34 THE METABOLISM OF ALGAE 



has been found to be dependent on the concentration of 

 the undissociated form even though both ionic forms are 

 present in considerable excess^^^ and direct measurements 

 have shown that only undissociated carbon dioxide can 

 penetrate into the vacuole of Valonia coenoc}tes.^^^ How- 

 ever, the results with Chlorella cannot be considered alto- 

 gether reliable since the buffer solutions used not only- 

 varied in hydrogen ion concentration but were very alkaline 

 for physiological media, those for the lower concentrations 

 of undissociated carbon dioxide having pH values of over 

 10.^^^ In Valonia the permeability of the vacuole membrane 

 towards carbon dioxide was studied whereas for photo- 

 synthesis it is the permeability of the plasma membrane, 

 which is almost certainly different, which is of more 

 importance. 



It is frequently observed that algae can carry on rapid 

 photosynthesis in alkaline bicarbonate solutions in which 

 the concentration of undissociated carbon dioxide is ex- 

 tremely low\ This may be explained if it is assumed that 

 the bicarbonate acts indirectly by maintaining the supply 

 of undissociated carbon dioxide, the rate of diffusion of 

 which would otherwise be limiting, in the vicinity of the 

 cells, but the effect might equally well be due to the absorp- 

 tion of the bicarbonate ions themselves. Active absorption 

 of anions is a property common to all growing plant cells 

 and there seems to be no reason for supposing that bicar- 

 bonate ions are exceptional in not being absorbed by this 

 mechanism. 



Unequivocal evidence of such direct utilization of 

 bicarbonate ions is difficult to obtain since the ratio 

 [C02]/[HC03~] in solutions cannot be altered without 

 changing hydrogen ion concentration too. However, over 

 three or four pH units around neutrality photosynthesis 

 appears not to be directly affected by hydrogen ion concen- 

 tration."^®' 2^^' ^^ Within this range it should be possible to 

 obtain information regarding the absorption of these two 

 forms of carbon dioxide by study of the performance of 

 algae in solutions in which the ratio of their concentrations 

 is varied. In this way evidence has been obtained which 



