1;Preface;6 1.1;References;8 2;Contents;10 3;Contributors;12 4;Free-Living Protozoa with Endosymbiotic Methanogens;16 4.1;1 Discovery;17 4.2;2 Distribution;18 4.3;3 Morphology and Life Cycles;20 4.4;4 Significance of the Association;20 4.5;5 Intracellular H2-Tension and Methanogens;22 4.6;6 Symbiotic Consortia as Natural Chemostats;23 4.7;7 The Role of Symbiotic Methanogenesis in Natural Habitats;23 4.8;References;24 5;Anaerobic Ciliates and Their Methanogenic Endosymbionts;27 5.1;1 Introduction;28 5.2;2 Methanogenic Endosymbionts Are Transmitted Vertically;29 5.3;3 Studies of the SSU rRNA Genes of Host and Symbiont;29 5.4;4 Endosymbiont Replacements;32 5.5;References;35 6;Symbiotic Methanogens and Rumen Ciliates;38 6.1;1 The Rumen and Ciliated Protozoa;39 6.2;2 Methanogens Associated with Rumen Ciliates;40 6.3;3 Detection of Methanogens Associated with Ciliates;40 6.4;4 The Effect of Ciliated Protozoa on the Composition of Methanogenic Archaea in the Rumen;44 6.5;References;46 7;The Methanogenic and Eubacterial Endosymbionts of Trimyema;48 7.1;1 Monoxenic and Axenic Cultures of Trimyema;49 7.2;2 Metabolic Features of Trimyema;52 7.3;3 Methanogenic Symbionts;55 7.4;4 Bacterial Symbionts;59 7.5;5 Perspectives;63 7.6;References;63 8;Termite Gut Flagellates and Their Methanogenic and Eubacterial Symbionts;67 8.1;1 Introduction;68 8.2;2 Methanogenic Endosymbionts of Termite Gut Flagellates;71 8.2.1;2.1 Phylogeny of Endosymbiotic Methanogens;71 8.2.2;2.2 Predicted Functions of Endosymbiotic Methanogens;73 8.3;3 Eubacterial Symbionts of Termite Gut Flagellates;77 8.3.1;3.1 Phylogeny of Ectosymbiotic Eubacteria;77 8.3.2;3.2 Phylogeny of Endosymbiotic Eubacteria;79 8.3.3;3.3 Predicted Functions of Eubacterial Symbionts;82 8.4;4 Genomics of Endosymbionts;83 8.5;5 Concluding Remarks and Future Perspectives;84 8.6;References;86 9;Methanogens in the Digestive Tract of Termites;92 9.1;1 Introduction;93 9.2;2 Methane as a Product of Symbiotic Digestion;94 9.3;3 Diversity of Methanogens in Termite Guts;96 9.4;4 Differences in Methanogenic Activities and Populations;99 9.5;5 Coexistence with Homoacetogens;100 9.6;6 Association with Gut Flagellates;102 9.7;7 Intercompartmental Transfer of Hydrogen;103 9.8;8 Relationship to Oxygen;105 9.9;9 Termites as a Source of Atmospheric Methane;105 9.10;10 Conclusions;106 9.11;References;107 10;Methanogenic Archaea in Humans and Other Vertebrates;112 10.1;1 Objective and Scope;113 10.2;2 Methanogens in Humans;113 10.3;3 Methanogens in Nonhuman Vertebrates;114 10.4;4 Diversity of Methanogens in Vertebrates;118 10.5;5 Diversity at the Subspecies Level;120 10.6;6 Methanogens in Vertebrates and Atmospheric Methane;120 10.7;7 Conclusions;121 10.8;References;122 11;Methanogens in the Gastro-Intestinal Tract of Animals;125 11.1;1 Introduction;126 11.2;2 Vertebrates;126 11.3;3 Arthropods;134 11.4;4 Conclusions;148 11.5;References;148 12;Syntrophy in Methanogenic Degradation;153 12.1;1 Introduction;154 12.2;2 Syntrophic Culture Systems, Microbiology, Biochemistry, and Molecular Biology;158 12.2.1;2.1 Ethanol;158 12.2.2;2.2 Butyrate;160 12.2.3;2.3 Propionate;162 12.2.4;2.4 Acetate;166 12.2.5;2.5 Branched Fatty Acids;167 12.2.6;2.6 Benzoate;167 12.2.7;2.7 Sugars;168 12.2.8;2.8 Amino Acids;169 12.2.9;2.9 Carrier Systems;171 12.2.10;2.10 Alternative Substrates for Pure Cultures and Technical Systems to Replace Methanogens;172 12.2.11;2.11 Anaerobic Methane Oxidation;174 12.3;3 Spatial Organization of Syntrophic Communities;174 12.4;4 Concluding Remarks;175 12.5;References;176 13;Hydrogenosomes;184 13.1;1 Introduction;185 13.2;2 The Hydrogenosomes of the Trichomonadina;187 13.3;3 The Hydrogenosomes of T. pyriformis;190 13.4;4 The Hydrogenosomes of P. lanterna;191 13.5;5 The Hydrogenosomes/Mitochondrion-like Organelles (MLOs) of Blastocystis sp.;193 13.6;6 The Hydrogenosomes of Chytridiomycete Fungi;195 13.6.1;6.1 Mitochondria versus Hydrogenosomes;196 13.6.2;6.2 Chytrids Perform a ``Mixed Acid Fermentation´´;196 13.6.3