438 



H. Gaffron, W. Wiessner and P. Homann 



acetate assimilation. Light at X 650 mfx alone showed the same 

 inefficiency betause the pigment which works mainly in the 

 infrared (and presumably is needed for acetate assimilation) 

 receives little energy. With both lights together the rate of 

 acetate assimilation is drastically enhanced. (At high intensities 

 of visible light we may assume a sufficient energy transfer so 

 as to produce some deviations from the behavior just described.) 



HYPOTHESIS 



The simplest interpretation of the phenomena described above 

 would be to say that the pigment absorbing at X 700 - 740 m\x 

 specializes in forming ultimately ATP through cyclic phosphory- 

 lation, and that it acts as pigment system I when coupled to 

 activated pigment system II. (See the articles by Witt, Kok, 

 Duysens in this book. ) No theoretical picture suits the photo- 

 metabolism of acetate in Chlamydobotrys better than that of two 

 pigments, presumably two chlorophylls, which specialize in 

 different photochemical reactions. The photosynthetic green 

 algae rely mainly on that one which is indispensable for oxygen 

 evolution. The acetate assimilating algae, however, depend 

 under aerobic conditions mainly on an infrared absorbing chloro- 

 phyll which specializes in the formation of ATP. 



But if the acetate assimilation has to proceed under anaerobic 

 conditions the rate limiting factor for this metabolic process is 

 its very low capacity for producing oxidative power at wavelengths 

 beyond X 700 m^. where as we assume ATP formation is the main 

 consequence of the primary light reaction. Correspondingly the 

 weak ATP formation at shorter wavelengths limits the rate of 

 acetate assimilation even under aerobic conditions. It follows 

 that illumination with light in the visible region, together with 

 that beyond X 700 m|j., is likely to promote the most efficient 

 assimilation of acetate. That red light beyond X 700 m\x supports 

 ATP formation in chloroplasts better than oxygen production has 

 already been shown by Hoch and quite recently by Arnon. 



The strength of the coupling between systems I and II may 

 vary from alga to alga according to whether they are in the habit 

 of assimilating mainly carbon dioxide, or acetate, or able to 

 switch reversibly between photosynthesis and photo reduction. A 

 drop in overall efficiency can be expected whenever the specific 

 metabolic processes connected to both systems are not properly 

 adjusted to one another either in respect to reaction velocities 

 or to the nature of the products. 



