General Introduction.- Microorganisms Involved in the Nitrogen and Sulfur Cycles (With 2 Figures).- Nitrogen Metabolism in General.- Nitrogen Metabolism in Plants (With 5 Figures).- The Assimilatory Reduction of Nitrate.- Dissimilatory Nitrate Reduction (With 10 Figures).- Dinitrogen Fixation.- Genetics of Dinitrogen Fixation (With 1 Figure).- Rhizobium Genetics (With 7 Figures).- Some Aspects of the Physiology of Dinitrogen Fixation (With 3 Figures).- The Biochemistry of Dinitrogen Fixation (With 3 Figures).- Pathways and Regulatory Aspects of N2 and NH4+ Assimilation in N2 -Fixing Bacteria (With 1 Figure).- The Hydrogenase-Nitrogenase Relationship in Nitrogen-Fixing Organisms (With 1 Figure).- Sulfur Metabolism in General.- Assimilatory Sulfate Reduction (With 7 Figures).- Ecology and Physiology of Some Anaerobic Bacteria from the Microbial Sulfur Cycle (With 1 Figure).- Dissimilatory Sulfate Reduction, Mechanistic Aspects (With 2 Figures).- Dissimilatory Sulfate Reduction, Energetic Aspects (With 9 Figures).- Photolithotrophic Sulfur Oxidation.- Oxidation of Ammonia by Nitrosomonas and of Inorganic Sulfur by Thiobacilli (With 3 Figures).- Contributed Papers: Nitrogen Metabolism.- Enzymatic Mechanism and Regulation of Nitrate Reduction in Rhodopseudomonas capsulata.- HCN and the Control of Nitrate Reduction. The Regulation of the Amount of Active Nitrate Reductase Present in Chlorella Cells (With 4 Figures).- Regulation of Nitrate Uptake in Green Algae and Duckweeds. Effect of Starvation and Induction(With 6 Figures).- Nitrate Uptake and Reduction in Chlorella - Characterisation of Nitrate Uptake in Nitrate-Grown and Nitrogen-Starved Chlorella sorokiniana (With 9 Figures).- Nitrogen Metabolism in Photosynthetically Inhibited Plants (With 5 Figures).- Investigations on the Reduction of Aliphatic and Aromatic Nitro Compounds by Clostridium Species and Enzyme Systems (With 6 Figures).- Fine Structure Analysis of the Gene Region for N2-Fixation (nif) of Klebsiella pneumoniae (With 7 Figures).- In vitro Associations Between Non-legumes and Rhizobium (With 13 Figures).- Limitations of Symbiotic and Associative Nitrogen Fixation by Developmental Stages in the Systems Rhizobium japonicum with Glycine max and Azospirillum brasilense with grasses e.g. Triticum aestivum (With 9 Figures).- Regulation of Nitrogenase Biosynthesis in Free-Living and Symbiotic N2-Fixing Bacteria: a Comparison (With 4 Figures).- Characterization of the Nitrite-Oxidizing System in Nitrobacter (With 8 Figures).- Contributed Papers: Sulfur.- The Role of Thioredoxins for Enzyme Regulation in Cyanobacteria (With 4 Figures).- Recent Results on the Assimilatory Sulfate Reduction: APS-Kinase and the reduction of APS to Cysteine in Higher Plants (With 1 Figure).- Aspects of S- and N-Metabolism in Tissue Cultures (With 4 Figures).- Regulation of Adenosine 5?-Phosphosulfate Sulfotransferase in Higher Plants (With 3 Figures).- On the Enzymatic System Thiosulfate-Cytochrome c-Oxidoreductase (With 6 Figures).