1180 THE LIGTTT FAf'TOU. Til. f'OLOU CHAP. 30 



trated by fig. 22.485). The main remaining differences between the "cell 

 spectrum" and the combined extract spectrum is the apparent broadening of 

 the red chlorophyll band in the intact cells, and a somewhat lower absorp- 

 tion of the latter at X < 510 mju. 



Chroococcus cells were used in carbonate buffer (85% 0.1 Af NaHCOs + 

 15% 0.1 Af Na2C03). These algae can live without potassium, but not 

 without sodium. The method of determination of y was the same as in 

 the work with Chlorclla. (Bands 6-10 mn ^\'ide were used in the rod and 

 15-20 m/x wide in the blue-violet; photosynthesis and respiration were 

 measured in alternating 10 minute periods of darkness and light, the value 

 of P hemg derived from the rate of oxygen production in the second half 

 of the illumination period.) Both "dense" (fully absorbing) and "thin" 

 (partially absorbing) suspensions were used. 



The quantum yield of photosynthesis in Chroococcus as a function of 

 wave length is shown in figure 30.10A. As in Chlorella, y is approximately 

 constant between 570 and 690 nifi (aside from a slight flat maximum at 680 

 m/x) — despite the fact that in Chlorella all absorption in this region is due to 

 chlorophyll, whereas, in Chroococcus, more than half the total absorption 

 in the region between 560 and 650 m^i must be attributed to phycocyanin. 

 Judging from figure 22.49, the absorption by phycocyanin at 600 m/u 

 should be at least six times as large as that by chlorophyll; but the quan- 

 tum yield is the same as at 660-680 m/x, where chlorophyll accounts for 

 practically all absorption. Thus, the photosynthetic efficiency of phyco- 

 cyanin in Chroococcus must equal that of chlorophyll (the maximum 

 possible difference being of the order of 10-15%). Another way of repre- 

 senting the same results is shown in figure 30.10B. Here, the absorption 

 spectrum of a thin suspension of Chroococcus cells is compared with the 

 quantized action spectrum of photosynthesis. The close parallelism be- 

 tween the two curves in the region above 570 mju shows the approximately 

 equal availability for photosynthesis of the light absorbed by both chloro- 

 phyll and phycocyanin. Particularly convincing are the two separate 

 maxima shown by both curves near 620 and 670 m^, which must be at- 

 tributed to phycocyanin and chlorophjdl, respectively. The large dis- 

 crepancy in the region 420-550 m/x indicates the inefficiency (or relatively 

 low efficiency) of the light absorbed by the carotenoids. The dotted line in 

 figure 30.10A shows, however, that assuming complete inefficiency of 

 the carotenoids leads to an underestimation of the yield between 450 and 

 550 m/x; the best agreement between measured and calculated yields can 

 be obtained by assuming a quantum yield y = 0.08 for the light absorbed 

 by chlorophyll and phycocyanin, and y = 0.016 for the light absorbed by 

 the carotenoids. 



The action spectra of several species of red algae were studied by Haxo 



