Serpentinization is the hydration and oxidation of ultramafic rock, which occurs as oceanic lithosphere is emplaced onto continental margins (ophiolites), and along the seafloor as faulting exposes this mantle-derived material to circulating hydrothermal fluids. This process leads to distinctive fluid chemistries as molecular hydrogen (H 2 ) and hydroxyl ions (OH − ) are produced and reduced carbon compounds are mobilized. Serpentinizing ophiolites also serve as a vector to transport sulfur compounds from the seafloor onto the continents. We investigated hyperalkaline, sulfur-rich, brackish groundwater in a serpentinizing continental ophiolite to elucidate the role of sulfur compounds in fuelling in situ microbial activities. Here we illustrate that key sulfur-cycling taxa, including Dethiobacter, Desulfitispora and 'Desulforudis', persist throughout this extreme environment. Biologically catalysed redox reactions involving sulfate, sulfide and intermediate sulfur compounds are thermodynamically favourable in the groundwater, which indicates they may be vital to sustaining life in these characteristically oxidant-and energy-limited systems. Furthermore, metagenomic and metatranscriptomic analyses reveal a complex network involving sulfate reduction, sulfide oxidation and thiosulfate reactions. Our findings highlight the importance of the complete inorganic sulfur cycle in serpentinizing fluids and suggest sulfur biogeochemistry provides a key link between terrestrial serpentinizing ecosystems and their submarine heritage.

Serpentinization of peridotites in Earth’s mantle is associated with the generation of hydrogen and low molecular weight organics that could support subsurface life. Studies of microbial metabolisms in peridotite-hosted environments have focused primarily on methanogenesis, yet DNA sequences, isotopic composition of sulfides and thermodynamic calculations suggest there is potential for microbial sulfate reduction too. Here, we use a sulfate radiotracer-based method to quantify microbial sulfate reduction rates in serpentinization fluids recovered from boreholes in the Samail Ophiolite, Oman and the California Coast Range Ophiolite, USA. We find that low levels of sulfate reduction occur at pH up to 12.3. These low levels could not be stimulated by addition of hydrogen, methane or small organic acids, which indicates that this metabolism is limited by factors other than substrate availability. Cellular activity drops at pH > 10.5 which suggests that high fluid pH exerts a strong control on sulfate-reducing organisms in peridotites.

Serpentinization is a low-temperature metamorphic process by which ultramafic rock chemically reacts with water. Such reactions provide energy and materials that may be harnessed by chemosynthetic microbial communities at hydrothermal springs and in the subsurface. However, the biogeochemistry mediated by microbial populations that inhabit these environments is understudied and complicated by overlapping biotic and abiotic processes. We applied metagenomics, metatranscriptomics, and untargeted metabolomics techniques to environmental samples taken from the Coast Range Ophiolite Microbial Observatory (CROMO), a subsurface observatory consisting of 12 wells drilled into the ultramafic and serpentinite mélange of the Coast Range Ophiolite in California. Using a combination of DNA and RNA sequence data and mass spectrometry data, we found evidence for several carbon fixation and assimilation strategies, including the Calvin-Benson-Bassham cycle, the reverse tricarboxylic acid cycle, the reductive acetyl coenzyme A (acetyl-CoA) pathway, and methylotrophy, in the microbial communities inhabiting the serpentinite-hosted aquifer. Our data also suggest that the microbial inhabitants of CROMO use products of the serpentinization process, including methane and formate, as carbon sources in a hyperalkaline environment where dissolved inorganic carbon is unavailable. IMPORTANCE This study describes the potential metabolic pathways by which microbial communities in a serpentinite-influenced aquifer may produce biomass from the products of serpentinization. Serpentinization is a widespread geochemical process, taking place over large regions of the seafloor and at continental margins, where ancient seafloor has accreted onto the continents. Because of the difficulty in delineating abiotic and biotic processes in these environments, major questions remain related to microbial contributions to the carbon cycle and physiological adaptation to serpentinite habitats. This research explores multiple mechanisms of carbon fixation and assimilation in serpentinite-hosted microbial communities.

Polyethylene plastics are a major source of industrial and household wastes, the majority of which end up in the environment or in landfills. These wastes pose challenges for microbial biodegradation due to their polymeric structure. There is a critical need for a process that aids in the breakdown and reuse of plastic compounds. Pyrolysis of high-density polyethylene (HDPE) has previously been used to induce chemical changes in plastic compounds, resulting in more structurally simplistic compounds. Here, we demonstrate the ability of pyrolysis to produce microbially biodegradable intermediate compounds. Biodegradation of pyrolysis-treated plastics has not previously been demonstrated. We found that enrichment cultures derived from six different environmental inocula were able to achieve extensive biodegradation of polyethylene pyrolysis products over the course of 5 days. We verified the biodegradation by quantifying residual compound concentrations of alkenes using gas chromatography/mass spectrometry (GC/MS). 16S rRNA gene amplicon sequencing results demonstrated that the most dominant taxa in the microbial community belonged to the phylum Proteobacteria. Many organisms in this phylum have previously been shown to metabolize hydrocarbons. Our results indicate that the coupling of chemical and biological processes can speed up the breakdown and conversion of polyethylene to bacterial biomass by microbial consortia.

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