PLANKTON 



41 



the chelating or complexing agents. These are biologically inert cyclic 

 organic compounds ("inner complex salts") resulting from the attach- 

 ment of a group at two points to the same metallic atom. They prevent 

 excessive absorption and precipitation. Ethylenediamine tetraacetic acid 

 (EDTA) is the chelating agent most commonly used. When a given 

 amount is added to a nutrient medium, it will permit the release of enough 

 ions through mass action to provide for cellular needs. By forming easily 

 reversible complexes of trace elements, it provides tonic concentrations at 

 preferred levels throughout the life of a culture. 



Besides the trace elements, there are other unidentified factors some- 

 times necessary for algal growth. It is common observation that some algae 

 grow with ease in impure culture, yet do not flourish in bacteria-free 

 media. Gymnodinium cannot be continuously subcultured without an 

 organic factor extracted from soil. Similarly Cryptomonas ovata, Synura 

 ovella, and Gloeotrichia ech'mulata demand the presence of certain soil 

 factors. Also, Dity Ilium brightivelli requires, for good growth in artificial 

 seawater, two organic substances from natural seawater. One of these is 

 a specific sulfur grouping, the other a substance also found in yeast or 

 algal extracts. 40 The exact nature of these various needed supplementary 

 compounds will doubtless become clearer with more experience in algal 

 culture. 



Although mass culture of phytoplankton for food and special organic 

 material has long been the subject of considerable discussional enthusiasm, 

 little specific investigation was done until World War II. 231 Most of the 

 experiments have been performed with some species of Chlorella, Scene- 

 desmus, or Nitzschia because they grow so fast and tolerate a diversity of 

 cultural conditions. Chlorella pyrenoidosa has been an especial favorite 

 because of its previous wide use in studies on photosynthesis. As listed by 

 Burlew in "Algal Culture, from Laboratory to Pilot Plant," 41 the elements 

 of a plant for growing algae on a large scale are relatively simple: 



1 . A container with a transparent upper surface. 



2. A means of circulating the culture medium, so that the algae do 



not settle. 



3. A means of controlling temperature. 



4. A means of introducing carbon dioxide and other nutrients con- 



tinuously. 



5. A means of harvesting the algae almost continuously. 



6. A means of preserving the harvest until used. 



