Environmental constraints of microbial carbon dynamics in hydrothermally impacted sediments of the Guaymas Basin

The Guaymas Basin, situated in the Gulf of California, is characterized by significant sediment deposition, active seafloor spreading, and strong hydrothermal fluid advection. These features create conditions conducive to development of diverse microbial communities, which actively mediate an array of metabolic processes. The dominant anaerobic microbial processes in hydrothermal systems (e.g. methane oxidation, sulfate reduction, etc.) have been intensively studied, but the environmental variables and geochemical factors regulating these processes are poorly understood. The unique environmental conditions of the Guaymas Basin create complex and variable geochemical profiles that, modulate microbial processes across a range of redox conditions. To constrain the effects of environmental factors (e.g. temperature, nutrients, substrate availability, etc.) on these processes, we quantified the rates of microbial activity with focus on the metabolism of low molecular weight (LMW) organic compounds, including methane, bicarbonate, acetate, and methanol from distinct habitats. We collected sediments from multiple sites exhibiting a range of temperatures (4 to ~80°C), measured a full geochemical suite, and used ¹⁴C radiotracers to quantify the metabolism (oxidation, methane production, assimilation) of these LMW carbon compounds. Variations in substrate availability and geochemical signatures revealed potential controls on metabolic rates. Similarly, in situ temperature was a primary control on metabolic activity; and the turnover of LMW carbon was rapid at elevated temperatures (45°C), and detectable up to ~80°C. Anaerobic methane oxidation rates were higher than sulfate reduction highlighting the importance of metabolic phasing, where oxidized species (e.g. nitrate or, Fe or Mn oxides) may act as alternative electron acceptors. These results highlight the importance of environmental drivers of LMW carbon cycling in hydrothermally impacted sediments of the Guaymas Basin.