244 SELMAN A. WAKSMAN AND J. S. JOFFE 



PHYSIOLOGY 



Source of carbon. The organism derives all its carbon need from 

 the CO2 of the atmosphere. TNTien carbon was introduced into the 

 culture in the form of carbonates and bicarbonates, the presence 

 of the former prevented growth due to the fact that they kept 

 the medium alkaline, thus preventing a normal development of 

 the organism, while the latter, if present only in small amounts, 

 allowed a good growth to take place. But since the growth was 

 not any better, and to some extent even worse than in the bi- 

 carbonate-free flasks, its use is superfluous. At this point, we 

 get a clear differentiation in the metabolism of two important 

 autotrophic organisms, the nitrifying and the sulfur-oxidizing 

 bacteria. While the former thrives best at an alkaline reaction, 

 the latter grow best at an acid reaction. Sodium bicarbonate 

 is considered to be indispensable for the nitrifying bacteria; 

 this was thought to be due to the utilization of the bicarbonate 

 as a source of carbon, but, as recently pointed out by Meyerhof 

 (1916), the bicarbonate merely serves the purpose of a buffer 

 in the medium, to keep the reaction alkaline (optimum pH 8.3- 

 9..3). In the case of the sulfur oxidizing bacterium, which has 

 its optimum at a distinctly acid reaction (pH 3.0-4.0), the bi- 

 carbonate is not necessary since its buffering properties will 

 tend to make the medium less acid and thus have an injurious 

 effect, while as a source of carbon, the CO2 from the atmosphere 

 seems to be sufficient. 



Source of energy. Sulfur is the all important source of energy 

 for this organism. The organism is strictly autotrophic and, 

 although glucose did not exert any injurious action, and perhaps 

 its action was even to some extent beneficial, the amount of 

 sulfur oxidized and acid produced were about the same in glucose 

 and in glucose-free cultures. 



In addition to sulfur, thiosulfate is also utilized, but to a 

 much smaller extent: while, with elementary sulfur, gi-owth 

 appears in four to five days, under favorable conditions, as demon- 

 strated by the turbidity and change in pH value, with thiosulfate, 

 growth appears only in ten to twelve days and is much slower. 



