228 RESPIRATION 



though the accumulation of malate or of fumarate by such a system 

 can be imagined, the cyclical character of the system as written would 

 not be maintained. Most of the enzymes required for Equation 15 

 are known in fungi, and it will be of considerable interest to see if 

 some such series of reactions participates in energy metabolism or in 

 the synthesis of dicarboxylic acids. A comparable reaction system in 

 Tetrahymena pyrifonnis converts succinate reversibly to acetyl phos- 

 phate (256). 



1 1 . CARBON DIOXIDE FIXATION 



Evidence for a physiological role of carbon dioxide in the growth 

 of fungi appears first in relatively crude experiments on Fusarium spp. 

 (180); retardation of early growth in the absence of carbon dioxide has 

 been observed in many other fungi (245, 246, 250). The carbon 

 dioxide requirement is enhanced by mineral deficiencies (269). 



An apparent effect of carbon dioxide on dehydrogenations (130) has 

 been shown to be exerted in fact on endogenous methylene blue reduc- 

 tion (97). 



Carbon dioxide stimulation of growth could, of course, be ascribed 

 to other phenomena than fixation. However, in Penicillium chryso- 

 genum there is good evidence that fixation is related to the essentiality 

 of carbon dioxide (106). In this organism, fixation is negligible un- 

 less both carbon and nitrogen sources are provided; this situation, 

 coupled with the fact that carbon dioxide is not essential in a complex 

 medium and that aspartic and glutamic acids are labeled very early 

 by C 14 from isotopic carbon dioxide, suggests strongly that the physio- 

 logical role of carbon dioxide is in the synthesis of essential amino 

 acids via the citric acid cycle. Carbon dioxide fixation in Neurospora 

 crassa is similarly enhanced by provision of nitrogen (275) and carbon 

 (128). 



It has been mentioned earlier in this chapter that under appropriate 

 conditions there is extensive incorporation of carbon dioxide into 

 citric acid. Labeled carbon dioxide is also incorporated into the 

 carboxyl group of pyruvic acid by Rhizopus nigricans (86), in the 

 carboxyl of lactate by R. oryzae (100), in oxalic acid by Aspergillus 

 niger (174), in succinic acid by Streptomyces coelicolor (60), in strepto- 

 mycin by 5. griseus (144), and in unidentified cell constituents by 

 several fungi (181). 



In organisms generally there are several different pathways by 

 which carbon dioxide is fixed (10, 118, 309). Indirect and not entirely 

 conclusive evidence suggests that fixation occurs into oxalacetate in 



