BACTERIAL CHEMOSYNTHESIS 111 



oxygen. However, we now see a more plausible explanation: If the rates of photo- 

 synthesis and respiration are limited by the supply of hydrogen through a common 

 enzyme system, every increase in photosynthesis must lead to a decrease in respiration. 

 The fact that under no circumstance does the organism switch over from oxygen con- 

 sumption to oxygen Uberation, agrees well with this picture, whereas it would be diffi- 

 cult to explain if photosynthesis and respiration were two independent processes, as in 

 the higher plants. 



Each fatty acid is decomposed by purple bacteria at a different characteristic rate — 

 the same in respiration and photosynthesis; and if a mixture of several acids is provided, 

 their total decomposition rate is additive. This proves that a specific enzyme is available 

 for each of the acids. 



We made this digression to the subject of the metabolism of purple 

 bacteria in the dark because the parallelism of respiration and photo- 

 synthesis provides an additional argument in favor of a partial direct 

 assimilation of the reductant: Since such an assimilation is known to 

 occur in the dark metabolism, it is also likely to occur in light. Thus, 

 in addition to the direct assimilation of overreduced intermediates, which 

 was suggested as an explanation of deviations from equations (5.11) and 

 (5.12), a part of the carbohydrates formed in the " photoassimilation " of 

 fatty acids, can also be due to a direct "heterotrophic" assimilation, 

 and not to photosynthesis. Van Niel (1941) considered, for the case of 

 acetate assimilation, the three possibilities (5.15), (5.16) and (5.17), to 

 which we may add (5.18) and (5.19) for the sake of completeness: 



To To 



photoreduction direct assimilation 



(5.15) H4C2O2 -t- 2 H2O >[8H + 2C02] 



(5.16) H4C2O2 -I- U H2O > [6 H + 1| CO2] + h ICH2O} 



(5.17) H4C2O2 + H2O > [4 H + CO2] + ICH2OI 



(5.18) H4C202+§H20 >[2H + ^C02] + U {CH2O) 



(5.19) H4C2O2 > 2 {CH2O) 



The first equation (5.15), represents pure photoreduction; the next 

 three represent photoreduction coupled with an increasing proportion of 

 direct carbohydrate assimilation; and the last one, direct assimilation 

 without photoreduction. 



B. Bacterial Chemosynthesis * 



From the pigmented photosynthesizing bacteria, there is but one step 

 to the nonpigmented " chemosynthesizing " bacteria, which often use the 

 same oxidation substrates (e. g., sulfide, thiosulfate or hydrogen), but 

 work with chemical energy instead of light energy. The discovery of 

 these organisms by Vinogradsky was mentioned on page 100. Some of 

 them can live in (and sometimes only in) purely inorganic media; their 

 autotrophic mode of life is thus easy to prove. These organisms are 



* Bibliography, page 126. 



