294 HOLM AND COX 



TABLE 1 



CONCENTRATIONS OF NITROGEN 

 IN AQUEOUS NITRIFICATION SYSTEMS* 



*The growth medium containing 10 mg/liter NH4^N 

 was inoculated with 5 ml of a mixed nitrifier population. 

 Abbreviation S is standard deviation of seven flasks. 



tMean of seven observations. 



acid. There was a small decrease in the oxidation rate at concentra- 

 tions of 1000 mg As/liter (Fig. 2). 



Nitrobacter-type organisms used in this study are sensitive to 

 arsenate (Fig. 3). Apparently there is a progressively longer lag phase 

 in the arsenic concentration range from 0.1 to 10 mg/liter, but the 

 only treatment statistically different from the control (based on 

 analysis of variance, a' = 0.05) is 10 mg/liter. Because only one point 

 from each oxidation curve was used for the statistical analysis and 

 only two replicate curves were obtained for each treatment, the 

 analyses for differences are probably conservative. At very high levels 

 of arsenate (100 and 1000 mg As/liter), Nitrobacter was inhibited, 

 and nitrite accumulated. 



Cacodylic acid does not affect Nitrobacter. Arsenic concentra- 

 tions as high as 1000 mg/liter as cacodylic acid did not alter the 

 oxidation rate of nitrite (Fig. 4). 



DISCUSSIOIM 



The technique we adapted for this study is useful for studying 

 the impact of pollutants on specific functional groups of bacteria. In 

 this study we enriched the medium for chemosynthetic autotrophs 

 important in the nitrification process. There are several advantages to 



