Microbial methanogenesis within hydrating peridotite
Professor Alexis Templeton, University of Colorado at Boulder, USA
The peridotite aquifers in Oman are actively undergoing low-temperature hydration and carbonation at near-surface temperatures. Several distinct fluid regimes exist that harbor distinct microbial communities and functional capabilities. Organisms such as Methanobacterium sp., methanogens who possess the genetic capability for the reduction of CO2 using molecular hydrogen, are notably widespread. However, the thermodynamic feasibility of CO2 reduction to methane is spatially and temporally variable, depending upon the extent of water rock reaction, fluid mixing, fluid pH, carbon availability and flux of dissolved hydrogen. Thus, it is challenging to determine the conditions under which rock-hosted methanogens convert dissolved inorganic carbon into methane, and to predict the scale of carbon fluxes controlled by in-situ methanogenesis. We are investigating the in situ coupling between water/rock reaction and methanogenic activity, in order to interpret the isotopic, lipid and mineralogical biosignatures of the peridotite-hosted subsurface biosphere. Detailed geochemical analysis of fluids and dissolved gases derived from subsurface sampling, coupled with methane production assays, genetic analysis of the methanogens, and laboratory experiments, together show that biological methane production can occur across an enormous range of conditions within the peridotite-water-CO2 system. Distinct isotopic and isotopologue compositions of the methane are produced that can be used to infer the carbon availability and energy state of the microbial communities. In particular, we can demonstrate that carbonate minerals might be critically important for sustaining slow biological methane production in hyperalkaline systems. Such findings are important in focusing our search for rock-hosted life on Earth and other rocky bodies in our solar system. Continuous biological methane production under hyperalkaline conditions should also be a quantitatively significant component of the carbon flux within serpentinites, shifting the balance from mineral carbonation to methanation.
Carbon sources shaping deep ecosystems in oceanic serpentinites
Professor Bénédicte Ménez, Institut de Physique du Globe de Paris/Université Paris Diderot, France
In serpentinizing alkaline environments, the lack of inorganic carbon sources and the reduced conditions are challenging for the development of deep ecosystems. Alternatively, abiotic organic compounds deriving from the serpentinization process could serve as carbon or energy sources, making organoheterotrophy a valuable strategy for microbes in serpentinization-related environments (1-3). Up to now, effective abiotic synthesis was only demonstrated for a restricted number of low-molecular-weight organic compounds in hydrothermal systems associated with serpentinization on Earth. These compounds include methane, short-chain alkanes and formate. In this talk, recent evidences for the presence of a larger diversity of abiotic organic compounds will be discussed. Special emphasis will be put on condensed carbonaceous matter predicted to be thermodynamically favored at low temperature where the formation of methane is kinetically inhibited (4). Condensed carbonaceous matter was recently shown to form simultaneously to the hydrothermal alteration of the oceanic crust and the growth of the host mineralogical assemblage. The local production of H2 associated with mineral formation and the presence of various catalysts appear to represent key parameters controlling the production rate and the chemistry of the associated carbonaceous matter. New pathways and their relevancy at the crust scale will be addressed during the talk. Consequences for ancestral metabolisms and microbial life strategies in the present-day deep biosphere will be discussed in light of the metabolic capabilities of microbes inhabiting such environments.
Identifying microbial activity in serpentinization systems using organics and isotopes
Assistant Professor Susan Q Lang, University of South Carolina, USA
The hydrogen produced during water-rock serpentinization reactions can fuel life in the rocky oceanic subsurface, and promote the abiotic synthesis of organic molecules. Microbial activity may be limited, however, by a combination of elevated temperatures, alkaline pHs, and limited bioavailable inorganic carbon. Our goal is to develop a mechanistic understanding of the relationship between microbial activity and the physical and geochemical characteristics of the rocky subsurface within serpentinizing environments. These associations can provide insights into where life may have developed on Earth or exist on other planetary bodies. Recent expeditions to the Atlantis Massif and to the Lost City Hydrothermal field (30°N, Mid-Atlantic Ridge) provide the opportunity to investigate carbon cycling and the presence of life in a zone of active serpentinization. The abundances, distributions, and isotopic compositions of carbon compounds are highly heterogeneous, and reflect varying degrees of subsurface water-rock reactions and microbial activity. In the subseafloor rocks, high concentrations of small organic acids are associated with serpentinized harzburgite and metadolerite lithologies in particular. Indications of biological activity, such as biologically derived amino acids and cellular abundances, have distribution patterns distinct from those of organic acids. Active metabolic pathways can be identified through a combination of incubation experiments and by comparing the radiocarbon content of lipid biomarkers to that of potential carbon sources. These findings may shed light on the feasibility of current theories that propose that the earliest metabolisms, and the Acetyl-CoA pathway in particular, may have developed by a transition from geoenergetics to bioenergetics within alkaline hydrothermal systems.
Serpentine and the search for life beyond Earth
Dr Steve Vance, California Institute of Technology, USA
Hydrogen from serpentinization is a potential source of chemical energy for life in the subsurface of Mars, and in the icy ocean worlds in the outer solar system. Much of the context of planetary serpentinization relies on inference from modeling and studies on Earth. While there is good evidence that serpentinization has occurred on Mars, the extent and duration of that activity has not been constrained. Similarly, current serpentinization might help to explain the abundant hydrogen in the ocean of the tiny saturnian moon Enceladus, but this raises questions of how long such activity has persisted. Titan’s hydrocarbon rich atmosphere may have derived from ancient or even present-day serpentinization at the bottom of its ocean, but this is difficult to confirm, and difficult to conceive under the high pressures in Titan’s interior. In Europa, volcanic activity and serpentinization may provide complementary sources of hydrogen as a redox couple to oxygen generated at the moon’s surface. Planned robotic exploration missions to these places can aid in the quest to understand the planetary context of serpentinization