The ability to fix nitrogen of 10 strains of the yeasts Rhodotorula, Bullera and Torulopsis and 4 strains of Pullularia, all isolated from soils and some supplied by other investigators was examined using both the heavy nitrogen (I~N~) and acetylene reduction techniques. Rigorous standards for aseptic culture, freedom from combined nitrogen and precision of analysis were maintained. No fixation was observed in any of the organisms and the ability of any eucaryote cell to fix nitrogen is doubted. Suggestions for the previous reports of fixation are made.
In recent years several reports have appeared in which fixation of nitrogen by free-living organisms other than blue-green algae and bacteria is described. Virtually all of the organisms are fungi or yeasts, and, with one exception, the claims are all based on the use of the conventional Kjeldahl procedure. The genera concerned are Photon (Jo~Es and SMITH, 1928), Pullularia (BgowN and M~TCALFV,, 1957), Rhodotorula and Saccharomyces (M~TCALFE et al., 1954), and Nocardia (METCALFV. and BROWZq, 1957). The actinomycete Nocardia fixes nitrogen with a speed and efficiency comparable with Azotobacter under similar conditions. This last information is useful as actinomycetes are generally agreed to be the endophytes of non-leguminous nitrogen fixers.
In the eucaryotic organisms the position is much less satisfactory. Fixation in Phoma was not confirmed when trials using 15N were carried out (WILson, 1952). A species of Pullularia was tested by METC~LLFE and BI~OWN (1957) using conventional Kjeldahl procedures upon whole cultures in liquid medium deficient in nitrogen. More studies have been carried out with yeasts than with all the other suspected genera. A marine Rhodotorula has been reported to fix nitrogen (ALLEN, 1963) but very few details of its physiology have been elucidated. Terrestrial species of Saccharomyces and Rhodoturula have received most attention. Two species of yeast were isolated from root nodules of lupin (NEM]~T~, 1959) and may give rise to enhanced growth by augmenting the nitrogen supply. Twenty strains of yeast were isolated from a heathland (MwT-CALFW et al., 1954) and fixed nitrogen when examined by the micro-Kjeldahl technique. Of these, two strains, one Rhodotorula and one Sac-