114 THE METABOLISM OF ALGAE 



but trace elements such as iron may be precipitated in an 

 unavailable form in alkaline media and frequently it is short- 

 age of this element which brings the growth of algae in 

 culture to a standstill. If precautions are taken to maintain 

 trace elements in an available form by means of complex- 

 forming agents, much higher population densities may be 

 achieved in culture.^^* In species with unrecognized re- 

 quirements for trace elements or organic growth factors 

 growth may cease after a short time as the traces of the par- 

 ticular substance concerned introduced with the inoculum 

 or by contamination become exhausted. (2) Preferential ab- 

 sorption of a particular ion may result in the reaction of the 

 medium becoming inimical to growth. Effects of this sort 

 produced by ammonium nitrogen have already been men- 

 tioned, another example is that of Chilomonas, which by 

 absorption of weak organic acids without equivalent intake 

 of cations causes the medium in which it is placed to become 

 too alkaline to support growth. ^^' (3) Accumulation of 

 growth inhibiting products of metabolism; autoinhibition 

 of growth by metabolic products accumulating in the me- 

 dium has been demonstrated for Nostoc punctiforme}^'^ a 

 strain of Chlorella 'vidgaris^'^^ and Nitzschia palea.^^ The 

 autoinhibitor of Chlorella, 'chlorellin', may be a product of 

 the photo-oxidation of fatty acids but rigorous proof of 

 this is lacking.^®^ Those of Nostoc and Nitzschia do not 

 appear to have been studied in detail. (4) The increasing 

 density of the culture may reduce the penetration of light 

 into the bulk of the medium so that photosynthesis be- 

 comes insufficient to maintain exponential growth. 



In natural environments interactions between different 

 species almost invariably occur and it is rarely possible to 

 ascribe the cessation of growth of a particular alga to the 

 operation of any single physico-chemical factor. However, 

 in an example which has been examined with particular 

 thoroughness, that of Asterioiiellu forniosa in the .Engli.sh. 

 lakes, it appears that it is depletion of silica in the water 

 which usually limits growth in the spring.^^^ 



When exponential growth ceases the course of metabolism 

 of an alga alters considerably. An example which may be 



