Exobiology Branch, NASA Ames Research Center, Moffett Field, California 94035-1000, USA1. Introduction 101.1. Antiquity and Ubiquity of H2 in the Microbial World 101.2. The Evolving Role of H2 in Biogeochemistry 112. H2 from the Planetary Matrix 132.1. Abiotic Mechanisms of H2 Production 132.2. Atmospheric Chemistry Involving H2 142.3. Abiotic H2 as an Energy Source for Photosynthesis-Independent Ecosystems 163. H2 Cycling in Anaerobic Ecosystems 173.1. Interspecies H2 Transfer 173.1.1. Organic Decomposition in AnaerobicEcosystems 173.1.2. Thermodynamics of Microbial H2 Metabolism 203.1.3. Obligate Interspecies H2 Transfer 213.1.4. Facultative Interspecies H2 Transfer 223.1.5. Enzymatic and Metabolic Reversal 233.1.6. H2 Leakage 253.2. Factors Controlling H2 Concentrations 263.2.1. The Production-Consumption Steady State 263.2.2. Thermodynamic Controls 273.2.3. Kinetic and Other Controls 303.3. Implications for Biogeochemistry 324. H2 Cycling in Phototrophic Ecosystems 324.1. H2 in the Metabolism of Phototrophic Microorganisms 324.1.1. Photosynthetic Metabolism 324.1.2. H2 Consumption 344.1.3. H2 Production 344.2. H2 Cycling in Photosynthetic Microbial Mats 364.3. Implications for Biogeochemistry 395. Summary 39Acknowledgments 41References 411. INTRODUCTIONHydrogen has played an important role in Earth’s geochemistry and biology sincethe earliest stages of the planet’s history. H2 is postulated to have been a signifi-cant component of the early, prebiotic atmosphere, where it had direct or indirecteffects on atmospheric redox chemistry and radiation budget, and contributed toplanetary oxidation by escape to space. With the origins and evolution of lifecame a broad variety of metabolisms involving H2, and an expanded role forH2 in planetary biogeochemistry. H2 levels in the modern atmosphere areperhaps three orders of magnitude or more lower than during Earth’s earliesthistory, but the significance of H2 cycling for planetary chemistry persiststhrough its central role in the microbial world.