LIPIDS 



149 



Study of the formation of fats and fatty acids by fungi has been 

 concentrated on those fungi which form them in large- amount, par- 

 ticularly species of Endomyces (Endomycopsis), Torula (Turulopsis), 

 and Oospora (Geotrichum, often miscalled Oidium) among the yeast- 

 like fungi, and Aspergillus, Fusarium, and Penicillium among the 

 filamentous fungi. Within any given species the formation of fats 

 varies quantitatively from one strain to another and even from one 

 clonal culture to another (193, 428). 



Fats are formed intracellularly, possibly in association with cyto- 

 plasmic particles (486). The possibility that different types of cells 

 differ in their capacity to form fat is raised by the work of Satina and 

 Blakeslee (461) on Absidia blakesleeana, but other explanations are 

 possible. It is usually assumed that fats are reserve substances, and 

 their disappearance — or, more commonly, partial disappearance — 

 during the later phases of the culture cycle support this generalization 

 (258, 428). It is doubtful whether starvation experiments permit 

 accurate differentiation of essential cellular lipids from non-essential 

 reserve lipids (39), although it seems probable that such a differentia- 

 tion is valid in general, the phosphatides and lipoproteins being essen- 

 tial and the true fats non-essential (443). 



For any given fungus which is genetically capable of forming fats, 

 environmental conditions determine the amount actually synthesized. 

 As would be expected from the mode of formation of fatty acids from 

 2-carbon fragments (p. 151), almost any utilizable carbohydrate can 

 support fat formation (50). The specific effect of xylose on Fusarium 

 spp. (194) is presumably attributable to the accumulation of acetate 

 during metabolism of the pentose (225). The most important single 

 factor in fat formation is the concentration of carbohydrate, more 

 accurately the carbon : nitrogen ratio (442). As the carbohydrate 

 concentration is increased, the total fat and the percentage of conver- 

 sion of glucose to fat both increase (Figure 3); in some organisms, of 

 course, high concentrations of glucose inhibit growth and thereby 

 reduce fat synthesis (540). Spores of Fusarium roseum given carbo- 

 hydrate but no nitrogen accumulate fat-staining globules (476). 



Corresponding to the carbohydrate effect, a level of nitrogen which 

 is suboptimal for growth is generally favorable for maximum fat 

 accumulation. Thus, Aspergillus fischeri utilizes glucose most rapidly 

 at a nitrogen level which is ten times that at which fat formation, 

 relative to dry weight, is optimal (428). Phosphate deficiency, if not 

 too severe, increases fat synthesis, probably by reducing the assimila- 

 tion of nitrogen (358). 



Other nutritional determinants include the essentiality of phosphate 



