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Microbial communities play a critical role in all key nutrient cycles on Earth and even more so in deserts where plants are scarce or even totally absent. In the most arid deserts, microbial communities find refuge inside translucent rocks (endoliths) as a survival strategy. Since the work of E. Imre Friedmann on microbial communities in the Dry Valleys of Antarctica, lithic communities have been described from most hot and cold hyperarid deserts around the world, and from multiple substrates. Yet, their diversity, metabolic activities, and interactions with their rocky habitat remain vastly unexplored. Here, we discuss the lithic habitat and explore the colonization strategies evolved by lithic microorganisms. We report that while recent studies have revealed the exceptional taxonomic and functional diversity of these highly adapted microbial communities, little is known about their in situ metabolic activities, interactions between community members, and the role of viruses in community dynamics. To address what factors impact the assembly of endolithic communities, we present the concept of “rock architecture,” that is, the space available for colonization, which, linked to light penetration and water retention, is ultimately a driver of community diversity and composition. Finally, we discuss how the study of microbial communities from hyperarid deserts can further our understanding of the dry limits for life as we know it and provide insights into the habitability of extraterrestrial environments.

Publisher Summary Microorganisms that are able to grow at temperatures above 90° are called as “hyperthermophiles.” They form a diverse group consisting of autotrophic and heterotrophic prokaryotes, including several bacteria, although the majority of hyperthermophiles are Archaea. Most of the conventional tools of genetic and physiological analysis are either not effective or very difficult to apply to these microorganisms because of their unusual growth conditions. As a result, relatively slow progress has characterized the field since its inception. A new paradigm has characterized the field recently, with the availability of complete genome sequences of five hyperthermophiles. A unique resource for comparative studies of hyperthermophiles—namely, the complete genomic sequences of three species in the genus Pyrococcus, is now accessible. Although these strains are quite similar in their fermentative, sulfur-reducing growth physiology and optimal growth temperatures, which are in the range 98-100°, significant issues of genome divergence are emerging from the ongoing study of their genomic sequences.

Publisher Summary This chapter focuses on a key enzyme of nitrogen metabolism in hyperthermophiles, glutamate dehydrogenase (GDH). GDHs are widely distributed enzymes involved in ammonia assimilation and catabolism of glutamate in microorganisms. GDH catalyses the reversible oxidative deamination of glutamate to 2-oxoglutarate and ammonia using NAD or NADP as cofactors. GDH represents an enzymatic link between major catabolic and biosynthetic pathways via the tricarboxylic acid (TCA) cycle intermediate 2-oxoglutarate. Analysis of GDH structures has provided the basis for comparative studies of extreme protein thermostability in this relatively complex enzyme. The chapter provides an overview of the purification and properties of thermostable GDHs and a review of the contributions that these studies have made to the understanding of the basis for exceptional enzyme stability.

Publisher Summary This chapter focuses on the enzymes at the interface between carbon and nitrogen metabolism in hyperthermophiles, a position occupied by glutamate dehydrogenase (GDH), glutamine synthetase (GS, glutamate- ammonia ligase), and aminotransferases (AT). This chapter addresses the structure and function of GDH as a central issue and attempts to relate this information to the less studied but related enzymes, GS and transaminases. Glutamate dehydrogenase catalyzes the reversible oxidative deamination of glutamate to 2-ketoglutarate and ammonia using NAD + or NADP + as cofactors. Glutamine synthetase is a ubiquitous biosynthetic enzyme with a key role in nitrogen assimilation. The enzyme catalyzes the biosynthesis of glutamine from glutamate and ammonia, in an ATP and Mn 2+ dependent reaction. The structure, biochemistry, and molecular biology of both these enzymes are explained in the chapter.