Award: OCE-2023575

Seafloor sediments are one of the largest global reservoirs for organic carbon on Earth. Low molecular weight (LMW) carbon compounds form during sedimentary organic matter alteration and these substrates are important energy and structural carbon sources for microbial communities. Methanogenesis is an important terminal metabolic process in marine sediments, especially sediments with low/no sulfate. Methane is generated from a number of LMW precursors, including hydrogen/carbon dioxide, acetate, formate, methanol and/or methylamines. 

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading and hydrothermalism that occurs in a region of extensive sedimentation. Hydrothermal heating of sediments drives biological and abiotic carbon transformations and results in interconnected biogeochemical cycles.  This project documented the ways through which heating of subsurface sediments affected the concentration and fate of reactive LMW dissolved organic carbon. We tracked the fate of key carbon species – including, formate, acetate, methanol, methylamine and carbon dioxide – through oxidative and methanogenic pathways. We also examined whether carbon monoxdide and/or hydrogen serve as substrates for microbial metabolism. We also interogated  patterns of stable sulfur isotopes to better understand interactions between carbon, sulfur and iron cycling. 

This project showed that methanogenesis is driven by the contemporaneous activity of several different pathways; that carbon monoxide may serve an important energy source in deep subsurface sediments; and that sulfur cycling is closely coupled to iron and carbon cycling in these deep, hot sediments.

Methanogenic activity from a number of substrates occurred over a wide temperature range (3°C to 80°C), highlighting the unexpectedly high metabolic versatility of methanogens in deep, thermally heated sediments. High methanogenesis rates were detected in near-surface sediments driven predominantly by methylotrophic methanogenesis, followed by hydrogenotrophic pathways. Rates declined sharply with depth, particularly within the 40–60°C interval, indicating a transition from moderate temperature loving to high temperature loving microbial communities. Methylotrophic methanogenesis remained detectable down to 320 meters below the seafloor and was the dominant methane-producing pathway at temperatures up to 60°C. In sediments increasingly influenced by magmatic intrusions, hydrogenotrophic and acetoclastic methanogenesis became the predominant modes of methane production. This deep, hot activity is attributed to the presence of active microbial biomass and the enhanced reactivity and bioavailability of organic matter in deep, hydrothermally-heated sediments, which provided abundant substrates for methanogenesis. These findings expand the current understanding of methanogenesis in the deep biosphere and reveal the discovery of the contemporaneous activity of multiple methanogenic pathways in deep, hydrothermally-influenced sediments.

We documented the importance of carboxydotrophy - carbon monoxide (CO) oxidation - as a metabolism in hydrothermally-impacted subsurface sediments of the Guaymas Basin, Gulf of California. We measured CO concentrations ranging from < 149 to 655 nmol L⁻¹ in sediments from 0.8 and down to 540 mbsf. Metagenomic and metatranscriptomic analyses show that carbon monoxide dehydrogenase (CODH) genes were present and widely expressed, with relative abundance and expression levels decreasing with depth. Incubation experiments using electron acceptors and inhibitors demonstrate that CO was largely consumed by sulfate reducers. These results reveal that CO is an energy source for microbes living in deep biosphere and carboxydotrophy is an efficient metabolic pathway not previously recognized in marine subseafloor sediments.

Microbial sulfur cycling plays an outsized role in organic matter degradation in marine sediments. The coupling of sulfur biogeochemistry with carbon and iron cycling in subsurface hydrothermal sediment remain poorly constrained. Here, we investigated carbon-sulfur-iron diagenesis in deep sediments of the Guaymas Basin in the Gulf of California. Sediments as deep as 370 meters below the seafloor had a low average carbon-to-sulfur ratio (C/S ≈ 1.6), particularly at sites impacted by active hydrothermal circulation (1.3–0.67). These values are well below the typical value of 2.8 for marine sediment and could reflect sulfur enrichment and/or elevated sulfate reduction stimulated by organic inputs driven by high geothermal gradients and sill intrusion. Reactive iron content correlates positively with elemental sulfur, indicating iron oxides facilitate elemental sulfur accumulation. High sulfur isotope fractionations of 60–65‰ contribute to ³⁴S-depleted pyrite (~ −40‰) in the organic-rich sediments. These results highlight the control of temperature and iron redox chemistry on sulfur dynamics in hydrothermal systems.

The broader impacts of this award included contributing to the geosciences workforce by training graduate students, hosting a summer camp for middle school students, and creating an educational cartoon. Joye developed the Ocean Discovery Camp which is taught through the UGA Summer Academy. This award provided three scholarships so that local middle schoolers could participate in the summer camp. Camp topics include ocean processes, ocean exploration, deep ocean extreme environments, ocean animals, and ocean conservation. We also created an episode of The Adventures of Zack and Molly, "To the Bottom of the Ocean and BEYOND" [https://youtu.be/lDWlxZ-AUKs?si=_mPyIJc-cJ17ir4a] in collaboration with Jim Toomey Education. A learning guide is available to teach the viewer more about hydrothermal processes and carbon cycling in marine sediments. 

 

Last Modified: 08/11/2025
Modified by: Samantha B Joye