80 CARBON NUTRITION 



(2, 45). It must, however, be concluded that there is no quantitative 

 and entirely satisfactory evidence for the utilization of chitin or of 

 N-acetylglucosamine. The work just mentioned suggests that an in- 

 vestigation of this problem in the fungi would be rewarding. Chitin- 

 splitting enzymes must be present in those fungi which autolyze ex- 

 tensively, and an exocellular chitinase is formed by Streptomyces sp. 

 and some fungi (Chapter 5). 



Pectic Substances. The pectic substances are polysaccharides of 

 plant cell walls, usually isolated as a mixture of which the major con- 

 stituent is a polymer of methylated galacturonic acids (235). Very 

 little attention has been devoted to these materials as carbon sources, 

 although pectin has been reported to provide carbon for the growth of 

 Sclerotinia cinerea (240) and of Psalliota bispora (216). From the 

 frequent occurrence in fungi of enzymes acting upon the pectic sub- 

 stances (Chapter 5) and from the employment of pectin as the carbon 

 source in experiments on enzyme production, it is probably safe to 

 assume that they are utilized in nature by many fungi, both parasitic 

 and saprophytic. 



5. ORGANIC ACIDS 



Organic acids are generally poorer sources of carbon for fungi than 

 are the carbohydrates. Apart from what might be called intrinsic 

 utilizability, however, three factors interfere with the utilization of the 

 organic acids in particular cases. First, cells are often impermeable to 

 organic acids at physiological pH levels. Impermeability is more often 

 invoked than demonstrated, but there is no doubt that it is a factor 

 in the utilization of acids. Second, the utilization of neutralized organic 

 acids causes a rise of the culture pH which may interfere with growth. 

 This phenomenon explains the repeated observation that growth with 

 organic acids is better if nitrogen is supplied as an ammonium salt 

 (115, 165, 207). Utilization of the ammonium cation tends to drive 

 the pH down, partially or wholly counteracting the alkalinizing effect 

 of acid utilization. In addition, it appears that the utilization of am- 

 monium ion is more rapid if the medium is neutral or alkaline (134). 

 It has been argued (24) that a more direct reaction exists between 

 organic acids and ammonium ion, but this appears unlikely and is 

 not necessary in order to explain the observed phenomena. 



A third, and probably only occasionally important, factor is the 

 chelation of inorganic ions by certain of the organic acids, e.g., citrate 

 and tartrate. This may, however, either promote growth by keeping 



