385 



Max H. Homme r sand 



Threshold light intensities for carbon dioxide fixation . 



In general, the photo synthetic rate measured on an oxygen 

 electrode is linear as a function of light intensity over a wide 

 range of intensities. Non-linearities have been observed with 

 the oxygen electrode in Chlorella in the region around 700mp by 

 Myers and Graham ('^) . In our strain of Chlorella , oxygen evolution 

 appears to be linear as a function of intensity in the far-red 

 region and does not show any threshold at low light intensities 

 (fig. 4). 



An examination of the rate of carbon dioxide fixation at wave- 

 lengths longer than 710mp using labeled bicarbonate showed that 

 while CO2 uptake is linear as a function of light intensity over 

 most of the intensity range, it has a distinct threshold at 

 moderately low light intensities (figs. 3 & 4) , If one uses 

 either younger cells, or successively longer wavelengths of far- 

 red light, this intensity threshold rises rapidly to a point 

 where very bright light is needed to effect a light promoted 

 fixation. We have chromatographed the products of C-'-402 fixation 

 at different light intensities given for different lengths of 

 time, and have found that at intensities at or below the thres- 

 hold the distribution pattern of the label is the same as in dark- 

 ness. At intensities only 10-15% greater than the threshold 

 intensity a labeling pattern characteristic of CO2 fixation in the 

 light, in which sugar mono- and di-phosphates, alanine, serine, 

 glycine, and sucrose are rapidly labeled, is discernible. 



Effects of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on 

 oxygen evolution . 



As can be seen in figure 5, CCCP does not have any significant 

 inhibitory effect on the steady-state, in-vivo photosynthesis 

 rate in C hlorella cells over a wide range of concentrations 

 (5 X 10 "^M to Id^M) in which CCCP stimulates respiration several 

 fold, presumably by uncoupling oxidative phosphorylation. CCCP 

 does, however, induce a lag in the onset of oxygen evolution, the 

 length of which varies depending on light intensity, light 

 quality, and on the concentration of CCCP (figs. 5 & 6) . The 

 lag observed in the induction of oxygen evolution is significantly 

 more sensitive to CCCP for photo synthetic rates above compensa- 

 tion, than it is for rates below compensation. At high CCCP 

 concentrations (5 x 10-5M) and low light intensities (less than 

 1000 ft. candles) even the onset of photosynthesis below the 

 compensation level may be affected. By using various combinations 

 of colored filters, and by adjusting the relative light intensity 



