30 THE METABOLISM OF ALGAE 



relative amounts of the individual pigments. It is also 

 evident that a species in which a particular type of pig- 

 mentation is determined genetically may become adapted 

 to an environment in which its pigmentation, although not 

 a disadvantage, is no longer an advantage. 



THE HYDROGEN DONOR 



The energy made available through absorption by the 

 pigments is used to effect reductions by means of hydrogen 

 derived from some specific substance. In normal photo- 

 synthesis by higher plants and by the majority of algae this 

 hydrogen donor is water. In certain algae, however, other 

 substances can be utilized at least as the ultimate source of 

 hydrogen. This ability to make use of a wider variety of 

 hydrogen donors in photosynthesis is a respect in which 

 the algae resemble the bacteria and which appears to be a 

 type of biochemical variation characteristic of the more 

 primitive classes of organisms. 



The process of 'photoreduction' by means of elementary 

 hydrogen, which can occur in various species of algae after 

 a period of adaptation under anerobic conditions, was dis- 

 covered by Gaffron in 1939.^^^' ^^"^ Adapted algae become 

 able to carry out various reactions which they were not 

 able to effect before. In the dark they are able to absorb 

 hydrogen from an atmosphere containing a high proportion 

 of this gas, providing that hydrogen acceptors such as 

 oxygen are available, or to liberate it into an atmosphere 

 of pure nitrogen by fermentation of a substrate such as 

 glucose. Oxygen and hydrogen can thus be absorbed 

 simultaneously by adapted algae and the reduction of carbon 

 dioxide may be coupled with this. In the light, exchanges 

 involving hydrogen are accelerated and photosynthesis 

 from carbon dioxide and hydrogen can occur. The value of 

 the quotient AHg 'ACOg is in accordance with the over-all 

 equation: 



CO2+2H2— >(CHoO)+H20 . . (2) 



i.e. the process is similar to the photosynthesis with hydro- 

 gen as hydrogen donor which occurs in the purple bac- 



