364 RESPIRATORY METABOLISM 



where D is the diffusion coefficient of O2, c is the O., tension at the 

 surface, and A is its rate of O., consumption. In the case of a cyhnder 

 (e.g., Spirostomum) the factor 5 should be 4. For Colpoda, Adolph 

 (1929) calculated the value of "'<«" to be l48 \x at atmospheric O2 

 tension, and 72 \\ at 40 mm. Hg partial pressure of O.,. In large ciliates 

 this factor might be important at low Oo tensions, even with a rather 

 low rate of O, consumption. 



3. EFFECT OF CO, TENSION ON O, CONSUMPTION 



Root (1930) showed that when CO, tension was raised from 1 mm. 

 Hg to 15-20 mm. Hg, the respiration of Paramecium increased slightly 

 (less than 15 percent), and it was believed that this increase might be 

 caused by increased activity of the organisms. As the CO, tension was 

 increased above 60 mm. Hg, O, consumption decreased continuously 

 to about 40-60 percent of the control when CO, tension reached 220- 

 360 mm. Hg. Similar experiments on fertilized Arbacia eggs did not 

 show increase at low concentrations, and all CO, tensions greater than 

 30 mm. Hg produced a decrease to less than 40 percent of normal. 

 Paramecit/m apparently was much more resistant to increase of CO, 

 tension than Arbacia. For both Paramecium and Arbacia only slight 

 effects were obtained with HCl, at pH values comparable to those 

 present during the CO2 experiment (4.5 to 7.5 for Paramecium). 



From the information available, it is not possible to determine the 

 mechanism of action of CO, on protozoan respiration, and results with 

 other organisms are few and variable. Apparently CO, is not involved 

 as an inhibitor or accelerator of any of the known mechanisms of 

 respiration (to be discussed below), and at present we can only say 

 that the effect on respiration seems to be indirect, and that the results 

 are not due to pH changes in the external fluid. However, internal pH 

 changes, as suggested by Root (1930), might account for the effect. 

 This is an explanation comparable to that given by Jahn (1936) for 

 the effect of the lack of CO., on growth of Chilomonas and bacteria. 

 It is well established that certain bacteria will not grow in the absence 

 of COg. Jahn (1936) studied the effect of COa-free media on growth 

 of Chilomonas and Colpidium and found a distinct inhibition with 

 Chilo7nonas and none with Colpidium. The explanation was offered 

 that the inhibition of growth might be caused by inadequate intracellular 

 buffering in those species which were affected by lack of CO,, and that 



