Todd O. Stevens

Lithoautotrophic Microbial Ecosystems in Deep Basalt Aquifers

Bacterial communities were detected in deep crystalline rock aquifers within the Columbia River Basalt Group (CRB). CRB ground waters contained up to 60 μM dissolved H2 and autotrophic microorganisms outnumbered heterotrophs. Stable carbon isotope measurements implied that autotrophic methanogenesis dominated this ecosystem and was coupled to the depletion of dissolved inorganic carbon. In laboratory experiments, H2, a potential energy source for bacteria, was produced by reactions between crushed basalt and anaerobic water. Microcosms containing only crushed basalt and ground water supported microbial growth. These results suggest that the CRB contains a lithoautotrophic microbial ecosystem that is independent of photosynthetic primary production.

Observations pertaining to the origin and ecology of microorganisms recovered from the deep subsurface of Taylorsville Basin, Virginia

To understand the conditions under which microorganisms exist in deep hydrocarbon reservoirs, sidewall cores were collected from a natural gas‐bearing formation, 2800 m below the surface in Taylorsville Basin, Virginia. Data from chemical and microbial tracers and controls indicate that the interiors of some sidewall cores contained microorganisms indigenous to the rock formation. The cultured microorganisms were composed primarily of saline‐tolerant, thermophilic fermenting, Fe(III)‐reducing, and sulfate‐reducing bacteria (1 to 104 cells/g). The physiological capabilities of the cultured microorganisms are compatible with the temperature (76°C), pressure (32 MPa), and salinity (≈0.8 wt.% NaCl equivalent) in the sampled interval. The petrological data indicated that the strata contain intercrystalline pores of micrometer size, that occur between late diagenetic cement in siltstone and within cross‐cutting, mineralized fractures in shale. These pores made up only 0.04% of the rock volume, were mostly gas‐f...

Biogeochemistry of anaerobic lacustrine and paleosol sediments within an aerobic unconfined aquifer

The geochemistry and the distribution and abundance of anaerobic bacteria were determined for sediments sampled in a deep borehole in south‐central Washington. The sampled sediments consisted of a 12‐m‐thick lacustrine sequence underlain by 8 m of paleosol grading into 5 m of silty sands, within an aerobic unconfined aquifer otherwise composed of transmissive sands and gravels. Concentrations of porewater sul‐fate varied systematically with depth, reaching a minimum of 3.9 mg L ‐1 in the central portion of the lacustrine sequence. Lacustrine sediments contained up to 1 wt% total organic carbon, whereas other sediments contained less than 0.2 wt% organic carbon. Fermentative bacteria were present throughout the sampled sequence, and were assumed to be responsible for primary degradation of organic carbon. Dissimilatory iron‐reducing bacteria (DIRB) were at maximum abundance where bioavailable Fe(III) and organic carbon were present at favorable combined concentrations. Sul‐fate‐reducing bacteria were cultu...

Bacterial mineralization patterns in basaltic aquifers: implications for possible life in martian meteorite ALH84001

To explore the formation and preservation of biogenic features in igneous rocks, we have examined the organisms in experimental basaltic microcosms using scanning and transmission electron microscopy. Four types of microorganisms were recognized on the basis of size, morphology, and chemical composition. Some of the organisms mineralized rapidly, whereas others show no evidence of mineralization. Many mineralized cells are hollow and do not contain evidence of microstructure. Filaments, either attached or no longer attached to organisms, are common. Unattached filaments are mineralized and are most likely bacterial appendages (e.g., prosthecae). Features similar in size and morphology to unattached, mineralized filaments are recognized in martian meteorite ALH84001.

Subsurface lithoautotrophic microbial ecosystems (SLMEs) in igneous rocks: prospects for detection

Evidence is accumulating to show that the Earths biosphere extends underground into deep igneous rock formations. In certain formulations, abiotic energy-yielding reactions between reduced rocks and groundwater provide a potential for in situ primary production by anaerobic microorganisms-- thus obviating any dependence on a surface ecosystem. Conceivably, such ecosystems could exist in the subsurface of other planets in the solar system. The main requirements are water, ferrous silicate minerals, carbon dioxide, and nitrogen. Unfortunately, observation of the subsurface is difficult. For example, current estimates suggest that the hydrosphere on Mars might be more than 2 km below the surface. Living SLMEs might be detected through conduits to the subsurface, such as wells, springs, or seeps in deep canyon walls. Signals produced by SLMEs might include cells, metabolic products (such as reduced gases) and their isotope ratios, and isotope ratios in residual substrates. Rocks in which now-defunct SLMEs once existed might be more accessible if they are brought to the surface by rock cycle processes. Signals of extinct SLME remnants have not yet been investigated, but might include microfossils, certain secondary mineralization patterns, and isotope ratios of secondary materials. Examples of both extant and extinct SLMEs have been identified on Earth, and are available for study and experimentation.