A method is described whereby the rate of increase in cell content of a particular material may be measured isotopically. The method is free from sampling errors, and gives accurate values in short experiments. It has been applied to the study of the effect of hydroxylamine on fixation of nitrogen and on respiration in Azotobacter vinelandii. Respiration and fixation are inhibited in series. The results have been interpreted in terms of a linked enzyme system, and the linkage is believed to occur through the medium of a high-energy product of respiration, possibly hydrogen peroxide acting as a source of hydroxyl radicals. In the course of preliminary investigations, the rate of decomposition of hydroxylamine in aqueous solution and in medium (both sterile and inoculated with A. vinelandii) were measured. The reaction is first order over a wide range of concentration; nitrite and ammonia appear among the decomposition products. Nitrogen does not interchange with nitrite or hydroxylamine. Hydroxylamine is not utilised by the organism.
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