1 Introduction

As an integral part of the hydrologic cycle, groundwater systems are influenced by a host of physical, environmental, hydraulic, and biogeochemical processes. These include seasonal changes in climate and precipitation, local to regional variations in the porosity and permeability of subsurface materials, the formation and dissolution of minerals along groundwater flow paths, as well as reactive mass transport and cycling of chemical substances by microorganisms. With the understanding that groundwater is a critical global resource of highly variable quantity and quality, the role of microbiology in biogeochemical processes has emerged as an essential topic in hydrogeology.

For many years, it was commonly thought that the subsurface of the Earth was mostly devoid of life. The presence of living organisms was believed to be limited mainly to soil and rhizosphere (plant root) environments. However, discoveries over the last few decades revealed a vast and diverse microbial biosphere extending several kilometers below the Earth’s surface (Kallmeyer et al., 2012; Bar-On et al., 2018; Magnabosco et al., 2018). These developments have emerged out of the growing realization that the ingredients considered necessary for life above ground – sunlight, oxygen, abundant organic carbon – are immaterial to many microorganisms. A wide variety of “unconventional” energy sources are utilized by different microbes, including a tremendous assortment of inorganic compounds as well as some organic compounds that are toxic to other organisms. These unique metabolic capabilities implicate microorganisms as key agents in the flow of energy and turnover of condensed matter (i.e., crystalline and amorphous solids and liquids, including liquid crystals, glasses, polymers, and gels) in reactive transport processes that link groundwater systems to surface environments in the hydrologic cycle (Falkowski et al., 2008; Lin et al., 2012).

As an exclusive habitat for microorganisms, the subsurface is characterized by a total absence of light, relatively constant temperatures, and a scarcity of nutrients including organic carbon. Microbial metabolism under such uninviting conditions may be far slower than on the Earth’s surface, calibrated more to the length and timescales of hydrogeological processes than daily and seasonal sunlight-dominated cycles. But the relatively stable conditions of the subsurface mean that populations of microorganisms can survive or even thrive over long periods of time, and in doing so exert a significant biogeochemical influence on their surroundings. This is especially important in a societal context because microbial activity directly affects the chemical composition and quality of groundwater, which constitutes a critical drinking water resource for much of the world’s population. The presence of microorganisms may be beneficial, for example by contributing to the degradation of toxic substances, or detrimental as in the case of aquifer contamination by pathogenic microbes from sewage wastes.

This book delves into the essentials of groundwater microbiology, beginning with the characteristic features of microorganisms and their overwhelming predominance in subsurface environments on Earth. Microbial cell structure, growth, ecology, and bioenergetics are introduced to build some familiarity with the expansive subject of microbiology. The concept of groundwater systems as a habitat for microorganisms is discussed in terms of physical limitations and the depth distribution of life in subsurface environments. Aspects of dissolved inorganic carbon equilibria and pH, oxidation-reduction (redox) processes, mineral precipitation-dissolution reactions, and the behavior of microbial cells as geochemically reactive solids are considered in an examination of the influence of microbial activity on chemical processes in groundwater. Final sections of the book address the transport and movement of microorganisms through groundwater systems, as well as some well-known and potential roles of microorganisms in contaminant hydrogeology and remediation.


Groundwater Microbiology Copyright © 2021 by F. Grant Ferris, Natalie Szponar, and Brock A. Edwards. All Rights Reserved.