22 THE METABOLISM OF ALGAE 



although, with the possible exception of zeaxanthin, the 

 xanthophylls typical of other Chlorophyceae are present, 

 two xanthophylls peculiar to this order are more abundant. 

 Only the Rhodophyceae and Myxophyceae possess phyco- 

 bilins, phycoerythrin being more characteristic of the 

 former and phycocyanin of the latter. The pigments pro- 

 duced by these two classes although similar are not identical, 

 and they are usually termed r- or c- forms according to 

 whether they occur in the Rhodophyceae or Myxophyceae 

 respectively. 



The variation to be found in the chromatophore pigments 

 of the algae stands in marked contrast to the conservatism 

 in this respect shown by higher plants. 



Evidence of the part played in photosynthesis by these 

 various pigments has been obtained principally by com- 

 parison of the absorption spectra of algae with their 'action 

 spectra' for photosynthesis.-^^ To a rough approximation 

 the spectral absorption curve of an alga, when effects due 

 to the scattering of light inevitable in a poly-phase system 

 are reduced to a minimum, is the sum of those of its indi- 

 vidual pigments. The action spectrum curve of photosyn- 

 thesis is obtained by determining the photosynthetic 

 efficiency of light, usually measured in terms of oxygen 

 production, at different wavelengths. These determinations 

 must be made at low light intensity since with light satura- 

 tion light of all wavelengths that can be utilized will produce 

 the same effect. A further point is that it is necessary to 

 make the comparisons for equal numbers of quanta not 

 for equal energy contents, since the photochemical effect 

 of a light quantum remains the same although the energy 

 per quantum varies with wavelength. If all the energy 

 absorbed by the pigments is available for photosynthesis 

 then the curves for the absorption and action spectra should 

 be super-imposable and since the different pigments have 

 absorption maxima at different wavelengths, it is sometimes 

 possible, by comparing the two curves, to determine 

 whether or not light absorbed by a particular pigment is 

 being utilized in photosynthesis or not (see Figs. 6 and 7). 

 Such comparison, however, is not straightforward be- 



