146 



ANAEROBICALLY ADAPTED ALGAE 



CHAP. 6 



to use cellular oxidants, R, as hydrogen acceptors in photosynthesis 

 instead of carbon dioxide. However, it was mentioned above that this 

 reaction can be eliminated if one assumes that the hydrogen absorbed 

 in light is conveyed to the fermentation carbon dioxide rather than to R. 

 (VI) and (VII). Photoreduction by Adapted Algae (Algae as Photo- 

 synthesizing Bacteria). — Hydrogen-adapted algae can reduce carbon 

 dioxide either at the cost of molecular hydrogen or of organic hydrogen 

 donors, the latter taking precedence as long as they are available. When 

 adapted algae are illuminated in presence of carbon dioxide, the photo- 

 chemical consumption of hydrogen sets in only after a certain delay, 



10 



I 20 



30 - 



40 



5 10 15 



Time, minutes 



Fig. 15. — ^Time course of photoreduction by adapted Scenedesmus D3 as a function 

 of the partial pressure of hydrogen (after Gaffron 1942). 



Culture medium. Adaptation 16 hours in hydrogen; 4% carbon dioxide. All 

 mixtures of hydrogen and nitrogen contained 4% carbon dioxide. D — 96% hydrogen; 

 0—22% hydrogen; A— 8% hydrogen; • — 4% hydrogen. 



during which the cellular hydrogen donors are used up. In agreement 

 with this explanation, the length of the "induction period" decreases 

 with increased light intensity (while in ordinary photosynthesis the 

 length of the induction period is independent of light intensity). The 

 induction effect is illustrated by the curves for 200 and 560 lux in figure 

 14; the curves for 1020 and 6000 lux show, in addition to induction, the 

 de-adaptation by excessive light. 



The "photosynthetic quotient," AH2/ACO2, of the hydrogen-adapted 

 algae was found by Gaffron (1940^) to be 1.97 (average of five measure- 

 ments with Scenedesmus D3, and seven measurements with Scenedesmus 

 obliquus, individual values varying between 1.84 and 2.17). This result 

 can be compared with the corresponding quotients found for purple 

 hydrogen bacteria and assembled in table 5.IIL In that table, some 



