ROBERT L. STARKEY 323 



Obligate autotrophic bacteria and facultative forms existing under autotrophic 

 conditions show certain distinctive physiological characteristics: 



1. They thrive on strongly elective, purely mineral media containing the specific in- 

 organic oxidizable substances. 



2. Their existence is dependent upon these substances which are oxidized in the per- 

 formance of the life-processes of the organisms. 



3. These oxidation processes furnish the only source of energy for the organisms. 



4. They require no organic nutrients as sources of energy. 



5. They use carbon dioxide (dissolved) as their exclusive source of carbon. This is re- 

 duced by means of the energy obtained from the oxidation of the inorganic foods.' 



The number of known organisms which live as obligate autotrophs is quite small. 

 Included in this group are nitrifying bacteria and some of the sulphur bacteria and 

 iron bacteria. There appears to be a greater abundance of forms which can live as 

 facultative autotrophs. These may either derive their energy from the oxidation of 

 inorganic substances and reduce carbon dioxide for synthesizing their organic struc- 

 tures or they may derive their energy, like the heterotrophs, from purely organic 

 substances. The facultative autotrophs have representatives in the groups of sulphur 

 bacteria, iron bacteria, and hydrogen bacteria. The transformations produced by 

 autotrophic bacteria may be represented by the following reaction: 



IL NH4++iiO. = NO.-+H,0+2H+(io-8) 



AF,98= —66,500 calories 



The oxidation of i gram molecule or 18 grams of ionized ammonia to ionized nitrous 

 acid results in the liberation of 66,500 calories. 



This group of organisms, called "autotrophic bacteria" (more accurately, auto- 

 trophic micro-organisms lacking chlorophyll), is composed of forms varying greatly 

 in morphological appearance as well as cultural habits. Some are minute single cells 

 identical in appearance with the Eubacteriales. Some are large multicellular fila- 

 mentous forms many micra in diameter. Some appear to be closely related to the 

 algae in morphology (shape and complex mode of division) and may even contain 

 pigments (as the bacteriopurpurin of the purple sulphur bacteria) which function in 

 photosynthetic processes. These organisms compose a physiological group and are 

 not distinctive morphologically from other bacteria. The known autotrophic bacteria 

 may be classified as follows: 



A. Bacteria which oxidize compounds of nitrogen 



a) Oxidize ammonia to nitrite (Nitrosomonas, Nitrosococcus) 



b) Oxidize nitrite to nitrate {Nitrohacter) 



B. Bacteria which oxidize sulphur or compounds of sulphur 

 a) Simple bacteria (genus Thiohacillus) 



I. Strictly autotrophic 

 {a) Aerobic 



(i) Develop at reactions close to neutrality (species Th. thiopanis Beijerinck) 

 (2) Develop under very acid conditions (species Th. ihiooxidans Waksman 

 and Joffe) 

 {b) Anaerobic (species Th. dcnitrificans Beijerinck) 



• Winogradsky, S.: Centralbl.f. Baktcriol. Abt. II, 57, 1-21. 1922. 



