125 



J. L. Rosenberg and Tevfik Bigat 



an oxidized direct or indirect product of Step (l) and is reduced 

 photo chemically "by the fluorescent state of System 2. We base 

 our interpretation on the model of Franck and Rosenberg, ^ 5j in 

 ■which the non- fluorescent System 1 efficiently sensitizes only 

 Step (l), as in Duysens ' scheme^ but in which System 2 may sensi- 

 tize either Step (l) or Step (2), Step (l) by way of a meta- 

 stable state and accompanying fluorescence and Step (2) by way of 

 the excited singlet state without fluorescence. With this dif- 

 ference we propose the following correlations with experiment: 



(a) The action spectrxim determined by Duysens for photo- 

 synthesis against a background of strong green light, \1) similar 

 to the action spectriom for cytochrome oxidation in the presence 

 of DCMQ, (^/ shows equal effectiveness of chlorophyll a and 

 phycoerythrin . We assign this as the action spectrum for Step 

 (l), not System 1, and expect it to be a combined action spectrxom 

 for System 1 plus System 2. 



(b) System 2 is capable of efficient photosynthesis by itself., 

 This correlates with the observed proportionality between the 

 action spectra for photosynthesis and for chlorophyll a fluor- 

 escence. (5 j Although good absolute quantxam yield data may not 

 exist showing a minimum quantum requirement of 8 for red algal 

 photosynthesis, there certainly is good evidence with Chi or ell a 

 and with brown and blue algae that the minimum quantum require- 

 ment may be achieved with monochromatic light over a wide wave- 

 length region. (d) If the same distribution of function between 

 two pigment systems exists in all the algae, this fact could be 

 explained only if one of the systems may sensitize both photo- 

 chemical steps. 



(c) A long-wave absorbing pigment, like P7OO, cannot act as a 

 sink for singlet excitation energy during normal photosynthesis 

 lander illumination absorbed by System 2, even when System 2 is 

 being used for Step (l). If we ass-ume that P7OO removes excita- 

 tion in connection with Step (l), then the observed chlorophyll a 

 fluorescence would have to arise by competition with Step (2). 

 This is in contradiction to the finding in our experiments and in 

 those of others that the fluorescence from System 2 is depressed 

 particularly when this system is working with high efficiency in 

 sensitizing Step (2), namely^ when simultaneous blue irradiation 

 provides adequate Q substrate. An attempt to retain the assump- 

 tion of trapping by P7OO by allowing a small fluorescence leak 

 during the flow of excitation from System 2 to P7OO would not 

 work, because a small fluorescence leak of chlorophyll a is 

 always accompanied by a many- fold greater leak into the 



