THE PHOTOTROPHIC ASSIMILATION OF CARBON 23 



cause it is only possible to determine the absorption spectra 

 of individual pigments after extraction from the plant 

 whereas it is known that the absorption properties as thus 

 determined may differ appreciably from those which a pig- 

 ment possesses in the living plant. For example, the main 

 absorption band in the red of chlorophyll has a peak at 

 about 660 mju in solvents such as alcohol and acetone 

 whereas in living plants this peak is seen to be shifted by 

 about 15 m/u towards longer wavelengths. The striking 

 change in colour, from brown to green, which brown algae 

 and diatoms undergo upon killing is due to a displacement 

 of the absorption band in the green of fucoxanthin towards 

 the position in the shorter wavelengths characteristic of the 

 extracted pigment. ^^^ Such displacements of absorption 

 bands in vivo are generally ascribed to chemical combina- 

 tion of the pigments with each other and with lipide and 

 protein. Allowance can be made for such alterations in 

 absorption properties in interpreting action spectra but 

 where the absorption bands of pigments in vivo overlap, 

 and it is consequently impossible to determine the positions 

 of peaks accurately, correction must be arbitrary and there 

 is a corresponding uncertainty in the conclusions derived. 

 A further assumption that must be made in the interpreta- 

 tion of action spectra is that the pigments are uniformly 

 distributed within the chromatophores. If there is unequal 

 distribution then one pigment may screen another and 

 estimation of the partition of light absorption among the 

 different pigments cannot be made correctly from know- 

 ledge of absorption properties and relative concentrations 

 only. 



Much of the early work on the role of algal pigments in 

 photosynthesis was made using colour filters which were 

 only capable of isolating broad regions of the spectrum. 

 Such work sometimes led to conclusions which have been 

 confirmed by more critical studies but it is unnecessary to 

 discuss it in detail here (for references see 248, 107). In 

 the more modern investigations action curves for photo- 

 synthesis have been determined using narrow spectral 

 bands isolated by means of monochromators. 



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