PHOTOSYNTHETIC REACTIONS OF AUTOTROPHIC BACTERIA 



103 



The relation between the quantities of carbon dioxide and hydrogen 

 sulfide consumed by bacteria was determined by van Niel (1930, 1931) 

 for a species of purple bacteria which oxidizes sulfide to sulfate. The 

 over-all reaction deduced from these experiments, was: 



(5.1) 



CO2 + I (HS)aq. + H2O 



-> ICHjOi + h (HS04)iq. - 7 kcal 



This and the following equations of bacterial photosynthesis have been rewritten 

 in the ionic form most suitable for reactions in aqueous phases. For the sake of uni- 

 formity all equations have been reduced to the assimilation of one molecule of carbon 

 dioxide, even if this necessitated the use of fractional coefficients. 



Formula (5.1) implies the following " photosynthetic quotients": 



(5.2) - ACO2 : - AH2S : AH2SO4 = 2:1:1 



The observed ratios are shown in table 5. II. These ratios are close to 



Table 5.II 



The Photosynthetic Quotients for Purple Sulfur Bacteria 



(after van Niel) 



the stoichiometric values, thus proving that carbon dioxide and hydrogen 

 sulfide take part in a common reaction, and not in two "coupled" meta- 

 bolic processes, as assumed by Buder. A further proof of the absence of 

 a separate photosynthetic reaction not involving the sulfide is the fact 

 that the consumption of carbon dioxide ceases as soon as the supply of 

 sulfide has been exhausted (instead of continuing with the liberation of 

 oxygen, as one would expect according to the theories of Vinogradsky 

 and Buder). 



Van Niel also measured the gas exchange of green sulfur bacteria, 

 and found it to be in agreement with the following equation: 



(5.2) 



CO2 + 2 H2S 



-> {CH2O} -I- H2O 4- 2 S -h 5.1 kcal 



As mentioned above, various species of purple sulfur bacteria are capable 

 of reducing carbon dioxide by means of compounds intermediate between 

 sulfide and sulfate, e. g., free sulfur, thiosulfate or sulfite. The following 

 over-all equations were suggested by van Niel for these forms of bacterial 



