112 PHOTO- AND CHEMOSYNTHESIS OF BACTERIA CHAP. 5 



analogous to the colored sulfur bacteria. Other species require the 

 presence of simple organic compounds, as methane, carbon monoxide, 

 formate, or methanol. They have thus the appearance of hetero- 

 trophants. However, evidence speaks in favor of assigning to the 

 organic substrates of these bacteria, the function of fuels rather than of 

 nutrients. There seems to be an analogy between the colorless bacteria 

 which use these substrates, and the purple bacteria which require organic 

 hydrogen donors for the reduction of carbon dioxide, except that, in the 

 case of the colorless bacteria, the organic substrate has to supply not 

 only hydrogen but also chemical energy. As in the case of the purple 

 Athiorhodaceae, conditions become complicated when colorless bacteria 

 use comparatively complex organic substrates, instead of methane or 

 similar "Ci compounds"; in this case, heterotrophic nutrition can be 

 superimposed upon the chemosynthesis. 



1. Types of Chemautotrophic Bacteria 



The main representatives of this class are the nitrifiers, the (colorless) 

 sulfur bacteria, the iron bacteria, the hydrogen (or "Knallgas") bacteria, 

 the carbon monoxide, methane and carbon bacteria. This list shows that 

 some kind of chemautotrophic bacteria has become associated with prac- 

 tically every oxidizable inorganic compound found on the surface of the 

 earth— ammonia, hydrogen sulfide, sulfur, ferrous iron, methane and 

 coal. Hydrogen and carbon monoxide are not found in natural habitats 

 of the bacteria. These gases are facultative components of the metabo- 

 lism of bacteria, whose mode of life under natural conditions is hetero- 

 trophic. Thiosulfate has occasionally been found in black mud, but 

 most thiosulfate bacteria also can live on organic substrates (that is, 

 they, too, are only facultative autotrophants). 



The following short description of the chemical activity of the chem- 

 autotrophic bacteria is based mainly on the review by Stephenson (1939), 

 although the thermochemical figures have been revised on the basis of 

 compilations by Kharash, Bichowski and Rossini, and Roth (c/., bibli- 

 ography to Chapter 3), and the over-all equations have been simplified 

 by the consequent use of ionic formulations. All equations are reduced 

 to the consumption of one mole of oxygen, to facilitate comparison with 

 the equation of carbon dioxide reduction, in which one mole of oxygen 

 is produced for each gram atom of assimilated carbon. 



(a) The Nitrifiers (Vinogradsky 1890) 

 These include Nitrosomonas, which oxidizes ammonia to nitrate, and 

 Nitrobacter, which carries the oxidation from nitrite to nitrate: 



(5.20a) O2 + ! (NH3)aq. > § (N02)aq. + f H2O + I Hjq. + 49 kcal 



(5.20b) O2 + 2 (N02)aq. > 2 (N03)lq. + 48 kcal 



