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

Abstract

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 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, information that could help in identifying biosignatures of microbial activity on other planets.

Low‐Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite

“…Recent work has identified low‐diversity microbial communities in well NSHQ‐14, just a few meters from Hole BA3A, as well as in other local wells containing hyperalkaline groundwater. These communities are dominated by Thermodesulfovibrionaceae (likely sulphate reducers), candidate phylum OP1 (potential hydrogenotrophic acetogens), Meiothermus (generally aerobic heterotrophs), and Methanobacterium (methanogens) likely utilizing H 2 as an electron donor (Kraus et al., 2020; Miller et al., 2016; Nothaft et al., 2020, 2021; Rempfert et al., 2017). To date, all the microbiological studies in the Samail ophiolite have focused on characterizing the community structure and activity of organisms present in the fracture fluids.…”

Aqueous Geochemical and Microbial Variation Across Discrete Depth Intervals in a Peridotite Aquifer Assessed Using a Packer System in the Samail Ophiolite, Oman

“…Mixing extents calculated for our samples are tabulated in Table S1. Other Samail Ophiolite data from (Canovas III et al, 2017;Kraus et al, 2021;Miller et al, 2016;Neal & Stanger, 1985;Nothaft et al, 2020;Paukert Vankeuren et al, 2019;Rempfert et al, 2017). atmospheric 2 CO into these moderately alkaline waters (Bruni et al, 2002;Cipolli et al, 2004;Leong & Shock, 2020;Paukert et al, 2012 was measured in the same analytical session as BA1A_2018_100-400, had  CO 2 c below the limit of quantitation in 2018 (<12    1 mol L ; Nothaft et al, 2020).…”

Procaryotic Diversity and Hydrogenotrophic Methanogenesis in an Alkaline Spring (La Crouen, New Caledonia)

“…Aerobic degradation of methylphosphonate by marine bacterioplankton has also been mentioned to contribute to CH 4 production in the sea and ocean waters [ 25 , 26 ]. In continental hyperalkaline springs, molecular studies have recently evidenced active microbial CH 4 production and oxidation in the serpentinite-hosted waters of the Voltri Ophiolitic springs (Italy) [ 27 ] and those of the Samail Ophiolite (Oman) [ 28 ]. Indeed, both anaerobic archaeal methanogens Methanobacteriaceae and aerobic bacterial methanotrophs Methylococcaceae were prevalent at these continental sites [ 27 , 28 , 29 ].…”

“…In continental hyperalkaline springs, molecular studies have recently evidenced active microbial CH 4 production and oxidation in the serpentinite-hosted waters of the Voltri Ophiolitic springs (Italy) [ 27 ] and those of the Samail Ophiolite (Oman) [ 28 ]. Indeed, both anaerobic archaeal methanogens Methanobacteriaceae and aerobic bacterial methanotrophs Methylococcaceae were prevalent at these continental sites [ 27 , 28 , 29 ]. At the alkaline site of La Crouen (New Caledonia), Deville and Prinzhofer [ 3 ] have measured a low CH 4 content (2.65−2.73%), with a δ 13 C signature of −39%, suggesting a possible thermogenic origin of CH 4.…”