ABSTRACT
The Earth’s crust is a solid cool layer that overlays the mantle, with a varying thickness of between 30 and 50 km on continental plates and 5 and 10 km on oceanic plates. Continental crust is composed of a variety of igneous, metamorphic, and sedimentary rocks that weather and reform over geologic cycles lasting millions to billions of years. At the crust surface, these weathered minerals and organic material combine to produce a
variety of soils types that provide suitable habitats and niches for abundant microbial diversity (see Chapter 4). Beneath this soil zone is the subsurface. Once thought to be relatively free of microorganisms, recent estimates have calculated that between 1016-1017 g, C biomass (2%–19% of Earth’s total biomass) may be present in this environment (Whitman et al., 1998; McMahon and Parnell, 2014). Microbial life in the subsurface exists across a wide range of habitats: in pores
5.1 Habitats / 69 5.2 Terrestrial Subsurface Biosphere Environments / 70
5.2.1Sedimentary Systems / 71 5.2.2Igneous and Metamorphic Rocks / 71 5.2.3Vadose Zone / 72
5.3Constraints and Strategies for Microbial Life in the Subsurface / 73 5.3.1Temperature and Pressure / 73 5.3.2Energetics / 74 5.3.3Electron Acceptors and Donors / 74 5.3.4 H2-Based Subsurface Biosphere / 77 5.3.5 Physiological Adaptations / 78 5.3.6Viruses and Eukaryotes / 78
5.4Probing the Subsurface / 79 5.4.1Drilling and Coring / 79 5.4.2Groundwater Wells / 81 5.4.3Molecular Technologies and Approaches / 81 5.4.4Ecological Analyses / 82
5.5Subsurface-Human Interactions / 82 5.5.1Gas and Hydrocarbon Production / 82 5.5.2Subsurface Contamination and Remediation / 84 5.5.3Waste Repositories / 85 5.5.4Geologic CO2 Sequestration / 85
5.6 Implication for Astrobiology and Early Life on Earth / 85 5.7Summary / 86 References / 87
associated with relatively shallow unconsolidated aquifer sediments to fractures in bedrock formations that are more than a kilometer deep, where extreme lithostatic pressures and temperatures are encountered. While these different environments contain varying physical and chemical conditions, the absence of light is a constant. Despite this, diverse physiologies and metabolisms enable microorganisms to harness energy and carbon for growth in water-lled pore spaces and fractures. Carbon and other element cycles are driven by microbial activity, which has implications for both natural processes and human activities in the subsurface, e.g., bacteria play key roles in both hydrocarbon formation and degradation. Hydrocarbons are a major focus for human utilization of the subsurface, via oil and gas extraction and potential geologic CO2 sequestration. The subsurface is also utilized or being considered for sequestered storage of high-level radioactive waste from nuclear power generation and residual waste from past production of weapons grade nuclear materials. While our understanding of the subsurface is continually improving, it is clear that only a small fraction of microbial habitats have been sampled and studied. In this chapter, we will discuss these studies in the context of the distribution of microbial life in the subsurface, the stresses that microorganisms must overcome to survive in these environments, and the metabolic strategies that are employed to harness energy in a region
of the planet far removed from sunlight. Finally, we will consider both benecial and deleterious effects of microbial activity in the subsurface on human activities.
