392 RESPIRATORY METABOLISM 



basis of either of the above two theories. Additional evidence, however, 

 might be gained for the second theory from the fact that one atmosphere 

 of O2 is more toxic at low temperatures (4-5° C.) than at high (23- 

 25° C.) , and that four atmospheres of Oo are more toxic at high tempera- 

 tures than at low. The greater solubility of O2 at 4-5° C. can account for 

 the greater toxic effect with one atmosphere pressure. However, the re- 

 verse effect at four atmospheres O2 pressure must, as suggested by Cleve- 

 land, be connected in some manner with metabolic processes. Super- 

 ficially, at least, these results seem to be explicable on the basis of the 

 second of the above theories, i.e., the organisms grew more rapidly and 

 produced more H0O2 at the higher temperatures. An examination of the 

 protozoa for catalase, or of the digestive contents of oxygenated insects 

 for H2O2, might yield pertinent information. 



The data of Cleveland (1925) on the toxicity of oxygen for the in- 

 testinal Protozoa of earthworms, salamanders, frogs, and goldfish, are 

 possibly open to this explanation, but here also we lack experimental 

 evidence. Where such defaunation procedures failed, as in the rat, we 

 might assume that the O2 tension of the digestive tract was not raised 

 in a manner comparable with that which occurred in the smaller or- 

 ganisms, or that the Protozoa present were more resistant. The former 

 theory seems much more probable. (For review of the chemistry of the 

 intestinal contents, see von Brand and Jahn, 1940.) 



The theory that the growth of anaerobes is dependent upon a low 

 oxidation-reduction potential in the medium was proposed by Quastel 

 and Stephenson (1926), and has gained considerable support among 

 bacteriologists (review, Hewitt, 1936) and some dissent (literature 

 cited by Broh-Kahn and Mirsky, 1938) . Positive evidence consists mainly 

 of the facts that (1) during the growth phases of anaerobic cultures, 

 especially the sporulating anaerobes, much lower oxidation-reduction 

 potentials are produced than during the growth phases of cultures of 

 aerobes; and (2) anaerobic forms do not start growing until the 

 potential is quite low (Eh < -f- 100 mv.). The first type of evidence 

 does not help to distinguish between cause and effect, and there are 

 some exceptions to the general trend. Most of these, however, are due 

 to the fact that the organism is only one factor which tends to determine 

 the Eh of the medium; the chemical composition of the medium certainly 

 determines, to a great extent, what potentials may be attained. The second 



