ENERGY TRANSFORMATION 



405 



The gas exchange in the autotrophic assimilation of carbon by non- 

 chlorophyll-containing bacteria is thus similar to the gas exchange 

 of green plants. On organic media, the organism grows in a manner 

 similar to heterotrophic bacteria. In the presence of organic matter 

 the organism oxidizes H 2 considerably less than in its absence. 



Fig. 22. Reaction of medium and oxidation of hydrogen by bacteria (Ruhland) 



The organisms oxidizing hydrogen are thus found to be facultative 

 autotrophic, i.e., they are able to develop heterotrophically in the absence 

 of hydrogen. Whenever hydrogen is present, however, they are cap- 

 able of developing autotrophically. The presence of iron (when only 

 2.4 x 10 -5 mgm. Fe are present in 50 cc. no growth or hydrogen 

 oxidation takes place) and a proper pH value (6.8 to 8.7 opt.) were 

 found to be essential for the respiration of organisms oxidizing hydro- 



