Endolithic and planktonic subsurface microbiome within zones of active low-temperature serpentinization in the Samail ophiolite of Oman, The

Abstract

The rock-hosted microbial ecosystems within serpentinizing subsurface environments are a prime target for future NASA missions searching for extant life in the solar system and constitute a unique habitat within Earth’s subsurface that is relatively uncharacterized. The water-rock interaction called serpentinization generates highly reduced and hyperalkaline fluids replete with hydrogen (H2) and reduced carbon compounds such as methane, which are potent electron donors and carbon sources used in microbial metabolism. This dissertation investigates the microbiome within a low-temperature terrestrial serpentinizing subsurface in the Samail ophiolite, Sultanate of Oman, from biomass collected from peridotite- and gabbro-hosted aquifer waters and drilled serpentinite rock cores. The metabolic pathways and key organisms of the Samail ophiolite’s microbial methane cycle and sulfur cycle are elucidated through multiple molecular methods with DNA and RNA sequencing and geochemical data. Results from this effort demonstrate a single active methanogen and methanotrophic populations in waters with hyperalkaline pH, representing an extension of the pH tolerance range of methanogens and suggesting biological involvement in the generation of unusual isotopic signatures of methane in the Samail ophiolite. The more complex subsurface microbial sulfur cycle is dominated by a few alkaliphilic sulfate reducing organisms and a single primary sulfur oxidizer. Biological sulfate reduction is a prominent metabolism in gabbro and hyperalkaline peridotite wells; polysulfide and elemental sulfur metabolism is important in moderately alkaline waters; and sulfur oxidation is limited to less alkaline gabbro waters. In addition to identifying planktonic microorganisms within well fluids, genomic material of endolithic microbiota residing within serpentinite rock was recovered using a novel core handling and DNA extraction protocol focused on contaminant minimization. Putative sulfate reducers, syntrophic organisms, methanogens, iron-metabolizing organisms, and other phylotypes constitute the endolithic microbiome based on small-subunit ribosomal RNA sequencing. Comparison of organisms in planktonic and endolithic biomass indicates a distinct rock-hosted community exists, potentially influenced by local fluid geochemistry, syntrophic interactions, and adjacency to Fe-bearing minerals. Taken together, this dissertation provides insight into the structure, distribution, and metabolism of the resident planktonic and endolithic microbiota of the Samail ophiolite and the habitability of deep subsurface environments of low temperature serpentinization. These results help to inform on the biogeochemical cycles within modern ophiolites, aid future missions by further constraining the habitability of other rocky subsurfaces such as on Mars, and refine protocols to process low-biomass rock in the presence of abundant contaminants.