58 THE METABOLISM OF ALGAE 



photosynthesis some other factor than carbon assimilation 

 limits the rate of growth. 



It is possible that light may have effects on growth other 

 than those arising from photosynthesis. If Chlorella vulgaris 

 is grown in the presence of glucose it is found that a weak 

 light intensity produces a considerable increase in the rate 

 of growth in the exponential phase as compared with that 

 in darkness, but that further increase in light intensity pro- 

 duces comparatively little effect even though carbon dioxide 

 is not in short supply.^^^ These effects may be indirectly 

 due to photosynthesis since in cultures of the type used to 

 obtain these results oxygen is limiting and the amount of 

 growth that can take place in older cultures is dependent 

 on the oxygen evolved in photosynthesis. In the exponential 

 phase the medium initially contains oxygen and it may be 

 that the small amounts produced by photosynthesis at the 

 lowest light intensities are sufficient to supplement this to 

 a level at which the maximum growth rate can be main- 

 tained. ^^^ However, the effect of light on the growth rate in 

 the exponential phase may equally well be explained in 

 other ways. For example, a substance which in small 

 amounts stimulates growth may be produced by a photo- 

 chemical reaction other than photosynthesis. Evidence for 

 such an effect may be obtained from studies of the effect 

 of light upon the growth of cultures in the absence of 

 carbon dioxide. For Chlamydomonas pseudococcum and 

 Hormidium nitens it has been found that cultures aerated 

 with carbon dioxide-free air grow better in the light than 

 in complete darkness. ^^^ However, really vigorous aeration 

 is needed to remove from a culture the carbon dioxide pro- 

 duced by respiration and it seems probable that in these 

 experiments a small amount of photosynthesis occurred 

 and that this might account for the better growth of the 

 light cultures. 2^^ Experiments with Chlorella pyrenoidosa 

 grown in cultures vigorously aerated with carbon dioxide- 

 free air have failed to show any stimulatory effect of light 

 on the rate of growth with glucose or acetate as substrate. 

 On the contrary, light retards the growth of Chlorella on 

 acetate in the absence of carbon dioxide.^^^ Thus there 



