46 THE METABOLISM OF ALGAE , 



ticular consideration because it is important whenever it is 

 necessary to measure the true rate of photosynthesis. There 

 is a certain amount of indirect evidence to show that the 

 view that is commonly held, i.e. that respiration continues 

 at the same rate during photosynthesis as under otherwise 

 comparable conditions in the dark, is correct. Thus, light 

 itself, in the absence of carbon dioxide, has no effect on the 

 rate of respiration of Chlorella.^'^ More direct evidence has 

 been obtained recently in experiments in which oxygen up- 

 take from air containing the heavy isotope of oxygen (O^^) 

 has been followed while photosynthesis using normal water 

 (HgO^^) was taking place.^^^ In Chlorella it was found that 

 respiration continued at the same rate in the dark and during 

 photosynthesis, the labelled oxygen being taken up at the 

 same rate regardless of whether or not oxygen was being 

 produced by photosynthesis at the same time. However, 

 with other algae low light intensities such as produce rates 

 of photosynthesis only a few times as great as that of 

 respiration were found sufficient to inhibit uptake of oxygen 

 from the air to a considerable extent. In Anabaena sp. 

 complete inhibition was observed. ^^^ Such inhibition may, 

 however, be only apparent since the oxygen produced in 

 photosynthesis may be used preferentially for respiration 

 and whether or not this occurs might well depend on 

 minor differences in cell or protoplasmic structure. 



The results of studies with radioactive carbon dioxide 

 have also been interpreted as showing that respiration is 

 inhibited by photosynthesis. In Chlorella exposed in the 

 dark to radioactive carbon dioxide, iso-citric and glutamic 

 acids, among substances participating in or closely con- 

 nected with the tricarboxylic acid cycle, acquire high 

 proportions of tracer but even after long periods of photo- 

 synthesis in the presence of radioactive carbon dioxide 

 scarcely any radioactivity can be detected in these com- 

 pounds.^^' ^- If it is assumed that respiration in Chlorella 

 occurs through the tricarboxylic acid cycle, this suggests 

 that respiration is suppressed by photosynthesis in this 

 organism. However, it must be remembered that other 

 processes than respiration and photosynthesis occur in 



