COMPARATIVE BIOCHEMISTRY 79 



HYDROGEN EVOLUTION AND NITROGEN FIXATION IN 



ATHIORHODACEAE 



Under anaerobic conditions growth in the Hght of 

 Athiorhodaceae which utilize organic compounds more oxi- 

 dized than carbohydrate resuhs in the production of carbon 

 dioxide; if organic compounds more reduced than carbo- 

 hydrate are utilized carbon dioxide is consumed and must 

 be supplied to obtain good growth. This may be represented 

 in a general equation for either monobasic or dibasic acids: — 



monobasic acid 

 dibasic acid 



CnHo.-iCOOH+'-^COa+^^HoO - ^-^^[CH^O] 



2 2 2 



Thus for the monobasic acid, formic acid (n = o), carbon 

 dioxide is produced whereas for acetic {n = i) and fatty acids 

 w ^ 2 carbon dioxide wdll be consumed. However, in Rhodo- 

 spmllum rubrum (Athiorhodaceae), in addition to the pro- 

 duction of carbon dioxide, hydrogen is also produced in the 

 light under anaerobic conditions in the presence of a variety 

 of organic substances such as fumarate, malate, pyruvate, 

 acetate, or oxalacetate. This invalidates the simple equation 

 given above. Further it has also been show^n that R. rubrum 

 fixes molecular nitrogen when illuminated in a medium con- 

 taining malate, biotin, and mineral salts, in which case the 

 photochemical production of hydrogen is decreased as is also 

 to a less extent the evolution of carbon dioxide. Addition of 

 ammonia also decreases the production of hydrogen and 

 carbon dioxide although in this case the decrease is greater. 



THE GENERALIZED NATURE OF THE LIGHT REACTION IN 



PHOTOSYNTHESIS 



According to van Niel a generalized equation for photo- 

 synthesis can be written as CO2+2H2A-* (CHgOj+HgO+aA, 

 thus representing photosynthesis as an oxidation-reduction 

 reaction in which carbon dioxide is reduced by an oxidiz- 

 able hydrogen donor. According to this equation referred to 



