OXYGEN AND GROWTH 



23 



193, 219), close to the value for resting cells of yeast (59). Bunning 

 (44) reports a lower internal pH, below 5.0, for Aspergillus niger. 

 Whether or not the internal pH responds to changes in the acidity 

 of the external medium is uncertain: there may be some effect (44, 

 228), but it is to be presumed that any large change would result in 

 injury and death. The principal effects of external pH are probably 

 on permeability and other surface phenomena. 



8. OXYGEN AND GROWTH 



Fungi are commonly thought of as strictly aerobic, and this opinion 

 is basically correct. However, the quantitative relations of growth 

 and oxygen supply vary considerably among different forms. 



Visually observable growth of fungi occasionally occurs under ni- 

 trogen or in a liquid-filled container (61, 191, 220). This limited 

 growth probably depends on oxygen previously dissolved in the 

 medium. Blastocladia pringsheimii grows well in tank nitrogen or 

 carbon dioxide but fails to grow if oxygen is chemically absorbed by 

 pyrogallol; the fungus has, therefore, a very low but real oxygen de- 

 mand (85). 



Achlya prolifera has been reported to grow in the presence of pyro- 

 gallol (134). However, this report cannot be accepted without con- 

 firmation, in view of the overwhelming evidence that other fungi 

 and species of Streptomyces fail to grow at very low or zero oxygen 

 pressures (69, 94, 119, 127, 166, 192, 195, 204, 234, 237, 257, 292, 342, 

 343). 



When growth on agar is the criterion, it appears that most fungi 

 grow as well at 20-40 mm oxygen pressure as at atmospheric (160 mm) 

 (69, 116, 118, 261, 295). However, the dry weight is usually affected 

 at somewhat higher oxygen pressures; growth of Ophiobolus graminis 

 is reduced at 105 mm oxygen (94), Aspergillus oryzae grows best at 

 oxygen pressures higher than atmospheric (292). Contradictory data 

 may be cited (12, 166), but it seems that mass increase is more sensi- 

 tive to oxygen deficit than is the more diffuse and thinner growth of a 

 colony on agar. 



In keeping with the relatively low oxygen requirement for growth, 

 fungi successfully colonize environments in which oxygen is limited, 

 e.g., relatively stagnant aquatic milieu. It is doubtful whether invasion 

 of woody tissues is ever prevented by lack of oxygen (295), and the 

 same is probably true of invasion by vascular pathogens and decay 

 fungi. The number of fungi usually falls off at lower soil depths 

 (31, 45, 147), but this may be caused more by high carbon dioxide 



