THE PHOTOTROPHIC ASSIMILATION OF CARBON 3I 



teria.247 The quantum yield of this process is of the same 

 order as that for normal photosynthesis.^*^ 



Hydrogenases, enzymes capable of introducing hydrogen 

 into cellular metabolism, are well known in bacteria^^"? and 

 it appears that in certain algae there are similar enzymes, 

 which, however, only become active after reduction. This 

 reduction occurs during adaptation under anaerobic con- 

 ditions and is reversed when de-adaptation takes place in 

 the presence of oxygen or in the presence of carbon dioxide 

 if the intensity of illumination exceeds a certain limit. It 

 seems likely that in photosynthesis by adapted algae the 

 photochemical reaction involves the decomposition of water 

 as in normal photosynthesis^*^ and that the hydrogen donors 

 provided by the hydrogenase reaction are used for the 

 reduction of the oxidized products of this reaction, which 

 would otherwise be disposed of by processes leading eventu- 

 ally to the liberation of oxygen. If the light intensity is high 

 then these oxidation products accumulate at a rate faster 

 than that at which they can be reduced by the hydrogenase 

 system and this leads to the oxidation and inactivation of 

 the hydrogenase.^*^ Certain substances such as hydroxyla- 

 mine and o-phenanthroline stabilize adapted algae against 

 de-adaptation in the light, evidently by inhibiting the 

 oxidation of the hydrogenase by these intermediates of 

 photosynthesis, but at the same time reduce the quantum 

 efficiency of the process by a half.^^^' ^^^ 



Not all algae can be adapted to hydrogen in this way. 

 The process has been studied principally in Scenedesmus sp. 

 and has been found to occur also in a number of other 

 algae, but many species have been found not to possess the 

 property (Table 3). A long period is required for adaptation 

 by the marine algae but Chlamydomonas-moewusii either 

 does not require adaptation or has an adaptation period 

 shorter than 10 minutes.^^^ It is to be noted that the distri- 

 bution of the capacity does not appear to depend on phylo- 

 genetic relationships. The capacity is present in members 

 of at least four algal classes but does not necessarily occur 

 in all members of a taxonomic group, e.g. it is absent from 

 Chlorella whereas it occurs in Scenedesmus. 



