VAN NIEL'S THEORY: THIRTY YEARS AFTER 11 



bigger. A while ago Bishop in our laboratory noted that a mutant of 

 Scenedesmus , which cannot evolve oxygen but handles photo reduction 

 very well, never produced hydrogen under conditions when we would 

 expect it to do so. In normal Scenedesmus evolution of hydrogen was 

 found to be sensitive to typical oxygen evolution inhibition. Putting the 

 old and the new experiments together, it seemed clear why light- 

 induced evolution of hydrogen is poor in algae and good in bacteria. 



The purple bacteria are set to use organic material, and if there is 

 a separation of |H| and [OH I the latter can easily be used to oxidize 

 the organic substrate. This is supported by Gest's recent studies where 

 hydrogen and carbon dioxide appear simultaneously and in proper 

 proportion. The green plants, on the other hand, have either put up a 

 permeability barrier against, or lost the enzymes for, the same 

 organic hydrogen donors. This apparently has been one condition for 

 the efficient evolution of free oxygen. What happens in the green 

 plants is that there will be back reactions whenever the reducing in- 

 termediates of the light reaction, such as reduced ferredoxin, are not 

 utilized. Some of these back reactions may be the ones that produce 

 cyclic phosphorylation. 



Assume now that we activate the hydrogenase in a normal green 

 plant, replace the hydrogen by nitrogen, remove the carbon dioxide, 

 poison the phosphorylating back reactions, and illuminate. What do 

 we get? A complete and direct photolysis of water by light. Hydrogen 

 and oxygen are evolved in about the right proportions and in impres- 

 sive amounts. 



If the oxygen evolution is stopped specifically by any of the three 

 possible methods— poison, manganese deficiency, or mutation— there 

 is no hydrogen evolution. No hydrogen without oxygen. It follows in 

 our opinion that this is the reaction nearest to the photochemical 

 process which gives you stable, usable products (Fig. 2). 



Thus we have reached the end of the line. This is what light is able 

 to produce. Once there is nascent hydrogen and nascent oxygen, 

 everything that follows falls under the category of enzymatic dark 

 reactions. Dr. Bishop will present these results at the Atlantic City 

 Federation meeting. 



Let us come back to the two-pigment problem and van Niel's theory. 

 The basic two-pigment skeleton of a general photo synthetic system 

 thus looks like the scheme of Fig, 3, It is of utmost importance, of 

 course, to fill in the arrows correctly with the proper enzymes, metals, 

 proton and electron transfer agents. Only then shall we know for sure 

 how the photolysis of water proceeds with two quanta. 



The water splitting is the result of the whole sequence and there may 

 be no one particular place to point to where we can say: here photo- 

 lysis happens. There are too many spots (at any one-electron transfer 

 metal catalysis, for instance) where we have to formulate the reaction 

 with the aid of water to balance the charges. 



