Microbial Ecology

The continental subsurface is composed of sediment and a rock matrix often deeper. In the saturated zone, water flows from the surface in recharge zones, into the subsurface at aquifer level (groundwater), then rises to the surface at discharge or resurgent zones. In underground habitats, there are planktonic microorganisms that live in water, sessile organisms that live attached to particles or on rock surfaces, and endolithic organisms that live in rock pores. These populations colonize both saturated and unsaturated areas in subsurface habitats, ranging from a few meters (shallow) to a few kilometers (deep).