VAN NIEL'S THEORY: THIRTY YEARS AFTER 9 



be approached by caHing attention to the differences. Which, and how 

 important, are they? 



Is the color difference important? Probably not, because the green 

 bacteria use quanta of very nearly the same energy level as the green 

 plants, yet do not evolve oxygen. 



Are the enzymes important that permit the purple and green bac- 

 teria to use inorganic or organic hydrogen donors for their photo- 

 synthesis? Again I believe the answer is no, because we have now a 

 dozen or more typical oxygen-releasing algae which can skip the oxygen 

 part and do a bacterial photo reduction with hydrogen just like obligate 

 anaerobes. Furthermore, Pringsheim andWiessner discovered a green 

 flagellate that, at best, shows only a marginal capacity for normal 

 photosynthesis. Like a purple bacterium, it cannot grow at all in the 

 light when presented only with carbon dioxide and water. It cannot 

 even grow like Rho do spirillum with acetate and oxygen in the dark. 

 It grows exclusively with acetate in the light. 



In short, the more we look around the more we find gradual transi- 

 tions between the photochemical capacities of a typical purple bac- 

 terium and that of the most specialized oxygen- evolving green plants. 



The only clear-cut difference I know, in respect to the ability to 

 release oxygen, is the need for manganese first shown by Kessler in 

 our laboratory, and confirmed for the Hill reaction by Eyster in this 

 laboratory, Wiessner later found that purple bacteria thrive on a low 

 manganese diet which forces the algae to turn heterotroph for dear 

 life. Then there is a difference in the quinones which might be im- 

 portant. Finally the absence of an Emerson effect in green algae after 

 adaptation to hydrogen, a condition that parallels again, as I have just 

 learned, the behavior of purple bacteria. In these van Niel and Blinks 

 could find no Emerson effect. 



The spectral investigations of Duysens, Kok, Rabinowitch; the gas 

 exchange measurements by French, Myers, Govindjee; and whoever 

 may by now have acquired the proper mo nochromators have established 

 that in photosynthesis two or more pigments which are activated by 

 distinctly different wavelengths have to cooperate. The products of 

 the two separate photochemical processes are chemical substances, 

 not physical states, because, according to Myers and French, they live 

 for seconds. And they both must be present in order that oxygen can 

 appear, and without oxygen evolution there is no reduction under 

 anaerobic conditions. 



The two-pigment problem has now merged with the two-quanta- 

 per-hydrogen problem, first clearly enunciated by Franck and Herzf eld 

 many years ago. Hill, Kok, Duysens and Witt, all independently, gave 

 reasons why one part of the tandem pigment should be assigned to the 

 reduction, the other to the oxidation side. A year ago Dr. Franck paid 

 us a visit. Dr, Clayton happened to be there too. Naturally we discussed 

 the Emerson effect, about which Franck had published some very in- 



