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X 



Fig. 2 Impact of the organic arsenical, cacodylic acid, on Nitro- 

 somonas. •, no arsenic; A, 100 mg As/liter; and A, 1000 mg As/liter. 



Our approach also has some disadvantages. First, selection of new 

 variants or species may occur, but this is not monitored. In fact, the 

 selection process may have occurred during the enrichment phase of 

 the study since the nitrification rate increased on successive transfer. 

 This phenomenon may also have occurred when populations of 

 Nitrobacter were exposed to arsenicals (Fig. 3). Second, interactions 

 of populations from different segments of the nutrient cycle are not 

 possible with enrichment techniques. This contrasts with work by 

 Lavegha and Dahm (1974), who used soil systems to monitor the 

 activity of several functional groups of bacteria after the test systems 

 were dosed with pollutants. 



Inhibition of Nitrosomonas and, consequently, inhibition of the 

 total nitrification process, can be economically and ecologically 



