Deep subsurface microbiology is a highly active and rapidly advancing research field at the interface of microbiology and the geosciences; it focuses on the detection, identification, quantification, cultivation and activity measurements of bacteria, archaea and eukaryotes that permeate the subsurface biosphere of deep marine sediments and the basaltic ocean and continental crust. The deep subsurface biosphere abounds with uncultured, only recently discovered and – at best - incompletely understood microbial populations. In spatial extent and volume, Earth's subsurface biosphere is only rivaled by the deep sea water column. So far, no deep subsurface sediment has been found that is entirely devoid of microbial life; microbial cells and DNA remain detectable at sediment depths of more than 1 km; microbial life permeates deeply buried hydrocarbon reservoirs, and is also found several kilometers down in continental crust aquifers. Severe energy limitation, either as electron acceptor or donor shortage, and scarcity of microbially degradable organic carbon sources are among the evolutionary pressures that have shaped the genomic and physiological repertoire of the deep subsurface biosphere. Its biogeochemical role as long-term organic carbon repository, inorganic electron and energy source, and subduction recycling engine continues to be explored by current research at the interface of microbiology, geochemistry and biosphere/geosphere evolution. This Research Topic addresses some of the central research questions about deep subsurface microbiology and biogeochemistry: phylogenetic and physiological microbial diversity in the deep subsurface; microbial activity and survival strategies in severely energy-limited subsurface habitats; microbial activity as reflected in process rates and gene expression patterns; biogeographic isolation and connectivity in deep subsurface microbial communities; the ecological standing of subsurface biospheres in comparison to the surface biosphere – an independently flourishing biosphere, or mere survivors that tolerate burial (along with organic carbon compounds), or a combination of both? Advancing these questions on Earth’s deep subsurface biosphere redefines the habitat range, environmental tolerance, activity and diversity of microbial life.

Obtaining and analyzing samples is challenging in subsurface science. This first-of-its-kind reference book addresses accomplishments in this field-from drilling to sample work-up. A collaborative approach is taken, involving the efforts of microbiologists, geochemists, hydrologists, and drilling and mining experts to present a comprehensive view of subsurface research. The text provides practical information about obtaining, analyzing, and evaluating subsurface materials; the current status of subsurface microbial ecology; and describes several applications that will interest a variety of readers, including engineers, physical, and life scientists.

The deep subsurface is, in addition to space, one of the last unknown frontiers to human kind. A significant part of life on Earth resides in the deep subsurface, hiding great potential of microbial life of which we know only little. The conditions in the deep terrestrial subsurface are thought to resemble those of early Earth, which makes this environment an analog for studying early life in addition to possible extraterrestrial life in ultra-extreme conditions. Early microorganisms played a great role in shaping the conditions on the young Earth. Even today deep subsurface microorganisms interact with their geological environment transforming the conditions in the groundwater and on rock surfaces. Essential elements for life are richly present but in difficultly accessible form. The elements driving the microbial deep life is still not completely identified. Most of the microorganisms detected by novel molecular techniques still lack cultured representatives. Nevertheless, using modern sequencing techniques and bioinformatics the functional roles of these microorganisms are being revealed. We are starting to see the differences and similarities between the life in the deep subsurface and surface domains. We may even begin to see the function of evolution by comparing deep life to life closer to the surface of Earth. Deep life consists of organisms from all known domains of life. This Research Topic reveals some of the rich diversity and functional properties of the great biomass residing in the deep dark subsurface.

Over the last two decades, exploration of the deep subsurface biosphere has developed into a major research area. New findings constantly challenge our concepts of global biogeochemical cycles and the ultimate limits to life. In order to explain our observations from deep subsurface ecosystems it is necessary to develop truly interdisciplinary approaches, ranging from microbiology and geochemistry to physics and modeling. This book aims to bring together a wide variety of topics, covering the broad range of issues that are associated with deep biosphere exploration. Not only does the book present case studies of selected projects, but also treats questions arising from our current knowledge. Despite nearly two decades of research, there are still many boundaries to exploration caused by technical limitations and one section of the book is devoted to these technical challenges and the latest developments in this field. This volume will be of high interest to biologists, chemists and earth scientists all working on the deep biosphere.

Until recently, much of our understanding of microorganisms in the subsurface was largely a matter of speculation. Recent advances in technology and methodology have enabled the discovery and study of microorganisms in deep subsurface environments. Researchers are now able to explore relationships between microbial physiology, taxonomy, and genetics, and the environment of these microorganisms, including geochemical, geological, and hydrological properties. Subsurface Microbiology and Biogeochemistry is a necessary learning tool that focuses on the integration of microbiology and the geosciences. The subsurface environment provides numerous niches for microorganisms and this book presents research in subsurface microbiology in addition to providing an understanding of the broad range and diversity of the previously "hidden" subterranean organisms. Contributing chapters from experts in their respective disciplines discuss the results of deep subsurface microbiology studies and enlighten the reader as to how the subsurface environment has grown to represent a true frontier for microbiological research. Subsurface Microbiology and Biogeochemistry reviews an important topic that is at the vanguard of subsurface environment studies with implications on the search for life on other planets. The discovery of subsurface microorganisms creates a multitude of opportunities for microbiologists and engineers in academia and biotechnology, with this book at the forefront of essential reading.