VAN KIEL'S THEORY: THIRTY YEARS AFTER 5 



eight quanta is too remarkable and complex a mechanism not to have a 

 long evolutionary history. And there are too many parallels in the be- 

 havior of photosynthetic bacteria and plants not to be intrigued by what 

 I am willing to call the more interesting and therefore more rewarding 

 hypothetical proposition. And perhaps I am biased because it took me 

 once so long to recognize its elegance. 



Certainly thirty years ago I simply could not see why I should accept 

 van Niel's proposition that organic substances serve purple bacteria 

 exclusively as hydrogen donors (just like H2S, sulfur or hydrogen) for 

 the reduction of carbon dioxide to carbohydrate, and thence to bacterial 

 substance. 



My OAvn results with purple bacteria did not show this at all. Quite 

 independently (never having heard of van Niel)Ihad started about 1929 

 on investigations on purple bacteria after Warburg had mentioned to 

 me that Stalfeld had told himof these strange organisms. As a chemist 

 I had never looked at a microbe before and knew only Warburg's great 

 discovery— the alga ChloreUa. Soon I discovered that the reddish 

 microbes behaved quite differently from green plants. They refused 

 to do photosynthesis but evidently ate organic acids in the light without 

 further ado, either with a stoichiometrically determined amount of 

 carbon dioxide, or, if available, also with hydrogen. The product of the 

 photometabolism was partly a substance (C4H6O2) (which Hans Fisher 

 later depolymerized into crotonic acid) and for the greater part just 

 more bacteria. Later, when working with Chromatium, the purple sul- 

 fur bacteria, I found that they produced lots of H2S in the dark, particu- 

 larly when previously illuminated in presence of butyrate. So I con- 

 cluded that the light reactions withsulfur were reversible and that this 

 was the mechanism by which they were able to attack organic sub- 

 stances. Many of you will remember that van Niel challenged this vigor- 

 ously. Years later Henley in my laboratory confirmed the fermentative 

 sulfide formation from internally stored sulfur but not from sulfate. 

 My observation of a particular accelerating effect of added sulfate was 

 indeed, as van Niel had shown, a nonspecific salt effect. 



In 1935 van Niel extended his special theory so that it included also 

 the metabolism of the heterotrophic purple bacteria. In this paper he 

 quotes Gaffron's statement that photosynthesis of the purple bacteria 

 involves the cooperation of a larger number of molecules and that 

 several intermediate reactions occur before the first stable reaction 

 products appear. Van Niel then wrote, "This statement seems to con- 

 tain an argument against a unified concept of photosynthesis in green 

 plants and purple bacteria." Because I could not see eye to eye with a 

 Dr. Roelofsen, working in Kluyver's laboratory, van Niel had spent a 

 year in Holland devising a good number of experiments to prove con- 

 vincingly that sulfur bacteria can use organic substances directly as 

 hydrogen donors, just like the Athiorhodaceae, He then came to Berlin 

 to see me. we set up one or two experiments, they were absolutely 



