SUMMARYNitrogen fixation has been obained with strains of Desulfovibrio vulgaris and D . gigas, organisms hitherto believed to be incapable of using molecular nitrogen. Fixation has been demonstrated by increases in total nitrogen and by uptake of 15N2. Fixation of N, may be widespread in this genus of the sulphate-reducing bacteria. I N T R O D U C T I O NThe research reported here was begun as a result of Postgate's comment (19653) in a review that 'Most strains of Desulfovibrio do not fix nitrogen'. Enriched cultures from the mud of Lake Mendota, Wisconsin, not completely free of contaminating bacteria, readily fixed N, ; however, the pure cultures intended as negative controls, Desulfovibrio vulgaris NCIB 8303 HILDENBOROUGH and D. gigas NCIB 9332, also fixed in some experiments. Accordingly, the attempt to purify the new isolates was discontinued in favour of a more thorough examination of this unexpected development. The results, reported briefly at the Colloquium on Nitrogen Fixation held at Sanibd Island, Florida (Silver, 1967), are given in more detail here. MATERIALS A N D METHODSCultures. The culture of Desulfovibrio vulgaris strain NCIB 8303 (HILDENBOROUGH) originally was obtained from Sister M. Regina Lanigan, Rosary Hill College (Buffalo, N.Y.); later, subcultures of this same strain were obtained from Professors L. L. Campbell (University of Illinois) and J. R. Postgate (University of Sussex). The purity of the cultures was routinely checked by examination in a phase microscope and inoculation of Postgate's medium A (1951). These subcultures are designated by the letters R, C and P after the strain number 8303. Crude cultures of this organism were isolated from Lake Mendota mud using the technique described by Postgate with 0.02 yo thioglycollate and 0.01 % ascorbic acid added. In the nitrogen fixation * Present address :
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