IMPACT OF ARSENICALS ON NITRIFICATION 293 



mg/liter were the quantities chosen for statistical analysis. Data were 

 analyzed using analysis of variance and Tukey's co procedure (Steel 

 and Torrie, 1960). 



RESULTS 



Nitrifying populations were easily cultured from the garden-soil 

 inoculum. For the initial enrichment, ammonia was oxidized to 

 nitrite by Nitrosomonas organisms within 2 weeks, and the nitrite 

 formed in the first step was oxidized to nitrate by Nitrohacter within 

 3 weeks. In successive transfers of 1-ml inocula, however, the rate of 

 oxidation of ammonia to nitrate was significantly increased. By the 

 second transfer, all the ammonia was oxidized to nitrite within 1 

 week. The entire nitrification process, involving the oxidation of 10 

 mg/liter ammonia nitrogen to 10 mg/liter nitrate nitrogen, was 

 shortened from 24 days to 10 days by the third transfer. 



No counts of nitrifiers were made as the ammonia was being 

 oxidized to nitrate. Nitrosomonas probably outnumbered Nitro- 

 hacter in each flask, however, because the oxidation of ammonia to 

 nitrite provides more energy for growth of cells than does the 

 oxidation of nitrite to nitrate (Tuffey, 1973; Curtis, Durrant, and 

 Harmon, 1975). 



These nitrification systems provide reproducible results. Data in 

 Table 1 were collected from seven replicate flasks, each receiving a 

 5-ml inoculum from a culture transferred twice previously. These 

 results demonstrate that aqueous systems in flasks can be used to 

 study the impact of soluble pollutants on nitrification. An estimate 

 of homogeneity of data, the coefficient of variation (CV), which is 

 100(standard deviation/mean), calculated from the data in Table 1 

 shows that these systems have CV values of 20% or less when the 

 ammonia or nitrite nitrogen concentration is above 1 mg/liter. 



Using these test systems, we observed that Nitrosomonas 

 organisms are not sensitive to arsenicals. At low levels of arsenate 

 (10 mg As/liter), the rate of ammonia oxidation was identical to that 

 of the arsenic-free controls (Fig. 1). The rate of ammonia oxidation 

 was significantly different from the control only at 1000 mg As/liter 

 added as arsenate, determined with Tukey's co procedure (a = 0.05). 

 These results show that arsenate should not have an inhibiting action 

 on Nitrosomonas in the natural environment because of the high 

 concentrations necessary before any effects are seen. 



Likewise, cacodylic acid should not have a significant direct 

 effect on Nitrosomonas. Nitrosomonas oxidized ammonia at normal 

 rates in media containing as much as 100 mg As/liter as cacodylic 



