THE PHOTOTROPHIC ASSIMILATION OF CARBON 35 



indicates that Scenedesmiis quadricauda is able to utilize 

 both undissociated carbon dioxide and bicarbonate ions, 

 the latter being more readily utilized, only 10 to 20 /^ moles 

 per litre being required for maximum growth whereas the 

 corresponding value for undissociated carbon dioxide is 

 80 1.1 moles per litre.^^^ Similar experiments with Chlorella 

 pyrenoidosa, on the other hand, show that this species is 

 not able to utilize bicarbonate and it thus appears that 

 there may be two types of algae with respect to carbon 

 dioxide absorption.^!*' 216 -Qqx^[i the Scenedesmus and the 

 Chlorella have been found to possess carbonic anhydrase 

 so that the difference between them cannot be ascribed to 

 differences in ability to convert carbon dioxide into bicar- 

 bonate.^!^ Experimental evidence confirms the expectation 

 that bicarbonate absorption is an active process of anion 

 absorption rather than one of simple diffusion and shows 

 that under certain circumstances it does not begin until 

 the alga has been activated by exposure to light for some 

 time.217 The mechanism of this induction effect is not 

 known. 



Carbonate ions evidently cannot serve directly as a source 

 of carbon dioxide and may have an inhibitory effect upon 

 gro^vth.^!^ 



By absorption of carbon dioxide algae may increase the 

 alkalinity of the medium surrounding them to the point 

 where calcium and magnesium carbonates are precipitated. 

 Many species, particularly those belonging to the Siphon- 

 ales, Rhodophyceae and Myxophyceae, are characterized 

 by the production of calcareous deposits which are often 

 incorporated in cell membranes, as for example in Coral- 

 lina.^^^' 1^^ Such calcification is generally supposed to be 

 brought about by photosynthesis, but the fact that calcareous 

 and non-calcareous species may grow side by side, appar- 

 ently in similar states of photosynthetic activity, suggests 

 that the mechanism may not be so simple.*^ 



In general the total concentration of all forms of carbon 

 dioxide in seawater is sufficient not to limit the photo- 

 synthesis of unicellular plants under conditions likely to be 

 met with in the open sea. This is shown by experiments 



