HYDROGEN ADAPTATION AND DE-ADAPTATION 129 



In studjdng this phenomenon Gaffron (1939, 1940) found that algae 

 which were capable of liberating hydrogen, were also able to absorb it, if 

 placed in an atmosphere containing a high proportion of this gas. 

 Hydrogen evolution and consumption can be observed even in darkness; 

 but both processes are accelerated by light. The hydrogen exchange 

 continues, gradually decreasing, until the available cellular "hydrogen 

 acceptors" are entirely saturated with hydrogen, or until the available 

 "hydrogen donors" are exhausted. In presence of an added hydrogen 

 acceptor, the absorption of hydrogen can continue for a much longer 

 time, and the same is true of hydrogen liberation in presence of an 

 added donor. 



Appropriate hydrogen acceptors are oxygen (in small quantities, 

 since larger quantities of this gas cause de-adaptation), and carbon 

 dioxide; while glucose and other organic substrates can act as hydrogen 

 donors. Thus, hydrogen-adapted algae are capable of bringing about 

 the following reactions: 



In the Dark: (I) and (11) .—Absorption of hydrogen from an atmosphere containing 

 a high proportion of this gas, and evolution of hydrogen into an atmosphere of pure 

 nitrogen (so-called "hydrogen fermentation"). 



(JU) .—Simultaneous absorption of hydrogen and oxygen (so-called " oxyhydrogen " 

 or "Knallgas" reaction). 



(lY).— Reduction of carbon dioxide coupled with the oxyhydrogen reaction (III), 

 a process analogous to the metabohsm of autotrophic hydrogen bacteria (page 116). 



In Light: (V) and (VI).— Enhanced hydrogen absorption in an atmosphere of hydro- 

 gen, and enhanced hydrogen evolution in an atmosphere of nitrogen. The first-named 

 process may, however, be identical with reaction (VII), i. e. it may represent the 

 photoreduction of carbon dioxide produced by acid fermentation, rather than the 

 hydrogenation of an organic hydrogen acceptor. 



(VII) and {Ylll).— Photosynthesis from carbon dioxide and hydrogen or from carbon 

 dioxide and organic hydrogen donors — processes reminiscent of the metabohsm of auto- 

 trophic and heterotrophic purple bacteria respectively. Gaffron designated these 

 reactions as "photoreductions," and although this term is not very specific, it may be 

 used as a short substitute for "photoreduction of carbon dioxide by reductants other 

 than water," while the term "photosynthesis" is retained to mean "photoreduction of 

 carbon dioxide by water." (To be consistent, one should use the term "photoreduction " 

 also when speaking of the metabohsm of purple bacteria, a terminology which was 

 not rigidly adhered to in chapter V.) 



2. The Mechanism of Hydrogen Adaptation and De-adaptation 



Before discussing the metabolic reactions of hydrogen-adapted algae, 

 we shall deal with the processes of adaptation and de-adaptation (the 

 latter called "reversion" by Gaffron). 



Not all unicellular green algae can be adapted to hydrogen. Experi- 

 ments with Chlorella, as well as with diatoms (two strains of Nitzschia) 

 and blue-green algae (Oscillatoria), gave no positive results. No generic 

 relationships are apparent: Scenedesmus, Ankistrodesmus and Raphidium 



