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 3 - ) and nitrite ( NO 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-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 3 - type fluids comprised mostly of NO 3 - , and Ca2+-OH- fluids comprised primarily of ammonia (NH3). We infer that fixed N is introduced to the serpentinite aquifer as NO 3 - . High concentrations of NO 3 - (>100 µM) with a relict meteoric oxygen isotopic composition (δ18O ~ 22‰, Δ17O ~ 6‰) were observed in shallow aquifer fluids, indicative of NO 3 - sourced from atmospheric deposition (rainwater NO 3 - : δ18O of 53.7‰, Δ17O of 16.8‰) mixed with NO 3 - produced in situ through nitrification (estimated endmember δ18O and Δ17O of ~0‰). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH3 (>100 µM) with little NO 3 - detectable. We interpret that NH3 in hyperalkaline fluids is a product of NO 3 - reduction. The proportionality of the O and N isotope fractionation (18ε / 15ε) measured in Samail Ophiolite NO 3 - was close to unity (18ε / 15ε ~ 1), which is consistent with dissimilatory NO 3 - reduction with a membrane-bound reductase (NarG); however, abiotic reduction processes may also be occurring. The presence of genes commonly involved in N reduction processes (narG, napA, nrfA) in the metagenomes of biomass sourced from aquifer fluids supports potential biological involvement in the consumption of NO 3 - . Production of NH 4 + as the end-product of NO 3 - reduction via dissimilatory nitrate reduction to ammonium (DNRA) could retain N in the subsurface and fuel nitrification in the oxygenated near surface. Elevated bioavailable N in all sampled fluids indicates that N is not likely limiting as a nutrient in serpentinites of the Samail Ophiolite.

Parapatric speciation of Meiothermus in serpentinite-hosted aquifers in Oman.

The factors that control the distribution and evolution of microbial life in subsurface environments remain enigmatic due to challenges associated with sampling fluids from discrete depth intervals via boreholes while avoiding mixing of fluids. Here, using an inflatable packer system, fracture waters were isolated and collected from three discrete depth intervals spanning >130 m in a borehole intersecting an ultramafic rock formation undergoing serpentinization in the Samail Ophiolite, Sultanate of Oman. Near surface aquifer waters were moderately reducing and had alkaline pH while deeper aquifer waters were reduced and had hyperalkaline pH, indicating extensive influence by serpentinization. Metagenomic sequencing and analysis of DNA from filtered biomass collected from discrete depth intervals revealed an abundance of aerobes in near surface waters and a greater proportion of anaerobes at depth. Yet the abundance of the putatively obligate aerobe, Meiothermus, increased with depth, providing an opportunity to evaluate the influence of chemical and spatial variation on its distribution and speciation. Two clades of Meiothermus metagenome assembled genomes (MAGs) were identified that correspond to surface and deep populations termed Types I (S) and II (D), respectively; both clades comprised an apparently Oman-specific lineage indicating a common ancestor. Type II (D) clade MAGs encoded fewer genes and were undergoing slower genome replication as inferred from read mapping. Further, single nucleotide variants (SNVs) and mobile genetic elements identified among MAGs revealed detectable, albeit limited, evidence for gene flow/recombination between spatially segregated Type I (S) and Type II (D) populations. Together, these observations indicate that chemical variation generated by serpentinization, combined with physical barriers that reduce/limit dispersal and gene flow, allowed for the parapatric speciation of Meiothermus in the Samail Ophiolite or a geologic precursor. Further, Meiothermus genomic data suggest that deep and shallow aquifer fluids in the Samail Ophiolite may mix over shorter time scales than has been previously estimated from geochemical data.

Molecular Evidence for an Active Microbial Methane Cycle in Subsurface Serpentinite-Hosted Groundwaters in the Samail Ophiolite, Oman.

Serpentinization can generate highly reduced fluids replete with hydrogen (H2) and methane (CH4), potent reductants capable of driving microbial methanogenesis and methanotrophy, respectively. However, CH4 in serpentinized waters is thought to be primarily abiogenic, raising key questions about the relative importance of methanogens and methanotrophs in the production and consumption of CH4 in these systems. Herein, we apply molecular approaches to examine the functional capability and activity of microbial CH4 cycling in serpentinization-impacted subsurface waters intersecting multiple rock and water types within the Samail Ophiolite of Oman. Abundant 16S rRNA genes and transcripts affiliated with the methanogenic genus Methanobacterium were recovered from the most alkaline (pH, >10), H2- and CH4-rich subsurface waters. Additionally, 16S rRNA genes and transcripts associated with the aerobic methanotrophic genus Methylococcus were detected in wells that spanned varied fluid geochemistry. Metagenomic sequencing yielded genes encoding homologs of proteins involved in the hydrogenotrophic pathway of microbial CH4 production and in microbial CH4 oxidation. Transcripts of several key genes encoding methanogenesis/methanotrophy enzymes were identified, predominantly in communities from the most hyperalkaline waters. These results indicate active methanogenic and methanotrophic populations in waters with hyperalkaline pH in the Samail Ophiolite, thereby supporting a role for biological CH4 cycling in aquifers that undergo low-temperature serpentinization.IMPORTANCE Serpentinization of ultramafic rock can generate conditions favorable for microbial methane (CH4) cycling, including the abiotic production of hydrogen (H2) and possibly CH4 Systems of low-temperature serpentinization are geobiological targets due to their potential to harbor microbial life and ubiquity throughout Earth's history. Biomass in fracture waters collected from the Samail Ophiolite of Oman, a system undergoing modern serpentinization, yielded DNA and RNA signatures indicative of active microbial methanogenesis and methanotrophy. Intriguingly, transcripts for proteins involved in methanogenesis were most abundant in the most highly reacted waters that have hyperalkaline pH and elevated concentrations of H2 and CH4 These findings suggest active biological methane cycling in serpentinite-hosted aquifers, even under extreme conditions of high pH and carbon limitation. These observations underscore the potential for microbial activity to influence the isotopic composition of CH4 in these systems, which is information that could help in identifying biosignatures of microbial activity on other planets.

High reactivity of deep biota under anthropogenic CO2 injection into basalt.

Basalts are recognized as one of the major habitats on Earth, harboring diverse and active microbial populations. Inconsistently, this living component is rarely considered in engineering operations carried out in these environments. This includes carbon capture and storage (CCS) technologies that seek to offset anthropogenic CO2 emissions into the atmosphere by burying this greenhouse gas in the subsurface. Here, we show that deep ecosystems respond quickly to field operations associated with CO2 injections based on a microbiological survey of a basaltic CCS site. Acidic CO2-charged groundwater results in a marked decrease (by ~ 2.5-4) in microbial richness despite observable blooms of lithoautotrophic iron-oxidizing Betaproteobacteria and degraders of aromatic compounds, which hence impact the aquifer redox state and the carbon fate. Host-basalt dissolution releases nutrients and energy sources, which sustain the growth of autotrophic and heterotrophic species whose activities may have consequences on mineral storage.

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 H2, CH4, Ca2+, Mg2+, [Formula: see text], [Formula: see text], trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and fermentation, as well as the oxidation of methane, hydrogen and small molecular weight organic acids utilizing nitrate and sulfate as electron acceptors. Dynamic nitrogen cycling may be especially prevalent in gabbro and alkaline peridotite fluids. These data suggest water-rock reaction, as controlled by lithology and hydrogeology, constrains the distribution of life in terrestrial ophiolites.

Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions.

Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO2 This study demonstrates for the first time the permanent disposal of CO2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. This result contrasts with the common view that the immobilization of CO2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO2 emissions through mineralization can be far faster than previously postulated.

Relative permeability experiments of carbon dioxide displacing brine and their implications for carbon sequestration.

To mitigate anthropogenically induced climate change and ocean acidification, net carbon dioxide emissions to the atmosphere must be reduced. One proposed option is underground CO2 disposal. Large-scale injection of CO2 into the Earth's crust requires an understanding of the multiphase flow properties of high-pressure CO2 displacing brine. We present laboratory-scale core flooding experiments designed to measure CO2 endpoint relative permeability for CO2 displacing brine at in situ pressures, salinities, and temperatures. Endpoint drainage CO2 relative permeabilities for liquid and supercritical CO2 were found to be clustered around 0.4 for both the synthetic and natural media studied. These values indicate that relative to CO2, water may not be strongly wetting the solid surface. Based on these results, CO2 injectivity will be reduced and pressure-limited reservoirs will have reduced disposal capacity, though area-limited reservoirs may have increased capacity. Future reservoir-scale modeling efforts should incorporate sensitivity to relative permeability. Assuming applicability of the experimental results to other lithologies and that the majority of reservoirs are pressure limited, geologic carbon sequestration would require approximately twice the number of wells for the same injectivity.