276 RADIATION BIOLOGY 



distillation in pyrex and in tin was used. Many of the nutrient solutions 

 in different laboratories have been made from well water ; and lake water 

 has been tried. Rieke (1939, 1949) and others reported that quantum 

 requirements in photosynthesis are affected by traces of certain chemical 

 elements. Emerson and Lewis (1941, 1942) studied this problem of the 

 trace elements and reported that the trace elements can be factors in the 

 carbon dioxide burst. The excellent agreement between workers in many 

 different laboratories seems to show that the ordinary methods of cul- 

 turing now in use give about the same results and that trace elements 

 needed for maximum photosynthesis are usually present. 



Suggestions have been made that centrifuging of the algal cells in the 

 dark or in the light, methods of washing the algal cells, and the sequence 

 of exposure to bright and dim Hght are all factors in developing algae 

 that will have an optimum photosynthetic efficiency. The conditions 

 set up in one laboratory can, however, be reproduced in another labora- 

 tory. There seems to be no proof that the type of algae or the conditions 

 of culture and growth constitute an important factor in the controversy 

 between the groups of laboratories which get high efficiencies and those 

 which get low efficiencies. 



Information concerning photosynthesis can be obtained from measure- 

 ments of the photosynthetic efficiency in bacteria (Van Neil, 1941) and 

 from the measurements of oxygen liberation in certain chemical solutions 

 in which nonliving chloroplast material was illuminated. French and 

 Rabideau (1945) obtained values of 12 78 photons per molecule of oxy- 

 gen released, and Ehrmantraut and Rabinowitch (1952) obtained values 

 from 9 to 15, averaging 12. These authors consider that the quantum 

 requirement of 10-12 for this chloroplast reaction as well as for photo- 

 synthesis represents the true measure of the efficiency of the common 

 primary photochemical process. 



It is interesting that the rate of respiration can be profoundy affected 

 by the addition of certain chemicals such as sucrose, but when proper cor- 

 rections are made for respiration, the rate of photosynthesis is unchanged. 

 The over-all energy requirements for photosynthesis may be reduced by 

 adding to the algae organic compounds that can be more easily reduced 

 to carbohydrates than carbon dioxide, but attempts to find such chemi- 

 cals have been unsuccessful thus far. It is clear that such an added 

 chemical would have to penetrate the cell and be incorporated in the 

 system of photosynthetic intermediates. 



7. PRACTICAL APPLICATIONS 



A maximum continuing photosynthetic efficiency of 35 per cent, corre- 

 sponding to 8 photons of red hght per molecule, can l)c obtained under 

 laboratory conditions. It is interesting to compare this efficiency with 



