Hydrogeologic Controls on the Deep Terrestrial Biosphere - Chemolithotrophic Energy for Subsurface Life on Earth and Mars
As exploration for gold, diamonds and base metals expand mine workings to depths of almost 3 km below the Earth's surface, the mines of the Canadian Shield provide a window into the deep biosphere as diverse, but to date less well-explored than the South African Gold Mines. To date investigations of the deep biosphere have, in most cases, focused on the marine subsurface, including deep sea sediments, hydrothermal vents, off-axis spreading centers and cold seeps. Yet the deep terrestrial subsurface hosted in the fracture waters of Archean Shield rocks provides an important analog and counterpoint to studies of the deep marine biosphere. Depending on the particular geologic and hydrogeologic setting, sites vary from those dominated by paleometeoric waters and microbial hydrocarbon production, to those in which H2 and hydrocarbon gases have been suggested to be a function of long-term accumulation of the products of water-rock interaction in the deepest, most saline fracture waters with residence times on the order of tens of millions of years. The hydrogeologically isolated fracture-controlled ground water system periodically generates steep redox gradients and chemical disequilibrium due to fracture opening, and episodic release of mM levels of H2 that support a redox driven microbial community of H2-utilizing sulfate reducers and methanogens. Exploration of these systems may provide information about the limits of the deep terrestrial biosphere, controls on the distribution of deep subsurface life, and the diversity of geochemical reactions that produce substrates on which microbiological communities at great depths survive. The geologically stable Precambrian cratons of Earth are arguably the closest analogs available to single-plate planets such as Mars. Studies of these Earth analogs imply that the habitability of the Martian crust might similarly not be restricted to sites of localized hydrothermal activity. While the presence of the Martian cryosphere and potential clathrates will affect the porosity and permeability, and net flux of gases from the Martian crust, the underlying principles of fracture-controlled energy sequestration and episodic release remain. Furthermore understanding the origin and distribution of biogenic and geologic sources of CH4 at these analog Earth sites will inform models and strategies for deciphering the origin of CH4 recently reported in the Martian atmosphere.
Variation in Isotopic Biosignatures From Carbonate Rich, Microbial Mats in Saline, Alkaline Lakes on the Cariboo Plateau, B.C.
Cyanobacteria dominated, carbonate rich microbial mats found in saline, alkaline lakes on the Cariboo Plateau, B.C. represent potential analogues of the evaporative systems that might have occurred on early Earth or Mars. These evaporative lakes generally have pH values > 10, salinities of up to 33 psu and alkalinities of > 15, 000 mg CaCO3/L but differ in other geochemical parameters. The ability to understand natural variations in microbial activity and biosignatures in such modern analogues is central to our understanding of the capabilities and limits of life, the interpretation of the geologic record and potentially one day to the interpretation of astrobiological data. Phospholipid fatty acid (PLFA) profiling, voltammetry, and stable isotope analysis of organic and inorganic carbon pools highlighted the spatial and seasonal variability that exists in modern evaporative microbial mat dominated lakes. Variations in microbial PLFA distribution demonstrated that Cariboo Plateau microbial mat community composition varied seasonally and spatially. Voltammetry results showed that photosynthetic oxygen production occurred in the upper 5 mm of mats resulting in supersaturation of oxygen in surface waters. Depletion of oxygen generally occurred just below 5 mm and sulfide production began at 10 - 15 mm from the mat surface. Isotope analysis (13C) of Cariboo microbial mats showed inorganic (dissolved inorganic carbon) to organic (bulk cell) isotopic discriminations of 23-25 ‰, indicating non-CO2 limited photosynthesis. These results are in contrast to high organic content analogue mats previously reported that show evidence of CO2 limitation. Further, the Cariboo mats demonstrated significant intra- and inter-mat variations in carbonate δ13C values with respect to dissolved inorganic carbon (DIC) ranging from enrichment to 13C-depleted carbonate. In Deer Lake, isotopic enrichment of surface water DIC by 2-3 ‰ above atmospheric equilibrium indicated microbial metabolic effects on entire lake system. Deer Lake carbonate δ13C values from the upper oxygenated 5 mm are similar to the lake water DIC δ13C values, however carbonate recovered from below this photosynthetic zone was 13C- depleted with respect to the DIC by 5 ‰ indicating dominant heterotrophic activity in this region. Recognition of heterotrophic versus autotrophic biosignatures in modern systems is important for interpretation of dominant processes that existed in past ecosystems. Observations of variable δ13C values between and within Cariboo Plateau microbial mats suggest that both autotrophic and heterotrophic metabolic processes are linked to biologically induced carbonate precipitation in these systems. Biologically influenced 13C-content of the local DIC pool and precipitated carbonate has the potential to be preserved in ancient or exobiological systems. Understanding variation in modern planetary analogue sites is crucial to interpretation of astrobiological carbonate deposits.
Early Earth as an Analogue Target for Astrobiology
To a rough approximation, the records of planetary evolution on Earth and Mars are mirror images of each other. The exposed Martian surface is dominated by ancient crust, with more than 40% of the planet covered by crust more than ~3.5 billion years old. On the other hand, the exposed surface on Earth is dominated by relatively young rocks, with Earth's early history contained within a sparse and fragmentary geologic record. Only the rarest terrestrial rock sequences are ancient and coherent enough to be used a model for early life in martian rocks. The recent discovery of the 4.3 Ga Nuvvuagittuq greenstone belt (NGB) in northeastern Quebec has essentially doubled the number of data points that can provide effective baselines for the Astrobiological evaluation of early Earth as an appropriate analogue for early Mars. This presentation will discuss promises and pitfalls of our current attempts to apply this approach in the NGB.
Supraglacial Sulphur Springs Supporting a Diverse Microbial Ecosystem, Borup Fiord Pass, Ellesmere Island.
Borup Fiord Pass (81°N, 81°W) is home to unique sulfur-rich springs which discharge onto the surface of glacial ice, releasing H2S and forming deposits of elemental sulfur (S°), calcite, and gypsum, as well as the rare carbonate mineral vaterite. Springs occur on the southern end of a prominent valley glacier, but discharge sites vary significantly from year to year. Stratified layers of elemental sulphur are observed in proglacial icings suggesting that spring discharge may be perennial. Springs are Na-Cl rich saline waters (7000 mg/l TDS) that discharge at up to 8.4 l/s. The measured level of dissolved H2S, 143 mg/l, is one of the highest reported for any sulphur spring in Canada. A thriving microbial community has been detected in the spring water and mineral deposits, with rapid cycling of sulfur between three oxidation states, as well as measured changes in S-isotopic signatures in the spring waters, indicating a complex series of biologically mediated redox reactions. Sulphur isotope data indicate that evaporites of the Otto Fiord Formation are the likely source of sulphur in the system, necessitating deep groundwater circulation under glaciated mountains in a region of over 500 m permafrost. Preliminary analysis of the clone library generated shows that some of the 16s rRNA sequences correlate well with known groups of sulfate-reducing microorganisms (e.g. classes of delta-Proteobacteria), while a small fraction of the total sequences do not correspond with any in the public databases. The large-scale annual precipitation of S° on the ice provide a geochemical biosignature extensive enough to be detected from orbital measurements.
Implications for global climate change from microbially-produced acid mine drainage
Microbial catalysis of sulphur cycling in acid mine drainage (AMD) environments is well known but the reaction pathways are poorly characterised. These reaction pathways involve both acid-consuming and acid- generating steps, with important consequences for overall AMD production as well as sulphur and carbon global biogeochemical cycles. Mining-associated sulphuric acid has been implicated in climate change through the weathering of carbonate minerals resulting in the release of 29 Tg C/year as carbon dioxide. Understanding of microbial AMD generation is based predominantly on studies of Acidithiobacillus ferrooxidans despite the knowledge that other environmentally common strains of bacteria are also active sulphur oxidizers and that microbial consortia are likely very important in environmental processes. Using an integrated experimental approach including geochemical experimentation, scanning transmission X-ray microscopy (STXM) and fluorescent in situ hybridization (FISH), we document a novel syntrophic sulphur metabolism involving two common mine bacteria: autotrophic sulphur oxidizing Acidithiobacillus ferrooxidans and heterotrophic Acidiphilium spp. The proposed sulphur geochemistry associated with this bacterial consortium produces 40-90% less acid than expected based on abiotic AMD models, with significant implications for both AMD mitigation and AMD carbon flux modelling. The two bacterial strains are specifically spatially segregated within a macrostructure of extracellular polymeric substance (EPS) that provides the necessary microgeochemical conditions for coupled sulphur oxidation and reduction reactions. STXM results identify multiple sulphur oxidation states associated with the pods, indicating that they are the sites of active sulphur disproportionation and recycling. Recent laboratory experimentation using type culture strains of the bacteria involved in pod-formation suggesting that this phenomenon is likely to be widespread in environments where both strains are present. These results will be presented highlighting the importance of identifying the roles of microbial communities in environmental processes and the advantages of using multiple, integrated techniques.
Geomicrobiology; inseparable from low temperature geochemistry & mineralogy
Bacteria play an important role in catalyzing a wide array of biogeochemical processes that affect the dissolution of minerals, the aqueous geochemistry of their surroundings and secondary mineral formation. Processes occurring at the bacteria-mineral interface can occur on the scale of nanoenvironments and will normally extend to microenvironments or even, to macroscopic features where extensive growth of bacteria is supported. The action of bacteria in these systems can produce a wide range of biomarkers that can be preserved over geologic time periods. Possible biomarkers include dissolution features in mineral substrates, fossil structures of individual cells to complex cell-cell associations, and chemical (isotopic and organic) signatures. In any system, we need to focus at the scale of the bacteria themselves to appreciate the actual chemistry of their surroundings and the kinds of reactions that they can catalyse. For example, photosynthetic microbial mats in an Atlin, BC wetland create ideal conditions for biologically induced precipitation of magnesium carbonates, specifically dypingite Mg5(CO3)4(OH)2•5H2O, which we were unable to reproduce abiotically. The preservation of biosignatures over geologic time presents obvious challenges, and the effect of diagenesis on fossils and their mineralogical assemblages deserves attention, especially with respect to the preservation and analysis of materials on (or from) Mars. For this, we need to rely on our Earth analogue sites as a way to triage the wide range of samples that are available for collection and analysis. The preservation of organic materials and cells in salts is particularly interesting. Conversely, the hematite nodules on Mars may not be good samples to target in the search for a Martian biosphere. The possibility of finding an extant biosphere increases with depth; however, evidence from Earth's deep subsurface demonstrates that it does not contain an abundant biosphere. Bacteria thrive in geochemical gradients where they take advantage of disequilibrium conditions to generate the energy needed to fight entropy. The selection of analogue sites needs to focus on the habitable conditions and preservation potential of these sites in order to increase our chances of finding evidence of life, to better understand how it is preserved on Earth, and by extension, to determine where it might be preserved on Mars.
Influences of Mn(II) and V(IV) on Bacterial Surface Chemistry and Metal Reactivity
Microorganisms in terrestrial and marine environments are typically bathed in solutions that contain a range of metal ions, toxic and beneficial. Bacteria such as Shewanella putrefaciens CN32 are metabolically versatile in their respiration, and the reductive dissolution of widely dispersed metals such as Fe(III), Mn(IV), or V(V) can present unique challenges if nearby bodies of water are used for irrigation or drinking. In redox transition zones, dissimilatory metal reduction (DMR) by bacteria can lead to generation of high concentrations of soluble metals. It has been shown that metals will associate with negatively charged bacterial membranes, and the mechanisms of metal reduction are well defined for many species of bacteria. The interaction of metals with the cell wall during DMR is, however, not well documented; very little is known about the interaction of respired transition metals with membrane lipids. Furthermore, bacterial surfaces tend to change in response to their immediate environments. Variations in conditions such as oxygen or metal presence may affect surface component composition, including availability of metal reactive sites. Our research seeks to characterize the biochemical nature of metal-membrane interactions, as well as identify the unique changes at the cell surface that arise as a result of metal presence in their environments. We have utilized scanning transmission X-ray microscopy (STXM) to examine the dynamics of soluble Mn(II) and V(IV) interactions with purified bacterial membranes rather than whole cells. This prevents intracellular interferences, and allows for near edge X-ray absorption fine structure (NEXAFS) spectroscopic analyses of cell surface and surface-associated components. NEXAFS spectra for carbon, nitrogen, and oxygen edges indicate that Mn(II) and V(IV) induce biological modifications of the cell membrane in both aerobic and anaerobic conditions. These changes depend not only on the metal, but also on the presence of oxygen. Results from NEXAFS spectroscopy revealed that oxygen presence had a strong impact on metal sorption, especially in the case of V(IV) association with membranes when oxygen is present. Bacterial membranes are necessarily dynamic, the membrane components are in a state of constant fluidity. Metal sorption to the cell surface, especially soluble metals which can fully engulf the cell, would limit the mobility of membrane components. Supporting this notion, CN32 cell membranes were observed via spectrofluorometry to become significantly stabilized when exposed to Mn(II) and V(IV) metals under anoxia. Despite stabilizing effects, cells are not adversely affected by metal presence in their growth environments, which is also supported by observations of metal coated cells by transmission electron microscopy (TEM). This supports STXM observations that cells counteract the metal effects on their surfaces by altering their membrane composition, and is enhanced by significant differences in cell membrane protein composition and quantity after SDS-PAGE separation. Our studies reveal several clear patterns in how cells interact with soluble metals in their environments, as well as the often overlooked subsequent effects that those metals, as well as oxygen, have on bacterial membranes.