30 



PHOTOCHEMICAL PRINCIPLES 



spectrum that are more effective than the reference beam they swim 

 to the spectrum side. At each wavelength there is an intensity at which 

 that wavelength is just as effective as the reference beam. Below that 

 point the algae all swim to the spectrum side; above it they swim to 

 the reference beam. By exposing a suspension of algae to these op- 

 posing beams for a few minutes a complete set of response data for 

 an action spectrum can be obtained. The intensity and wavelengths 

 had previously been measured for the whole surface of the projected 

 spectrum. All that remains to make the resulting automatic plot truly 

 quantitative is to take a picture of the response pattern, or to mark the 

 dividing Hne on the front of the vessel, and carry through the calcula- 

 tions. 



500 too 



WJVIIIHCI" I" "I 



Fig. 9. Phototaxis action spectra. 



Figure 9 for the phototaxis action spectra of some motile algae so 

 measured looks very much like most phototaxis and phototropism 

 action spectra. Their shape could very well be attributed to light 

 absorption by carotenoids with some lowering in the blue through 

 internal screening by chlorophyll. In Fig. 10, however, the action 

 spectrum for phototaxis of Prorocentrum shows a narrow peak at 570. 

 This is a highly unusual discovery. The absorption curve for this 

 organism does not show anything particular at this wavelength, and 

 the nature of the pigment responsible for phototaxis in this organism 

 is not yet known. The fact that the absorbing substance for the action 

 in this species cannot be found in the absorption spectrum shows the 



