THE CHEMISTRY OF PHOTOSYNTHESIS 165 



The addition of O2 results in glutamic acid being resynthesized and photosyn- 

 thesis starting again. This is further evidence of the absolute necessity of Oo 

 in photosynthesis (see § 42). 



Under appropriate cultivation conditions, the ratio between the chloro- 

 phyll content, the Oo capacity and the CO2 removed in the dark after the 

 addition of NaF is 1:1:1. This very important observation has been con- 

 firmed by Vishniac and Fuller (2) with Chlorella, Scenedesmus and Euglena, 

 using continuous and fluctuating illumination and cultivation at pH 6.8 in- 

 stead of 3.8. After removing the COo with NaF, the algae were washed and 

 allowed to recover ^^COo in the presence of Oo. After further addition of 

 NaF the COo produced was radioactive : it was concluded that the CO2 fixed 

 during the recovery is located at the same site as the COo removed by NaF. 

 Paper chromatography showed 90% of the radioactivity in the a-carboxyl 

 group of glutamic acid. Chlorophylls a and b did not exhibit any radio- 

 activity. 



§ 62 Lactic Acid Fermentation in Chlorella 



As has been stated in § 15 and § 61, the glutamic acid content of the cells 

 depends upon the kind of cultivation. The behavior of the X-cells is particu- 

 larly atypical (§ 15). They are produced in very strongly illuminated cul- 

 tures — without day light — with four 200 watt metal filament lamps. The 

 action of light in the X-cells is at least 10 times smaller than in other types of 

 cells, probably to counteract over-illumination during cultivation. Anaero- 

 bically, the X-cells produce lactic acid in far greater amounts and at a higher 

 rate than ordinary cells (13). Indeed, it has been found that anaerobic lactic 

 acid fermentation depends upon cultivation conditions (8). The immediate 

 inhibition of photosynthesis upon withdrawal of Oo has, however, no con- 

 nection whatever with this lactic acid formation. This inhibition is solely 

 the result of the fact that induced respiration cannot occur in the absence of 



Oo. 



In 1936 Warburg and C'hristian (7) demonstrated that lactic acid fermen- 

 tation proceeds as follows (see also § 44) 



CH;rCOCOOH + DPNH + H+ ;=^ CH3CHOHCOOH -f DPN + 

 pyruvic acid lactic acid 



It should be noted that the active enzyme of Chlorella, lactic acid dehydrogen- 

 ase, is a D-enzyme and the lactic acid produced is d-( — )-lactic acid. As yet, 

 no other D-enzyme has been observed in Chlorella. Glutamic acid decar- 

 boxylase and malic acid dehydrogenase, which also occurs in Chlorella, are 

 L-enzymes. The alanine present in large amounts in the alga is L-alanine. 

 The significance of lactic acid in photosynthesis probab.'y lies in its regu- 

 lation of the pH in Chlorella to assure the optimal value for the activation of 

 glutamic acid decarboxylase. Figure 62 shows that D-lactic acid — but not 

 L-lactic acid — exerts an influence upon light absorption. Both isomers of 



