into sub-objectives. Within the main part of the report, the importance to science and/or engineering of each objective is described, critical measurements that would address the objectives are specified, and the kinds of samples that would be most likely to carry key information are identified. These seven objectives provide a framework for demonstrating how the first set of returned Martian samples would impact future Martian science and exploration. They also have implications for how analogous investigations might be conducted for samples returned by future missions from other solar system bodies, especially those that may harbor biologically relevant or sensitive material, such as Ocean Worlds (Europa, Enceladus, Titan) and others. Summary of Objectives and Sub-Objectives for MSR Identified by iMOST: Objective 1 Interpret the primary geologic processes and history that formed the Martian geologic record, with an emphasis on the role of water. Intent To investigate the geologic environment(s) represented at the Mars 2020 landing site, provide definitive geologic context for collected samples, and detail any characteristics that might relate to past biologic processesThis objective is divided into five sub-objectives that would apply at different landing sites. 1.1 Characterize the essential stratigraphic, sedimentologic, and facies variations of a sequence of Martian sedimentary rocks. Intent To understand the preserved Martian sedimentary record. Samples A suite of sedimentary rocks that span the range of variation. Importance Basic inputs into the history of water, climate change, and the possibility of life 1.2 Understand an ancient Martian hydrothermal system through study of its mineralization products and morphological expression. Intent To evaluate at least one potentially life-bearing “habitable” environment Samples A suite of rocks formed and/or altered by hydrothermal fluids. Importance Identification of a potentially habitable geochemical environment with high preservation potential. 1.3 Understand the rocks and minerals representative of a deep subsurface groundwater environment. Intent To evaluate definitively the role of water in the subsurface. Samples Suites of rocks/veins representing water/rock interaction in the subsurface. Importance May constitute the longest-lived habitable environments and a key to the hydrologic cycle. 1.4 Understand water/rock/atmosphere interactions at the Martian surface and how they have changed with time. Intent To constrain time-variable factors necessary to preserve records of microbial life. Samples Regolith, paleosols, and evaporites. Importance Subaerial near-surface processes could support and preserve microbial life. 1.5 Determine the petrogenesis of Martian igneous rocks in time and space. Intent To provide definitive characterization of igneous rocks on Mars. Samples Diverse suites of ancient igneous rocks. Importance Thermochemical record of the planet and nature of the interior. Objective 2 Assess and interpret the potential biological history of Mars, including assaying returned samples for the evidence of life. Intent To investigate the nature and extent of Martian habitability, the conditions and processes that supported or challenged life, how different environments might have influenced the preservation of biosignatures and created nonbiological “mimics,” and to look for biosignatures of past or present life.This objective has three sub-objectives: 2.1 Assess and characterize carbon, including possible organic and pre-biotic chemistry. Samples All samples collected as part of Objective 1. Importance Any biologic molecular scaffolding on Mars would likely be carbon-based. 2.2 Assay for the presence of biosignatures of past life at sites that hosted habitable environments and could have preserved any biosignatures. Samples All samples collected as part of Objective 1. Importance Provides the means of discovering ancient life. 2.3 Assess the possibility that any life forms detected are alive, or were recently alive. Samples All samples collected as part of Objective 1. Importance Planetary protection, and arguably the most important scientific discovery possible. Objective 3 Quantitatively determine the evolutionary timeline of Mars. Intent To provide a radioisotope-based time scale for major events, including magmatic, tectonic, fluvial, and impact events, and the formation of major sedimentary deposits and geomorphological features. Samples Ancient igneous rocks that bound critical stratigraphic intervals or correlate with crater-dated surfaces. Importance Quantification of Martian geologic history. Objective 4 Constrain the inventory of Martian volatiles as a function of geologic time and determine the ways in which these volatiles have interacted with Mars as a geologic system. Intent To recognize and quantify the major roles that volatiles (in the atmosphere and in the hydrosphere) play in Martian geologic and possibly biologic evolution. Samples Current atmospheric gas, ancient atmospheric gas trapped in older rocks, and minerals that equilibrated with the ancient atmosphere. Importance Key to understanding climate and environmental evolution. Objective 5 Reconstruct the processes that have affected the origin and modification of the interior, including the crust, mantle, core and the evolution of the Martian dynamo. Intent To quantify processes that have shaped the planet's crust and underlying structure, including planetary differentiation, core segregation and state of the magnetic dynamo, and cratering. Samples Igneous, potentially magnetized rocks (both igneous and sedimentary) and impact-generated samples. Importance Elucidate fundamental processes for comparative planetology. Objective 6 Understand and quantify the potential Martian environmental hazards to future human exploration and the terrestrial biosphere. Intent To define and mitigate an array of health risks related to the Martian environment associated with the potential future human exploration of Mars. Samples Fine-grained dust and regolith samples. Importance Key input to planetary protection planning and astronaut health. Objective 7 Evaluate the type and distribution of in-situ resources to support potential future Mars exploration. Intent To quantify the potential for obtaining Martian resources, including use of Martian materials as a source of water for human consumption, fuel production, building fabrication, and agriculture. Samples Regolith. Importance Production of simulants that will facilitate long-term human presence on Mars. Summary of iMOST Findings: Several specific findings were identified during the iMOST study. While they are not explicit recommendations, we suggest that they should serve as guidelines for future decision making regarding planning of potential future MSR missions. The samples to be collected by the Mars 2020 (M-2020) rover will be of sufficient size and quality to address and solve a wide variety of scientific questions. Samples, by definition, are a statistical representation of a larger entity. Our ability to interpret the source geologic units and processes by studying sample sub sets is highly dependent on the quality of the sample context. In the case of the M-2020 samples, the context is expected to be excellent, and at multiple scales. (A) Regional and planetary context will be established by the on-going work of the multi-agency fleet of Mars orbiters. (B) Local context will be established at field area- to outcrop- to hand sample- to hand lens scale using the instruments carried by M-2020. A significant fraction of the value of the MSR sample collection would come from its organization into sample suites, which are small groupings of samples designed to represent key aspects of geologic or geochemical variation. If the Mars 2020 rover acquires a scientifically well-chosen set of samples, with sufficient geological diversity, and if those samples were returned to Earth, then major progress can be expected on all seven of the objectives proposed in this study, regardless of the final choice of landing site. The specifics of which parts of Objective 1 could be achieved would be different at each of the final three candidate landing sites, but some combination of critically important progress could be made at any of them. An aspect of the search for evidence of life is that we do not know in advance how evidence for Martian life would be preserved in the geologic record. In order for the returned samples to be most useful for both understanding geologic processes (Objective 1) and the search for life (Objective 2), the sample collection should contain BOTH typical and unusual samples from the rock units explored. This consideration should be incorporated into sample selection and the design of the suites. The retrieval missions of a MSR campaign should (1) minimize stray magnetic fields to which the samples would be exposed and carry a magnetic witness plate to record exposure, (2) collect and return atmospheric gas sample(s), and (3) collect additional dust and/or regolith sample mass if possible

Enzyme specific isotope effects of the Nap and Nar nitrate reductases

AbstractDissimilatory nitrate reduction (DNR) to nitrite is the first step in denitrification, the main process through which bioavailable nitrogen is removed from ecosystems. DNR fractionates the stable isotopes of nitrogen (14N, 15N) and oxygen (16O, 18O) and thus imparts an isotopic signature on residual pools of nitrate in many environments. Data on the relationship between the resulting isotopic pattern in oxygen versus nitrogen isotopes (18ε / 15ε) suggests systematic differences exist between marine and terrestrial ecosystems that are not fully understood. DNR can be catalyzed by both cytosolic (Nar) and periplasmic (Nap) nitrate reductases, and previous work has revealed differences in their 18ε / 15ε isotopic signatures. In this study, we thus examine the 18ε / 15ε of six different nitrate-reducing microorganisms that encode Nar, Nap or both enzymes, as well gene deletion mutants of the enzymes’ catalytic subunits (NarG and NapA) to test the hypothesis that enzymatic differences alone could explain the environmental observations. We find that the distribution of the 18ε / 15ε fractionation ratios of all examined nitrate reductases form two distinct, non-overlapping peaks centered around a 18ε / 15ε proportionality of 0.55 and a 18ε / 15ε proportionality of 0.91, respectively. All Nap reductases studied to date cluster around the lower proportionality (0.55) and none exceed a 18ε / 15ε proportionality of 0.68. Almost all Nar reductases, on the contrary, cluster tightly around the higher proportionality (0.91) with no values below a 18ε / 15ε proportionality of 0.84 with the notable exception of the Nar reductases from the genus Bacillus which fall around 0.62 and thus closely resemble the isotopic fingerprints of the Nap reductases. Our findings confirm the existence of two remarkably distinct isotopic end-members in the dissimilatory nitrate reductases that could indeed explain differences in coupled N and O isotope fractionation between marine and terrestrial systems, and almost but not fully match reductase phylogeny.</jats:p

Activity and abundance of sulfate reducting bacteria and environmental parameter in serpentinizing fluids of Coast Range ophiolote (USA) and Samail ophiolote (Oman)

We have measured microbial sulfate reduction rates by ultra sensitive radio tracer incubations in fluids from 11 wells of the actively serpentinizing the Coast Range ophiolite (CA, USA) and Samail ophiolite (Oman). We also determined environmental parameter of the serpentinizing fluids (pH, redox potential and chloride, sulfate, hydrogen, methane and organic acid conentrations. We also determiend the abundance of sulfate reducing taxa in the these fluids