528 LIGHT AND LIFE 



TABLE 6 



Equivalence of ATP and Light in the Assimilation of C"-acetate by 



Cell-free Preparations of Chromalium 



(LosADA, Trebst, Ogata, and Arnon, 97) 



Carbon'^ fixed in 

 Treatment soluble compounds 



(Thousands of counts /min) 



1. Dark, control 27 



2. Dark, ATP 180 



3. Dark, ATP, hexokinase 186 



4. Dark, ATP, hexokinase, glucose 6 



5. Light, control 414 



6. Light, hexokinase 348 



7. Light, hexokinase, glucose 20 



Each vessel included, in a final volume of 1.5 ml, cell-free extract, containing 0.3 mg 

 bacteriochlorophyll and the following in micromoles: Tris buffer, /(H 7.8, 80; cysteine, 

 20; magnesium chloride, 5; manganese chloride, 2; potassium chloride, 20; coenzyme 

 \, 0.3; oxaloacetate, 10; carboxyl-labelled C'-acetate, 3. 1.5 mg hexokinase, type III 

 (Sigma Chemical Co.), 10 micromoles glucose, and 4 micromoles ATP were added as 

 indicated. In treatments 5, 6, and 7, no addition of ADF and phosphate was neces- 

 sary to supplement the catalytic amounts present in the cell-free extracts. 



particularly significant because it was found in such photosynthetic 

 bacteria as CJiromatium, that are unique in the living world in being 

 strict phototrophs. CJiromatium, unlike, for example, Chlorella or 

 photosynthetic bacteria of the genus RJiodospirillum, cannot replace 

 its light-dependent mode of life by a heterotrophic, aerobic metabolism 

 in the dark (158, 109, 159) . CJiromatium grows only in the light (158, 

 109) , and being an obligate anaerobe, does not possess an alternative 

 way for forming ATP by the mechanism of oxidative phosphorylation. 



As regards the photoassimilation of acetate in another photosyn- 

 thetic bacterium, the facultative anaerobe R. nibrum, a similar view 

 that the contribution of light is limited to cyclic photophosphorylation 

 was recently expressed, on the basis of independent evidence, by 

 Stanier, Doudoroff, Kunisawa, and Contopoulou (141) . 



In certain circumstances, ATP formation may be the sole contribu- 

 tion of the photosynthetic process, not only in bacteria but also in 

 higher plants. We have suggested elsewhere (18) that in green plants 

 cyclic photophosphorylation may continue forming ATP when CO2 

 assimilation is, for one reason or another, reduced or even stopped 

 altogether. This might arise during the well-known midday closure 

 of stomata in leaves of higher plants (140, 69), a closure which re- 

 stricts the suj:)ply of CO^. The closure of stomata often coincides 

 with an abundance of starch and an incipient water deficit in the 

 photosynthesizing cells. Under these conditions cyclic photophos- 



