PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 37 1 



Light phase 



Cyclic photophos- 



phorylation: n-ADP + n-P >n-ATP 



Dark phase 



DPN reduction : 2DPN + 2H0 > 2DPNH.3 



COo assimilation: CO0 + aDPXHo + n-ATP >(CH,0) + HoO + 



2DPN + nADP + nP 



Sum : CO, + 2H, ^^ (CH,,0) + H^O 



Several algal species are known to contain hydrogenases and to acquire, 

 after adaptation to hydrogen, a capacity to photoassimilate CO, with the 

 aid of molecular hydrogen [133, 134, 135]- This process, which Gaffron 

 named photoreduction [134], appears to be the same type of photo- 

 synthesis as that in CJiromatiiim when it is supplied with hydrogen gas. It 

 seems likely that photoreduction by algae is a case of reversion to a 

 primitive photosynthesis of an earlier epoch when hydrogen gas was 

 present in the environment and the sole contribution of light was the 

 formation of ATP by cyclic photophosphorylation. 



12. The photoreductant in bacteria 



Although photosvnthetic bacteria when supplied with hydrogen gas do 

 not require light energv for the production of DPXH, (or TPNH,), a 

 different situation arises when photosynthetic bacteria are grown with 

 such hvdrogen donors as succinate or thiosulphate [122, 123]. Electrons 

 donated by these substances have an insufficient reducing potential for 

 reducing DPN (or TPX) in the dark. 



Additional energy is then required to bring about the reduction of 

 DPN (or TPN) and, in a photosvnthetic mode of life without oxygen 

 which is characteristic of photosynthetic bacteria, this additional energy 

 must come from light. If the electron flow mechanism is fundamental to 

 the conversion of light into chemical energy, how can it apply to the 

 photoreduction of pyridine nucleotides by thiosulphate or succinate ? 



An attractive hypothesis was to consider bacterial photosynthesis with 

 thiosulphate and succinate as an extension of bacterial photosynthesis with 

 hydrogen gas, when the photochemical events proper are restricted to the 

 formation of ATP by cvclic photophosphorylation. The primary photo- 

 chemical act that results in the generation by the excited chlorophyll of a 

 high energy electron and of the ultimate electron acceptor, [Chl+], would 

 be the same in both cases. But in the thiosulphate and succinate type of 



