110 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



sition approximately represented by the formula C5H7O2, corresponding 

 to a L value of 1.15. Gaffron (1933) isolated from the purple bacteria 

 a substance which could be depolymerized to crotonic acid, C4H6O2, 

 L = 1.18, and considered it as a direct product of photosynthesis. 

 However, the results obtained by Foster with isopropanol indicate that 

 the true photosynthetic quotient of purple bacteria probably corresponds 

 to the formation of carbohydrates, rather than to the production of any 

 more completely reduced substances. If this is true, deviations from 

 equations (5.11) and (5.12) in the assimilation of fatty acids are due 

 to a direct assimilation of intermediates, whose reduction level is higher 

 than that of the carbohydrates, rather than to the 'photosynthesis of 

 " overreduced " substances. (Dismutations, which often occur in enzy- 

 matic oxidation, can produce such high-energy intermediates even if the 

 original oxidation substrate itself is not overreduced.) 



This is not the only argument in favor of a partial heterotrophic nutrition of purple 

 bacteria. Another argument can be derived from the consideration of the metabolism 

 of these organisms in the dark. 



Barker (1936), Giesberger (1936), Clifton (1937), CUfton and Logan (1939), Winzler 

 and Bamberger (1938) and Winzler (1940) showed that the oxidation of organic sub- 

 strates by respiring bacteria often is coupled with their partial assimilation, e. g., in the 

 case of acetate, according to one of the equations: 



(5.14a) C2H3O2- + O2 > {CH2O} + HCO3- or 



(5.14b) 2 C2H3O2- + 3 O2 > {CH2O} + 2 HCO3- + O2 + H2O 



In the case of the purple bacteria, the mechanism of respiration has a particularly 

 close bearing on that of photosynthesis, because van Niel demonstrated that the first 

 stages of both processes are probably brought about by the same enzymatic system. 

 A few words may be said here about this pecuhar relationship. 



All Thiorhodaceae (as well as some Athiorhodaceae) are anaerobic, i. e., their dark me- 

 tabolism is of the nature of fermentation. This metabohsm was studied by Gaffron (1 934, 

 1935), Roelefson (1935), French (1937'' 2) and Nakamura (1937); but its chemistry has 

 not yet been clarified, mainly because it is preponderantly " auto fermentative " (although 

 Nakamura observed the dismutation of formate into hydrogen and carbonate by purple 

 bacteria). The relationship, if any, between this dark metabolism and the metabolism 

 of the same bacteria in light, is as yet not clear. 



However, some Athiorhodaceae are aerobic (or not strictly anaerobic), and their 

 dark oxidative metabolism was found by van Niel to bear a remarkable relation to 

 their photosynthesis. Respiration and photosynthesis, which are independent (al- 

 though contra-acting), in green plants, appear to be competitive in aerobic Athiorhodaceae. 

 The investigations of Nakamura (1937'''', 1938^'^) with Rhodobacillus palustris showed 

 that the same substances which are readily used as substrates of photosynthesis, also 

 are eagerly consumed as respiration substrates in the dark. Van Niel (1941) found 

 that the uptake of different fatty acids by Spirillum rubrum occurs at the same rate in 

 the dark and in fight — although only oxygen is consumed in the dark, while both oxygen 

 and carbon dioxide are taken up in moderate fight, and carbon dioxide alone is consumed 

 in strong fight. 



As mentioned on page 100, Nakamura interpreted the decrease in oxygen con- 

 sumption by purple bacteria in fight as evidence of a photochemical production of 



