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Enzymatic assays of cell-free extracts of the ammonia-oxidizing bacterium Nitrosocystis oceanusfailed to establish that the biochemical basis of its obligate autotrophy stemmed solely from a metabolic defect. All of the Embden-Meyerhof enzymes except phosphofructokinase, and all of the tricarboxylic acid-cycle enzymes, as well as reduced nicotinamide adenine dinucleotide oxidase, were found in these extracts. A phosphoenolpyruvate-CO2-fixing system was also demonstrated. Resting cells incubated with 14C-d-glucose and 14C-l-glutamate and cells grown in the presence of 14C-labeled glucose, glutamate, pyruvate, and methionine incorporated these compounds into cellular material, but at a level too low to provide the cells' major carbon and energy needs.