530 LIGHT AND LIFE 



chromatophores. The Chromatkim hydrogenase reduced DPN and 

 TPN with molecuhir hydrogen in tlie dark in the presence of benzyl 

 viologen. The enzyme was more active toward DPN than TPN 

 (Table 7) . 



These results indicated that in the presence of hydrogen gas 

 Chromatium cells do not require light for the reduction of pyridine 

 nucleotides. The role of light is, then, limited to ATP formation, 

 without which CO2 assimilation cannot occur (97) . Photosynthesis 

 by Chromatium in the presence of molecular hydrogen may, there- 

 fore, be summarized as follows: 



LIGHT PHASE 



Cyclic photophos- 



phorylation: wADP + "'P — * «-ATP 



DARK PHASE 



DPN reduction: 2 DPN + 2 H2 -^ 2 DPNHa 



CO2 assimilation: CO. + 2 DPNHo + n-KI? -^ 



(CH2O) + H2O + 2 DPN + n-hDV + n-V 



SUM: CO2 + 2 H2 -^ (CH2O) + H2O 



Several algal species are known to contain hydrogenases and to ac- 

 quire, after adaptation to hydrogen, a capacity to photoassimilatc 

 CO2 with the aid of molecular hydrogen (54, 55, 52) . This process, 



TABLE 7 



Pyridine Nucleotide Reduction with Molecular Hydrogen by 



Cell-free Preparations of Chromatium 



(Ogata, Nozaki, and Arnon, 115) 



Each vessel included, in a final volume of 3.0 ml, a cell-free preparation (PS) con- 

 taining 0.3 mg bactcriochlorophyll and the following in micromoles: Tris buffer, 

 pH 7.8, 80; MgClo, 5; potassium phosphate, 5; KCl, 50; and when added, DPN, 4; 



ri'N, 4; and ben/yl viologen, 0.1 (a gift of Dr. H. C;est). 0.1 mi of 20 per cent 

 K.OH was present in the center well. The reaction was carried out in an atmos- 

 phere of hydrogen at 25°C. Illumination, when given, was 3r),()()() Lux. At tlic end 

 of the reaction, an aliquot of the reaction mixture was precipitated with satinated 



(NH4).SO„ /;H 8, centrifugcd, and the optical ticnsity of the clear supernatant fluid 

 was measured at .'MO ni^. 



