THE NUTRITIONAL REQUIREMENTS OF BACTERIA 61 



The Nutritional Requirements of Bacteria 



Some bacterial species cau live and multiply when provided only with very 

 simple food materials ; others require far more complex substances. Orla-Jensen 

 (1909) distinguished three main bacterial groups on the basis of their food require- 

 ments. The groups are : 



(1) Bacteria, obtaining both their carbon and nitrogen from inorganic sources, 

 such as CO2, CH4, NH3 or atmospheric nitrogen itself. 



(2) Bacteria obtaining nitrogen from inorganic sources but requiring organic 

 substances as a source of carbon. 



(3) Bacteria demanding organic substances as sources for both carbon and 

 nitrogen. 



The first group consists of autotrophic, the second and third groups of hetero- 

 trophic bacteria. The limitations of this grouping have been discussed at some 

 length by Knight (1936), who points out that the difference between autotrophs 

 and heterotrophs is quantitative, the autotrophs using the far more costly means 

 of obtaining their energy ; but that the quantitative difference is so large that 

 it amounts to a qualitative difference. This we express in terms of the character- 

 istic methods whereby energy is obtained for carbon assimilation ; namely, in the 

 case of autotrophs by the use of radiant energy or the oxidation of inorganic com- 

 pounds, in the case of heterotrophs by use of carbon compounds already partly 

 synthesized on the paths leading to protoplasm. Knight's monograph, which 

 has been drawn on freely in this and the following sections, should be consulted 

 for details of the argument. Like those who have previously attempted a classifi- 

 cation of bacteria upon nutritional grounds, he arranges bacterial species in classes 

 corresponding to increasing complexity of nutritional requirements, but maintains 

 that these are to be regarded only as stages, which merge into one another ; there 

 are, for example, heterotrophic bacteria that with training can adapt themselves 

 to an autotrophic existence. We shall proceed to discuss the different nutritional 

 types under the headings of Knight's four stages. 



Stage 1.- — Carbon is assimilated as CO^, and N 2 from inorganic sources, especially 

 ammonia. The energy required for this assimilation {the reduction of the CO^ and 

 the sijnthesis of protoplasm) is derived from the oxidation of simple inorganic com- 

 pounds, or from the use of radiant and chemiccd energy. 



The classical studies of autotrophs in Stage 1 have been made upon soil organ- 

 isms, especially those concerned in the nitrogen -cycle. The nitrosifying bacteria 

 of the soil obtain energy by the oxidation of ammonia to nitrites, the nitrifying 

 bacteria by the oxidation of nitrites to nitrates. The relation between the ammonia 

 or nitrite oxidized, and the CO2 assimilated, is a quantitative one (Winogradsky 

 1890a, b, c). Among the substances oxidized by autotrophs are molecular hydrogen 

 (Kaserer 1905, 1906, Niklewski 1908) and sulphur and sulphur compounds 

 (Winogradsky 1889). A special class of bacteria obtains energy by a photosynthetic 

 mechanism depending on the action of light on a cell pigment, and in this respect 

 forms a link with the blue-green algae and the chlorophyll-containing higher plants. 

 These photosynthesizing species include the purple and green sulphur bacteria 

 (see van Niel 1931, 1933, 1944, MuUer 1933, Eoslofson 1934, 1935, and Gaffron 

 1934, 1935, 1944). The purple sulphur bacteria contain a red and a green pigment, 

 the green alone being concerned in photosynthesis ; HjS is oxidized to sulphur, 



