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Until recently it was believed that Klebsiella pneumoniae fixed nitrogen only under anaerobic conditions (see Postgate, 1971). However, Klucus (1972) showed that a species of Klebsiella produced active nitrogenase in the presence of 0, in a chemostat culture provided that the medium contained yeast extract (I g/l) as a fixed N source and the dissolved oxygen tension was at 10 mmHg or below. Klucus (1972) suggested that the tolerance of nitrogenase activity to 0, arose because of a high respiratory rate at low dissolved oxygen tension (Harrison & Pirt, 1967; Harrison & Loveless, 1971) which could maintain the local oxygen tension at a very low level, although he did not measure respiratory rates. The obligate aerobe Derxia gummosa is particularly sensitive to 0, when fixing N2 (Hill & Postgate, 1969 ; Hill, 1971) and shows colony dimorphism on N-free media in air (Jensen, Petersen, De & Bhattacharya, 1960). Only the ' massive' colonies which develop among many 'thin' whitish colonies fix nitrogen (Jensen et al. 1960) or reduce acetylene (Hill, 1971). The numbers of colonies developing into the 'massive ',type could be increased by either adding fixed nitrogen (Tchan & Jensen, 1963) or lowering the atmospheric oxygen concentration (Hill, 1971). Colony dimorphism probably arises in air because nitrogen fixation and subsequent growth to the 'massive' colony type takes place only where the local oxygen tension can become depressed (Hill, 197 I). A methane-oxidizing bacterium also showed dimorphism during growth on a N-free medium in air (De Bont & Mulder, 1974). The present work demonstrates that K. pneumoniae, although probably more sensitive to 0, when fixing N, than D. gummosa, also shows colony dimorphism associated with N, fixation when incubated in air if the medium is supplemented with low levels of fixed nitrogen.

Acetylene Reduction by Klebsiella pneumoniae in Air Related to Colony Dimorphism on Low Fixed Nitrogen