206 RKSPIRATION 



2. FACTORS IN RESPIRATION 



In general, it is probable that the most important factor in respira- 

 tion is the species. Few comparative data are available, but we may 

 assume that fungi which can grow on a given substrate can also 

 oxidize it (291), so that the omnivorous saprophytes, e.g., Aspergillus 

 spp., may be expected to oxidize a greater variety of compounds than 

 do nutritionally more specialized forms. Within a given species, age 

 and developmental status are the primary non-environmental factors; 

 the permeability of the cell is also presumed to be important by the 

 usual indirect evidence. The principal external factors are acidity, 

 temperature, oxygen pressure, carbon dioxide, and accessory nutrients. 



Age and Developmental Status. As mentioned in Chapter 1, the 

 highest respiratory activity is usually found during the period of most 

 rapid growth. Subsequent to this period, respiration, especially endog- 

 enous respiration, falls off with increasing age. This may, of course, 

 reflect only the fact that more and more dead cells accumulate in the 

 older mycelium; quite possibly, the respiration of the growing tips of 

 the thallus is independent of the age of the culture so long as nutrients 

 are available. 



The data of Figure 1 are typical, and many others of the same type 

 have been reported in studies of fungi and aerobic actinomycetes (52, 

 70, 95, 96, 115, 134, 163, 249). Ustilago zeae appears unusual, in that 

 once the maximum respiratory rate is reached it is maintained (260). 



Spore respiration is considered briefly in Chapter 12; there is as yet 

 no reason to think that it differs fundamentally from that of mycelium, 

 although it would not be surprising to find generally higher activity in 

 germinating spores than in older cultures. The yeast phase of Blasto- 

 myces brasiliensis exhibits an endogenous oxygen uptake greater than 

 that of the mycelial form (215), and the same relation prevails between 

 cell types of Mucor guillermondii (179). In Blastocladiella emersonii 

 the two different types of mature thallus differ in their content of 

 important respiratory enzymes and, presumably, in their respiratory 

 pathways (44). Oxygen consumption of Dictyostelium discoideum 

 is in part a function of developmental stage (116). 



Cell Permeability. It is difficult to distinguish between intrinsic 

 inability of a cell to respire an exogenous substrate and failure of the 

 compound to penetrate to the interior of the cell. As in the utiliza- 

 tion of organic acids for growth (Chapter 3), it is often found that a 

 low pH is required for or accelerates the oxidation of organic acids 



