PHOTOHETEROTROPHIC BACTERIA 109 



Gaffron's results showed the expected general increase of Qp with n, 

 with the notable exception of n-butyric acid. He saw in the different 

 vahies for the two butyric acids an indication that the mechanism of 

 assimilation may depend on the structure of the molecule. Van Niel 

 (1941) made similar experiments with Spirillum rubrum (another Athio- 

 rhodacea). The individual values again scattered over a considerable 

 range; for example, for acetate, in 48 single experiments, they varied 

 from — 0.15 to + 0.28, but the averages, as given in table 5. VI, showed 

 the expected regular increase from acetate through propionate to 

 n-butyrate. 



A comparison of the observed values with the calculated ones in 

 table 5.V shows a regular deficiency of carbon dioxide consumption. 

 In the case of acetate, some carbon dioxide is liberated, although the 

 formula of acetic acid, C2H4O2, allows of a quantitative conversion into 

 a carbohydrate (corresponding to Qp = 0). The average "coassimila- 

 tion" of carbon dioxide with the higher fatty acids, amounts to only 

 60% of the theoretical value. One possible explanation of this fact is 

 that assimilation produces compounds which are more reduced than the 

 carbohydrates. 



For compounds consisting only of C, and H atoms (and not con- 

 taining peroxide bonds), an appropriate measure of the reduction level 

 is provided by the respiratory quotient, Qr (which is defined on page 32 

 as the ratio ACO2/— AO2) or, more conveniently, by its inverse value, 

 the reduction level L, which is equal to the number of molecules of oxygen 

 required for the complete combustion of a molecule, divided by the 

 number of carbon atoms in it. It can be calculated by means of the 

 equation 



,. ,„, y 1 2nc + Inn - no 



^ ^ Qr 2nc 



where nc, wh and no are the numbers of carbon, hydrogen and oxygen 

 atoms in the molecule, respectively. (We shall see in chapter 9 how 

 closely L determines the energy content of compounds of this type.) 



The value of L for carbohydrates is 1. A simple calculation shows 

 that products obtained by the coassimilation of fatty acids and 60% of 

 the quantity of carbon dioxide required for the formation of a carbo- 

 hydrate, must have L values between 1.43 {n = 0) and 1.15 (71 = =»). 

 Analyses of the dry matter of purple bacteria are in good agreement 

 with this calculation. Van Niel (1936) found in Spirillum rubrum, 

 Rhodomonas, Streptococcus varians and Chromatium (sulfur-free) an 

 average of 55.7% C, 7.4% H, 15.1% O and 11.8% N. Assuming that 

 all nitrogen is present in the form of amino groups, and substituting an 

 equivalent quantity of hydroxyl groups for them, we arrive at a compo- 



