40 THE METABOLISM OF ALGAE 



substance and its reduction by illuminated chloroplasts may- 

 be coupled with a reductive carboxylation such as that of 

 pyruvic acid to malic acid, the sequence of reactions being: 



light 



HoO+coenzyme II >• coenzyme II.H2+^02 . (6) 



'malic' enzyme 

 CHa.CO.COOH + COa+coenzyme II. H2 > 



COOH.CHa.CHOH.COOH+coenzyme II (7) 



The so-called 'malic' enzyme which catalyses reaction 

 (7) has been shown to occur in a number of plant 

 species. ^^®' "^^' ^^ Although an extracellular photosynthetic 

 reaction has thus been brought about, it is doubtful whether 

 this particular reaction is concerned in normal photo- 

 synthesis.^®^' ^^ Malic acid does indeed appear among the 

 products of short-term photosynthesis but photosynthesis 

 has been found to be little affected by the presence of in- 

 hibitors such as malonic acid which completely suppress 

 the formation of malic acid.^^' ^^ 



Further information regarding the link between the 

 photochemical reaction and the reduction of carbon dioxide 

 comes from observations on the enhanced dark fixation 

 which occurs in algae and in other plants immediately 

 following a period of normal photosynthesis or a period of 

 illumination in the absence of carbon dioxide and oxygen. 

 In both cases the enhanced dark fixation shows character- 

 istics of photosynthetic fixation. ^^' ^^^> ^^* For Chlorella^ 

 studies using radioactive carbon dioxide have shown that 

 the products of this dark fixation following illumination 

 are the same as those of short-term photosynthesis. This 

 has been used as evidence in favour of the view that a 

 reducing product, perhaps, for example, reduced coenzyme 

 II, survives from the photochemical reaction and is capable 

 of complete carbon dioxide reduction in the absence of 

 light. Such a substance might be concerned in reductions 

 at more than one point in metabolism, for example in the 

 formation of hexose sugar from phosphoglyceric acid or in 

 the regeneration of the Co acceptor.^^ However, it has been 

 pointed out that although the products of dark fixation 

 following illumination are qualitatively the same as those 



