Chapter 14 : Spatial Segregation: The Deep Subsurface Story

Abstract:

This chapter focuses on the physical and chemical characteristics of the terrestrial subsurface that are most likely to influence microbial evolution, with emphasis on the spatial segregation issue, and suggests possible directions for future research. Subsurface environments are any terrestrial environments situated beneath the topsoil zone of the Earth's crust. Microorganisms are present in many different types of subsurface environments. Some subsurface microbes may be physically isolated from other microorganisms because they are surrounded by layers of dense, impermeable material. Spatial segregation could even be an important issue within some kinds of geological formations, especially those with very low porosity, in which the movement of groundwater, chemicals, and microbial cells is greatly restricted. Phylogenetic analysis of 16S rRNA gene sequences may provide evidence for long-term spatial segregation of microbes in some types of subsurface environments. Physical characteristics like permeability may affect spatial segregation of microbes in the subsurface by determining whether microbes, water, and nutrients can move from one location to another. Microbes and microbial communities in ultra-deep environments may have evolved unique combinations of traits in response to long-term spatial segregation. Analysis of antibiotic resistance traits of subsurface microbes might provide preliminary evidence for gene transfer in some kinds of subsurface environments. Sometimes the physical nature of the environment leads to spatial segregation, such that subsurface microbes may be physically isolated from those at the surface for very long periods.