Some requirements of biological nitrogen fixation.

Clues to the mechanism of biological nitrogen fixation have been sought through experiments aimed at defining the substances specifically required by organisms to effect the process. The nitrogen-fixing sequence in bacteria is known to proceed through ammonia (Zelitch et al., 1951; Newton et al., 1953; Wilson and Burris, 1953; Virtanen, 1953). Thus, the effect of adding ammonium salts to a fixing culture should be to reduce the requirement for those substances that are needed by the microorganism to convert elemental nitrogen to ammonia. Accordingly, a search for chemical factors involved in nitrogen fixation was made by comparing the nutritional requirements of bacteria and algae during growth on molecular nitrogen and on ammonia. Substantially higher requirements for biotin and for iron were found in the anaerobic bacterium, Clostridium pasteurianum strain W-5, when fixing nitrogen than when assimilating ammonia. An increased requirement for iron during nitrogen fixation was also found in the blue-green alga, Nostoc muscorum G. The chemical functions of these substances remain to be identified.