130 ANAEROBICALLY ADAPTED ALGAE CHAP. 6 



(three genera which have been successfully adapted) are no more closely 

 related between themselves than they are to Chlorella. 



Adaptation requires at least two hours of anaerobic incubation at 

 20° C, less at higher temperatures. At 35°, the hydrogen metabolism 

 of Scenedesmus starts almost immediately upon the removal of oxygen; 

 however, this temperature rapidly causes an irreversible injury to the 

 algae. During the adaptation period, the algae ferment, as all green 

 plants do when the oxygen pressure is below that corresponding to the 

 Pasteur effect (c/. Genevois 1927), producing carbon dioxide and non- 

 volatile acids; the rate of this "acid fermentation" is about the same in 

 hydrogen and in nitrogen. When fermentation has proceeded for a 

 certain time, Scenedesmus and similar algae become capable of picking 

 up hydrogen and this rapidly completes their adaptation. 



Gaffron suggested that an enzyme, which is usually present in the 

 chloroplasts in an inactive, oxidized form (we may designate it by EhO) 

 is reduced by a fermentation product H2F : 



(6.1) EhO + HjF >Eh + F + H20 



and by this acquires the properties of a hydrogenase, i. e., of a reversible 

 acceptor for molecular hydrogen : 



(6.2) E H + H2 . H2E H 



capable of transmitting this hydrogen to other substrates: 



(6.3) H2EH + R > H2R -1- Eh 



The effect of a hydrogenase on photosynthesis can be understood if 

 one assumes that the role of the oxidant R in (6.3) can be played by the 

 oxidation intermediates of photosynthesis, whose conversion into free 

 oxygen is thus effectively blocked (c/. Scheme 6.1, page 136). 



If we use the most general formulation of the primary photochemical 

 process (Eq. 7.10a) and designate the primary oxidation product as Z, 

 we may postulate that in hydrogen-adapted algae, reaction (6.4) : 



(6.4) 2Z + H2EH >2HZ + Eh 



takes the place of reaction (7.10b) and photosynthesis is thus converted 

 into "photoreduction." 



To explain the long "incubation period" and the rapid completion 

 of adaptation after the absorption of hydrogen has finally set in, Gaffron 

 suggested that the activation of the hydrogenase is accelerated auto- 

 catalytically by the reaction: 



(6.5) HjEh + EhO >2Eh + H20 



which is the reverse of a dismutation, and may perhaps be designated as a 



