Identifying Putative Subsurface Microbial Drivers of Methane Flux on Earth and Mars
Authors: Sapers Haley , Orphan Victoria , Moores John , Whyte Lyle , Côté Mathieu , Fecteau Daniel , Grandmont Frédéric , Innanen Alex , Rusley Calvin , Roux Michel 
Published: October 2023

On Earth microorganisms are critical drivers of the methane cycle, both producing and consuming methane (Boetius et al. 2000, Knittel and Boetius 2009, Orphan et al. 2001). Molecular and isotopic-based investigations of archaeal-bacterial consortia catalyzing the anaerobic oxidation of methane (AOM) in marine methane seeps identified the pivotal role of these microorganisms in mitigating the release of methane into the atmosphere (Knittel and Boetius 2009, Orphan et al. 2001). In the marine environment, AOM is predominantly carried out by closely associated consortia of methanotrophic archaea (ANME) and sulfate reducing bacteria (SRB) coupling methane oxidation to sulfate reduction in the absence of oxygen.
Wolf Spring (WS), Axel Heiberg Island, Nunavut is a hypersaline cold spring methane seep and the only known terrestrial permafrost hosted methane seep known to host ANME-1 archaea associated with AOM (Niederberger et al. 2010, Magnuson et al. 2022). Wolf Spring is an unparalleled analogue for putative subsurface brines and sites of methane release on Mars. Enigmatic observations of methane in the near-surface Martian atmosphere remain a tantalizing potential biosignature.
The combination of field site characterization, microbial microcosm experiments, and in situ methane monitoring represents a coordinated interdisciplinary effort to identify methane driven microbial metabolisms not only critical to understanding methane flux in the Arctic, but also as possible drivers to the methane cycle on Mars. Detailed microbial characterization of these springs has identified a chemotrophic community dominated by sulfur cycling (Altshuler et al. 2022, Niederberger et al. 2010). To date, microbial and geochemical characterization has been carried out on sediment samples to a few centimeters depth. This study expands on these initial studies, with the successful collection and analysis of deeper sediment cores at WS focusing on AOM activity to better understand the microorganisms involved and the methane cycling capacity at depth.
Two decades of observing methane on Mars (Mumma et al. 2009) have generated data indicative of a dynamic, geochemical system characterized by a profile similar to the release of methane from seeps on Earth (Etiope and Oehler 2019) producing both distinct pulses known as plumes and slow background seepage. These observations suggest as of yet unknown geochemical and potentially geobiological methane sources and sinks.
While methane can be produced abiotically (Etiope and Lollar 2013), on Earth most methane is biogenic. Determining the biogenicity of CH4 is non-trivial and requires a correlated approach including determination of carbon isotopes. In terrestrial systems, biogenic CH4 is 13C depleted. To characterize methane sources and sinks on Mars, near surface measurements at a frequency not possible with existing instrumentation are required.
We are currently developing off-axis integrated cavity-enhanced output (OA-ICOS) spectrometry as a portable trace gas analyzer capable of obtaining high frequency measurements of methane at the sub-ppb level (Sapers et al. 2021). Optimizing OA-ICOS trace methane measurements at WS will help refine sensitivity and measurement cadence in a Mars-like environment as well as providing new remote methane monitoring capabilities for Arctic methane emissions. We are currently developing in situ 12CH4:13CH4 capabilities using OA-ICOS technology. The importance of δ 13C as a biosignature is summarized in Fig. 1.