652 



N. E. Tolbert 



glycolaldehyde phosphate dehydrogenase system producing NADPH, ATP 

 and phosphoglycolate was reported in abstract (25). However, the 

 results have not been consistently reproducible. No data exist 

 which implicates xylulose diphosphate in photosynthesis. When 

 the ribulose diphosphate area from paper chromatograms of C-'^'^Oa 

 photosynthesis experiments was treated with acid phosphatases 

 free of 3-epimerase, 12 to 25% of the C-"-^ followed xylulose on 

 ion exchange borate columns (G. Orth and N. E. Tolbert, unpub- 

 lished). The formation of xylulose diphosphate during photosyn- 

 thesis needs to be confirmed by a method which will prevent epi- 

 merization of RuDP during isolation and identification. The 

 problems of phosphoglycolate synthesis, insufficiency of aldolase 

 in chloroplasts and even whether ribulose diphosphate is the only 

 precursor for COg fixation, all seem to be interrelated and cer- 

 tainly require further investigation. 



Glycolate Excretion 



By algae : When photosynthesizing Chlorella or Chlamydomonas 

 utilize C^'^Oa, nearly all of the glycol ate-C^'* produced is found 

 in the supernatant fluid as the main C^"* product therein and not 

 in the cells (5, 26, 27, 28, 29). Whittingham' s group has reported 

 excretion rates for Chlorella of 0.35 \xg glycolate per [il cells 

 per hour, and Warburg has reported 6.8 [ig per [il cells per hour. 

 Originally, we had found a maximum glycolate accumulation of 4 mg 

 per liter for actively growing cultures. Old cultures contain 

 little glycolate in the medium. Fogg (30) has extended the con- 

 cept that glycolate excretion by phytoplankton may represent a 

 reservoir of chemical energy for the plankton which is built up 

 during active photosynthesis for use at other times. 



In this work it is necessary to separate the algae from the 

 supernatant fluid by centrif ugation or by filtration on a 1 cm 

 thick celite pad. The latter process requires about 15 to 30 

 seconds. The glycolate-C^^ pool from photosynthesis was meta- 

 bolically active as judged by the fact that it had been rapidly 

 formed and was partially converted to glycine and serine. When 

 the algae were placed in the dark after 0^^*02 fixation, the gly- 

 colate-C^'^ was located inside the cells for it was not removed 

 from the algae by filtration. The cells will rapidly absorb free 

 glycolic acid at pH 3, but glycolate excretion occurs at neutral 

 pH values. Moreover, if one were to add glycolate-C''^'* externally 

 to such algae cultures in the dark or light, the rate of entry is 

 much slower than the excretion and in fact at pH 6 several hours 

 of light are required for glycolate-C^"* uptake (26, 28). Thus 

 glycolate-G^'* produced by photosynthesis and glycolate-C^^ added 

 externally are not metabolically equivalent, even though glyco- 

 late-C^'^ produced by photosynthesis could be captured outside the 



