50 THE METABOLISM OF ALGAE 



in photosynthetic organisms operates or how the energy- 

 yielding reactions are linked with the reduction of carbon 

 dioxide. ^'^^ 



FACULTATIVE CHEMO-ORGANOTROPHY 



Many algae possessing photosynthetic pigments are able 

 to utilize preformed organic substances and are thus enabled 

 to grow in the dark or in the absence of carbon dioxide. 

 These are facultative chemo-organotrophs. Other algae, 

 having no photosynthetic pigments, are obligate chemo- 

 organotrophs. It may be noted that, although most experi- 

 mental work has been carried out on the assimilation of 

 organic substances from solution, certain forms included 

 in the algae can assimilate particulate food, i.e. show 

 holozoic nutrition. ^^^ Sometimes, as in Ochromojias^ capaci- 

 ties for photosynthesis and holozoic nutrition are present 

 in the same organism. ^*^" 



There have been many investigations in which the effects 

 of organic substances on the final amount of growth 

 achieved by algae in culture have been recorded in qualita- 

 tive or semi-quantitative terms (a historical review of early 

 work is given in ref. 258). Such work has established that 

 many algae possessing photosynthetic pigments are able 

 to grow in darkness upon substrates such as sugars, 

 alcohols, organic acids, amino-acids, peptones and pro- 

 teins. These same substances also generally stimulate the 

 growth of the algae in the light. The biological advantage 

 of this ability to make use of organic substances to algae 

 inhabiting soils and polluted waters and to symbiotic forms 

 need not be emphasized. So far, investigations of the 

 assimilation of organic compounds by algae have been con- 

 fined to those forms which are easily obtainable in pure 

 culture and very little information is available about chemo- 

 organotrophy in the larger marine algae although it has been 

 shown that numerous organic substances, especially fatty 

 acids and amino-acids, can serve as substrates for the 

 respiration of several species of Chlorophyceae, Phaeo- 

 phyceae and Rhodophyceae.^^^ Ulva lactuca, however, does 

 not appear capable of chemotrophic growth.^"^^ 



