Nutrition 115 



they require particulate food, dead or alive. The carnivores Stylonychia 

 p^lst^data and Pleurotricha lanceolata, for example, need live Protozoa as food, 

 in addition to some vitamins. ^'^ Amoeba proteiis grows well on Chilomonas 

 hut not on Paraviecium as food, whereas Amoeba dubia seems to need chloro- 

 phytes. ^'^^ Whether the need for live food represents need for particular 

 compounds as carbon source or for some special growth substances is not 

 known. Holotrichous ciliates of the Colpidium-Tetrahymena group can ob- 

 tain their food from solution. Tetrahymena geleii can use some sugars and is 

 stimulated by acetate, whereas some fatty acids are toxic; it can ferment dex- 

 trose, levulose, mannose, and maltose, but it cannot ferment some thirteen 

 other sugars and cannot use sucrose. *^" 



The salts of fatty acids, together with necessary amino acids, can supply 

 some of the carbon needs of multicellular animals. Certainly the monosaccha- 

 ride sugars are adequate, and numerous adult insects live exclusively on a 

 diet of sugars. Adult honeybees can survive on any of seven sugars which 

 are sweet to the bee and six which are tasteless, but they cannot use five others 

 which are tasteless. ^^^ Blowflies can live on a-glucosides and a-galactosides, 

 but not on other glycosides. '^^ The waxmoth larva (Galleria) has no need for 

 carbohydrate but can grow and metamorphose on a diet of beeswax. Fatty 

 acids are formed by the action of the symbiotic bacteria and protozoans in 

 termites and ruminants; these fatty acids are absorbed and used as a principal 

 carbon source (see Chapter 6). Acetate can be substituted for part of the 

 carbohydrates in the diet of a rat. It is difficult to see how an organism which 

 obtains its energy by oxidation via the Krebs tricarboxylic acid cycle (see Fig. 

 63, Ch. 8) could survive long without a carbohydrate source of pyruvic acid. 

 However, this minimal carbohydrate can be formed from protein, and rats can 

 survive on a diet of protein and minerals only. Acetone bodies, however, are 

 formed when protein is substituted for carbohydrate. Fatty acids can serve as 

 sole carbon source for some leucophytes, but in all other animals fatty acids 

 are used only along with sugars, although protein can be substituted as a 

 sugar source. Caloric requirements of animals will be considered in Chapter 8. 



NITROGEN REQUIREMENTS 



There is a general relation between utilization of carbon and nitrogen com- 

 pounds, as shown in Table 19. Any organism which can use an inorganic 

 source of nitrogen to build its own amino acids can also use the more complex 

 nitrogen compounds. The simplest usable nitrogen sources have been exam- 

 ined for several phototrophic and heterotrophic flagellates.-"- ^''' Nitrates 

 can be used by Euglena gracilis, E. stellata (if much Ca is present), and E. 

 klebsii, whereas ammonia but not nitrate can be used by E. anabaena. Euglena 

 deses requires amino acids, and E. pisciformis is said to grow only if nitrogen 

 is supplied in the form of peptones or polypeptides. It is not known whether 

 this need of E. pisciformis is for some growth substance in the peptones or for 

 a particular combination of amino acids. Some microorganisms require specific 

 polypeptides (e.g., strepogenin). Growth of several species of Euglena in dif- 

 ferent single amino acids is shown in Table 20, and it appears that some amino 

 acids, such as tyrosine and tryptophane, are not adequate nitrogen sources 

 even for species that can use ammonia or nitrate. It would be of interest to 

 test these mesotrophic and metatrophic species of Euglena in amino acid mix- 



