542 PHOTOCHEMISTRY OF PIGMENTS 7A^ VIVO CHAP. 19 



Fan, Stauffer, and Umbreit (1943) carried out interesting experiments 

 on the production of oxygen by Chlorella pyrenoidosa in the absence of 

 carbon dioxide, but in the presence of various organic oxidants. In 

 experiments analogous to those with ferric salts (cf. above), they obtained 

 0.05-0.10 milliliter of oxygen by the addition, to illuminated C02-free 

 suspensions of Chlorella cells (about 100 mg. dry weight), of acetaldehyde, 

 benzaldehyde, parabanic acid, and nitrourea. It will be noted that all 

 these compounds contain a carbonyl group. Similar experiments gave 

 negative results with formaldehyde, butylaldehyde, dimethylglyoxime, 

 cystine, alizarin, quinalizarin, methylene blue, urea, methylurea, cyanuric 

 acid, allantoin, uracil, xanthine, alloxan, succinate, citrate, fumarate, 

 lactate, acetate, malate, isocitrate, pyruvate, glucose, xylose, arabinose, 

 hexose diphosphate, hexose monophosphate, or phosphogluconic acid. 



Quantitative studies of the photoreduction of carbonyl compounds by 

 Chlorella cells were carried out with benzaldehyde as oxidant. To obtain 

 a maximum amount of oxygen from a given quantity of benzaldehyde, 

 it had to be added immediately after the start of illumination. It it was 

 added earlier in the dark, some benzaldehyde was used up by a dark 

 reaction. Since this nonphotochemical decomposition was accompanied 

 by the release of an equivalent quantity of carbon dioxide, if carbonate 

 was present (but not if it was absent), the authors interpreted it as a 

 dismutation of benzaldehyde (into benzoic acid and benzyl alcohol), with 

 benzoic acid liberating carbon dioxide from the carbonate. If benzalde- 

 hyde is added immediately after the beginning of illumination, and the 

 algae are young and in good condition, one molecule of oxygen can be 

 obtained from two molecules of benzaldehyde, in accordance with the 

 equation : 



(19.3) 2 CeHsCHO + 2 H2O > 2 CeHsCHaOH + O2 



If benzaldehyde is added later, after the illumination in absence of carbon 

 dioxide has lasted for one-half hour or more, the oxygen yield is smaller. 

 This could be caused by a photochemical accumulation of substances 

 capable of reducing benzaldehyde {i. e., a substitution of H2R for H2O in 

 reaction 19.3). In older cell suspensions, the quantity of liberated 

 oxygen is further reduced — apparently, by a continuation, in light, of the 

 nonphotochemical dismutation of benzaldehyde into benzyl alcohol and 

 benzoic acid. 



As always in "photosynthesis with substitute oxidants," the question 

 arises whether benzaldehyde is used as such, or is first oxidized to carbon 

 dioxide. The authors considered the latter alternative as improbable, 

 because they were unable to "catch" any carbon dioxide by alkali. 

 The amount of liberated oxygen was unaffected by the presence of 

 potassium hydroxyde. The maximum observed rate of oxygen produc- 



