GROWTH 165 



organisms, includes the nitrite and nitrate bacteria, 

 sulphur bacteria, the thiosulphate bacteria and the 

 Hydrogenomonas group (see also p. 168). 



Generally speaking, these prototrophic bacteria are 

 very similar to algae, the difference resting mainly in the 

 substitution of the chlorophyl mechanism by a chemical 

 method of energy supply. The amount of energy 

 required to make organic matter from inorganic is quite 

 large, much larger than that needed by the common 

 saprophytes. 



Another compound with but one carbon atom is the 

 formic acid. This is often stated to be a rather unsatis- 

 factory source of carbon for growth, though it can be 

 used by some molds and bacteria. Most experiments 

 on the decomposition of formic acid, (and of many 

 other compounds) have been carried on in a medium 

 containing peptone as a ^^ source of nitrogen"; these 

 results indicate the utilization of the acid for energy, 

 but not for cell construction. The fitness of a com- 

 pound as building material is not dependent upon 

 the amount of energy needed to reduce it to cell con- 

 stituents, but is largely determined by its chemical 

 constitution. 



There is for example the carbonic acid, with three oxygen atoms 

 coupled to the carbon. This compound can be used by a goodly 

 number of bacteria. Most of these can use neither carbon-monoxide 

 nor methane, though these compounds are much more easily reduced, 

 and methane would even liberate energy if changed to carbohydrate 

 or organic acid. The structure seems to play the essential role. 



Formaldehyde is a strong disinfectant, much stronger 

 than formic acid. But there is evidence of its being used 

 as a source of carbon. Kaserer (1906) observed that 

 Hydrogenomonas required both hydrogen and carbon 



