Chapter 6 

 THE METABOLISM OF ANAEROBICALLY ADAPTED ALGAE * 



1. The Adaptation of Algae to Hydrogen and Hydrogen Sulfide 



In the preceding chapter, we found that the photosynthesis of bacteria 

 is strikingly adaptable. The photosynthesis of green plants, on the other 

 hand, has long been considered as a rigid process, which can be accelerated 

 or retarded by external influences, but whose chemical mechanism is 

 unalterable. This is, however, not universally true. Nakamura (1937, 

 1938) found that certain diatoms (Pinnularia) and blue-green algae 

 {Oscillatoria) can use hydrogen sulfide for the reduction of carbon dioxide 

 —in other words, can adopt a metabolism similar to that of the purple 

 sulfur bacteria. Ordinarily, the photosynthesis of green plants is 

 inhibited by hydrogen sulfide (c/. page 315); but Nakamura's algae 

 consumed carbon dioxide even in presence of this gas. The evolution of 

 oxygen, however, was replaced by the deposition of sulfur globules in 

 the cells. 



This interesting phenomenon certainly deserves more than the cursory 

 attention it has received in Nakamura's work. Much more detailed has 

 been the study which Gaffron devoted to certain unicellular green algae 

 which, after a period of anaerobic incubation, become able to utiHze 

 molecular hydrogen or organic hydrogen donors as reductants in photo- 

 synthesis, that is, adopt a metaboHsm reminiscent of the autotrophic or 

 heterotrophic Athiorhodaceae. 



The adaptation of green algae to molecular hydrogen was discovered 

 by Gaffron in 1939, and investigated in a series of important papers 

 (Gaffron 1939, 19401-2; 194212; Gaffron and Rubin 1942, reviews Franck 

 and Gaffron 1941, Gaffron 1943). In studying "induction effects" in 

 plants after anaerobiosis in the dark, Gaffron found that some unicellular 

 green algae {Scenedesmus, for example) do not react to this "anaerobic 

 incubation" by a temporary inhibition of gas exchange in light, as do 

 the higher plants, but by a more-lively-than-usual liberation of gas. 

 This "inverse induction" was later found to be caused by a liberation of 

 hydrogen, in addition to (or instead of) the usual exchange of carbon 

 dioxide and oxygen. 



* Bibliography, page 148. 



128 



