B52B-01 INVITED
Energy as a Constraint on Habitability in the Subsurface
All living things must obtain energy from the environment to grow, to maintain a metabolic steady state, or simply to preserve viability. The availability of energy sources in the environment thus represents a key factor in determining the size, distribution, and activity of biological populations, and ultimately constrains the possibility for life itself. Lacking the abundant energy provided by solar radiation or the products of oxygenic photosynthesis, life in subsurface environments may be limited by energy availability as much as any other factor. The biological requirement for energy is expressed in two dimensions - analogous to the power and voltage requirements of electrical devices - and consideration and quantification of these requirements establishes quantitative boundary conditions on subsurface habitability. The magnitude of these requirements depends significantly on physicochemical environment, as does the provision of biologically-accessible energy from subsurface sources. With this conceptual basis, we are developing an 'energy balance' model that is designed to ultimately predict the habitability of a given environment, with respect to a given metabolism, in quantitative terms (as 'biomass density potential'). The model will develop from conceptual to quantitative as experimental and observational work constrains and quantifies, in natural populations adapted to low energy conditions, the magnitude of the biological energy requirements and the impacts of physicochemical environmental conditions on energy demand and supply.
B52B-02
In situ Determination of Physiological States Under Conditions Characteristic of the Subseafloor Microbial Biosphere
Studies of water samples from the deep sea have revealed organisms specifically adapted to the low temperatures (~2° C) and elevated pressures (~100 MPa) native to these environments and elucidated genetic and physiological adaptations of life to high pressures. Investigation of the subsurface biosphere, at pressures exceeding those of the deep sea, has pushed the depth limits of microbial ecosystems to at least 1000 meters below the seafloor and 3-4 km into the continental lithosphere. In many of these environments, organisms are confronted with multiple stressors including not only high pressures, but high temperatures and low energy fluxes. Subsurface microorganisms live at a precarious boundary between geologically-supported growth and cell death and remineralization. As a result of these factors the calculated average growth rates of these organisms have challenged our notions of what is biologically possible. A limitation to our study of deep ecosystems has been an inability to accurately characterize microbial physiology under conditions found in subsurface habitats. This paper describes a strategy to distinguish the physiological status of microorganisms indigenous to the deep subseafloor at environmentally relevant temperatures (50 - 200° C) and pressures (30 - 300 MPa). Preliminary results using chemolithoautotrophic bacteria and hyperthermophilic Archaea show that four distinct physiological states (active and growing, active but non-growing, viable but inactive, and dead) can be distinguished in situ using biomolecular probes coupled to geochemical measurements. Further refinements of this approach can be used to expand our understanding of microbial adaptations to high pressure environments, and be linked to both genetic and geochemical studies at high pressure. The data produced by such experiments will be important to deciphering both the extent and the biogeochemical consequences of a deep subsurface biosphere.
B52B-03
Geological Sources of Hydrogen for Subsurface Microbial Communities
Subsurface microbial communities can be conveniently divided into two general types: heterotrophic communities that rely primarily on input of photosynthetically derived organic matter, and autotrophic communities that rely on inorganic chemical sources of energy. In situ production of H2 has been proposed to support subsurface autotrophic microbial communities within basalts and ultramafic rocks in both subaerial and submarine settings. The extent and activity of such communities, and even their very ability to inhabit subsurface environments, depends largely on the balance between the supply of H2 for metabolic energy and the energetic costs of existence (T. Hoehler, Geobiology, 2004). As a consequence, the capacity for H2 generation in basaltic and ultramafic environments places significant constraints on the distribution and productivity of microbial populations. At present, however, geochemical reactions that might generate H2 in basaltic and ultramafic systems at temperatures sufficiently low to allow life to exist (<~150 ° C) remain very poorly known. I will summarize the currently available experimental data on H2 production during low-temperature alteration of basaltic and ultramfic rocks, including ongoing laboratory studies to refine the chemical reactions responsible for H2 generation. In addition, potential thermodynamic constraints on H2 production will be considered. Overall, the presently available data indicate that H2-based communities are likely to be considerably more productive in basaltic than ultramafic systems.
B52B-04
Bioinorganic chemistry and mineralogy of biogenic iron oxides from Loihi seamount: A case study relevant to the crustal deep biosphere
Establishing linkages between mineral transformations – dissolution and precipitation – and biological processes is critical for evaluating the biological influences on important deep-biosphere processes such as the alteration of the ocean crust. Crustal alteration includes hydration and oxidation reactions involving principal components of basalt, such as Fe and S. However, the means to establish firm linkages is non trivial and requires approaching the problem from several distinct and complementary methodological and analytical approaches. At Loihi seamount, hydrothermal venting of fluids bearing high-Fe concentrations occurs over a range of conditions and includes high T (60°C) vigorous venting, low temperature seeps (15°C or less), and extinct, dead vents (for 3 or more years), which have been concretely linked with present or former biological processes through cultivation and molecular methods. Here we present evaluation of extinct and actively produced biogenic Fe oxides, using high- and low-energy synchrotron-based X-ray adsorption spectroscopy. Results show a range of mineralogical and bioinorganic properties of the oxides appear to track environmental parameters relating to the recency of active biological properties; recently produced vs. "old" oxides produce distinct and potentially tractable signatures that may be highly useful in evaluation of oxide alteration products in crustal rocks from the deep biosphere.
B52B-05
Insights into the Deep Biosphere from Petroleum Geology
Petroleum reservoirs provide a unique and direct portal into the deep biosphere enabling microbiological and geochemical studies of microorganisms, carbon cycling processes, microbial reaction kinetics and the spatial distribution of the organisms. Detailed mapping of oil fluid property and chemical composition shows that microbially and geologically induced oil compositional gradients are ubiquitous at regional to sub-reservoir scales (1 m to 105 m) in heavy and super heavy oil deposits of the Alberta basin. Some oil columns shows a cascading series of gradients in one through three ring alkylaromatic hydrocarbons with increasingly deeper locations of complete compound removal related to increasing resistance of degrading hydrocarbon component to biodegradation and decreasing component diffusivity. Similar compositional profiles have been seen in other heavy oilfields around the globe. Vertical 1D compositional diffusion models coupling oil charge, diffusive mixing and the effects of geological barriers can match observed gradients when the active biodegradation zone extends across the oil-water transition zone and well into the oil column. We observe steepening compositional gradients at the base of the oil column coincident with up to 10-15 m thick, downward increasing water saturation zones, thicker than a capillary pressure controlled oil-water transition zone. These "burnout zones" possess the hallmarks of a vertically extensive bio-reactor with increased concentrations of biogeochemical parameters of microbial processes and commonly an immobile and probably discontinuous oil phase. In this zone and across an oilfield, the relative biodegradation susceptibility of different hydrocarbon components varies greatly, indicating a suite of biodegradation reaction pathways dependent in part on local mass transport controls. 1D models show that the top of the degradation zone is coincident with complete depletion of reactive components, when oil charge is active. Thus in heavy oil reservoirs, biodegradation processes extend into the oil leg well above the base of the oil leg (5-10 m) rather than just near the base of the oil column as previously proposed. Thick biodegradation zones especially develop where high oil viscosity results in high relative water mobility, enabling efficient transport of essential nutrients through the mobile water phase to microorganisms. Burnout zones may be the key to determining biodegradation reaction kinetics over human and geological time and reconstructing oil charge histories, optimizing microbial enhanced oil recovery strategies and identifying low mobility thief zones in heavy oil reservoirs. We discuss the profound implications of this greater reaction volume of the petroleum degrading biosphere on microbial processes, and the implications for the spatial distribution of microorganisms in subsurface environments and its impact on sampling and study protocols.
B52B-06
Geochemical and hydrological constraints on the deep subsurface terrestrial ecosystems
Pore water and fluid inclusion compositions were determined on pristine rock cores from the Ventersdorp and Witwatersrand Supergroup by leaching experiments in order to constrain the origin of the chemical nutrients in the fracture water of the Witwatersrand basin. Subsequent chemical extractions including fusion analyses of the rock cores constrained the redox relevant mineral abundances of these strata. The resulting data set was used in mixing models with meteoric water from the overlying Transvaal dolomitic aquifer. The model also incorporated dissolved gas concentrations, radiogenic and radiolytic reactions, mineral dissolution and oxidation reactions, mineral equilibria and microbial redox reactions at rates that were varied over the course of one million years. Results revealed pore water and fluid inclusion leachates rich in Na, Ca, Si, Cl, acetate, SO42- and formate, with the resulting inferred fluid inclusion concentrations exceeding pore water concentrations, which in turn exceeded fracture water concentrations in nearly all observed elements. The model successfully simulated dissolved He and Cl concentrations with a mixing rate of 1e-5 L/year, corresponding to a fracture water velocity of 1 mm/year. Upon examining the viability of Fe reduction, SO42- reduction, acetogenesis, methanogenesis, NO3- reduction, anaerobic NH3 oxidation and HS- oxidation, model results indicate that SO42-reduction is the dominant metabolic process at high and low salinities and can be sustained at rates of 1.2e-12 M SO42- s-1 for biomasses of 2e8 cells/mL water. Final electron acceptor and donor concentrations and Free Energy Flux (FEF) calculations suggest that acetogenesis is not the source of acetate, nor is the syntrophic degradation of light hydrocarbons the source of the observed carboxylic acids. Ambient concentrations are instead likely the result of production via the thermally activated step-wise decarboxylation of abiogenic hydrocarbons and consumption by carboxylic acid utilizing metabolic reactions consistent with phylogenetic data showing few acetogens, no hydrocarbon oxidizers and a significant abundance of acetoclastic methanogens.
B52B-07
What lies beneath? New perspectives on the deep icy biosphere
The deep sub-surface regions of continental ice sheets represent vast gaps in current understanding of the Earth's biosphere, a reflection of the difficulties of access and concerns over contamination of these pristine ecosystems. Here, we draw upon existing and new biogeochemical datasets for subglacial meltwaters sampled from a spectrum of ice masses, including the Greenland Ice Sheet and Lake Vostok accretion ice, to provide the first assessment to date of biogeochemical processes beneath ice sheets. We demonstrate that microbial-processes (lithoautrophy and heterotrophy) are ubiquitous beneath glaciers and ice sheets and that ice mass size is a critical control on the balance of biogeochemical processes that take place. Hence, the beds of large ice sheets are driven towards anoxia by the microbial oxidation of organic carbon and sulphide minerals, and enhanced silicate dissolution arises as a result of calcite saturation. We demonstrate that this sequence of reactions creates favorable conditions for methanogenesis beneath ice sheets, where methane is stored as clathrate at the ice sheet base. These findings have significance for subglacial biodiversity and the role of ice sheets in global biogeochemical cycles. They may also be used to inform the first sampling of Antarctic Subglacial Lakes and other sub-ice sheet environments over the next decade.
B52B-08
Earth's Deep Biosphere and Life in the Solar System
Studies of Earth's deep biosphere over the past decade have shown that remarkable varieties of environments previously thought to be uninhabitable actually harbor significant quantities and a great diversity of microbial life. It has become apparent that many of the microorganisms living below the Earth's surface rely on geochemical reactions rather than photosynthesis as metabolic energy sources. Because of similarities in geochemical processes that are likely occurring, there are other examples of celestial bodies in our solar system that may also harbor subsurface biospheres, most notably Mars and the jovian moon, Europa. We can use lessons learned through investigations of Earth's deep biosphere to assess the possibility that life may exist below the surface of these worlds. In this presentation, we discuss some of what we know about Earth's deep biosphere and what it may tell us about the nature of life in similar environments outside of Earth. As plans are made for missions to search for deep biospheres on other worlds, we also explore what may be the most appropriate analogs here on Earth and how the methods being developed to study Earth's deep biosphere may aid in our search. Comparisons will be made between habitats on Earth that may be analgous to those postulated to exist on Mars and Europa.