HYDROGEN ADAPTATION AND DE- ADAPTATION 131 



"commutation." As soon as some reduced and hydrogenated enzyme, 

 H2EH, has been formed, reaction (6.5) allows the organism to dispense 

 with the slow incubation reaction (6.1). 



Enzymes capable of introducing molecular hydrogen into cellular 

 metabolism have been discovered by Stephenson and Stickland (1931) 

 in certain colorless bacteria (as B. coli) and later found by Roelefson 

 (1934), Gaffron (1935) and Nakamura (1937, 1938) in the nonsulfur 

 purple bacteria Rhodovibrio, Rhodohacillus palustris and Rhodospirillum 

 giganteum, and in the sulfur bacteria Thiocystis and Chromatium minutis- 

 simum. In each case, a variety of organic and inorganic substrates 

 (methylene blue, fumarate, nitrate, oxygen, etc.) were found to be 

 suitable as hydrogen acceptors. Their assortment is different for 

 different organisms, so that one can conceive of the existence of a number 

 of different, acceptor-specific hydrogenases. However, Yamagata and 

 Nakamura (1938) concluded, from comparative cyanide inhibition 

 experiments with B. coli formicum, Rhodohacillus palustris and B. del- 

 hruckii, that the hydrogenase in all these organisms is the same, and that 

 it donates hydrogen to a common intermediate acceptor, after which 

 specific oxidoreductases (or an oxidase) transfer it to different final 

 acceptors. 



We may assume that the same hydrogenase and the same intermediate 

 acceptor are present also in anaerobically incubated algae. 



We designate the primary hydrogen acceptor by Ah, and spht equation 

 (6.3) into the two equations (6.6a) and (6.6b); and equation (6.4) — into 

 the two equations (6.6a) and (6.6c): 



(6.6a) HzEh + Ah ^ HaAn + Eh 



(6.6b) H2AH + R > HoR + Ah or 



(6.6b') H2R' + A H > H2A H + R' 



(6.6c) H2AH + 2Z > 2HZ + Ah 



Reaction (6.6a) must be reversible (since adapted algae can either 

 absorb or liberate hydrogen). As for reaction (6.6b), its direction may 

 depend not only on concentrations, but also on the specific nature of the 

 metabolites R present in the cell (as expressed by the alternative equation 



6.6b'). 



De-adaptation occurs, in presence of carbon dioxide, if the intensity 

 of illumination is raised beyond a certain threshold. The further this 

 threshold is exceeded, the more rapid is the return to normal photosyn- 

 thesis (c/. Fig. 14, p. 145). The photochemical de-adaptation is irrevers- 

 ible, i. e., hydrogen absorption is not resumed upon return to low light 

 intensity (Fig. 8). However, the adapted state can be restored much 

 more rapidly immediately after de-adaptation than after a prolonged 

 period of aerobic photosynthesis, probably because the autocatalytic 



