CATALYSTS IN PHOTOSYNTHETIC PROCESSES 451 



a good deal of evidence remains which indicates that in the intact cell the 

 oxygen is liberated with the help of a catalyst which is inhibited by hydroxyl- 

 amine and, although to a smaller degree, by cyanide and azide (042,951,974, 

 2198,3029). The system of Warburg and Liittgens {29Jfo) which resembles 

 that of Hill but uses quinone instead of ferric oxalate, was inhibited by 

 o-phenanthroline. 



Hydrogenase in photoreduction of carbon dioxide in bacteria and algae. The 

 photoreduction of carbon dioxide in bacteria was discovered by Roelefsen 

 {2326) and Gaffron {987) in 1934.. and has been studied extensively by van 

 Niel and Gaffron. Green bacteria and purple bacteria are able to reduce 

 carbon dioxide by a photochemical process using hydrogen sulfide {Thwrho- 

 daceae) or simple organic compounds such as lower fatty acids {Athwrho- 

 daceae) as hydrogen donors. In the purple bacteria bacteriochlorophyll fills 

 the role played by chlorophyll in the green algae and higher plants. 



Molecular hydrogen can be used as hydrogen donor for the photoreduction 

 of carbon dioxide by some species of purple bacteria {950,968,2326,3038) . 

 In the dark the following reactions can be observed: in nitrogen molecular 

 hydrogen is developed {2007,2326), while in the presence of hydrogen and 

 oxygen (of low pressure) the "Knallgas" reaction is catalyzed {968). It can 

 therefore be assumed that hydrogenase takes part in these reactions and also 

 in the photoreduction of carbon dioxide, light being required solely for the 

 reduction of carbon dioxide {2050). 



Similar observations .have been made with certain green algae by GafTron 

 {969,970,972,975). If Scenedesmus is exposed to several hours of anaerobiosis, 

 in hydrogen as in nitrogen, its metabolism becomes similar to that of purple 

 bacteria. On illumination with weak light intensities, the alga reduces carbon 

 dioxide^ith molecular hydrogen. In the dark hydrogen is taken up or 

 evolved (in nitrogen); the "Knallgas" reaction is catalyzed and, if carbon 

 dioxide is present, is coupled with a dark reduction of this substance (Gaffron, 

 971). The latter reaction has also been observed to occur in some bacteria 

 (Ruhland, 2392). 



Adaptation and deadaptation. If the anaerobically adapted alga (in the 

 "reduced state") is subjected to intense light in the presence of carbon dioxide, 

 normal photosynthesis with oxygen development returns, and photochemical 

 reduction with hydrogen will now not reappear if the light is dimmed again. 

 This is known as deadaptation. Exposure to oxygen in the dark also causes 

 deadaptation, but requires a higher o.xygen pressui'e; in the "Knallgas" 

 reaction oxygen disappears without abolishing the reduced state. 



The study of the effect of inhibitors on adaptation and deadaptation 

 gives further support to the assumption that valency changes of 

 hematin-Hke enzymes are involved in these reactions. Carbon mon- 

 oxide not only inhibits the photoreduction of carbon dioxide {i.e., 

 the hydrogenase), but also deadaptation. Cyanide and hydroxyl- 

 amine, on the other hand, which hardly inhibit the photoreduction 



