IMPACT OF ARSENICALS ON NITRIFICATION 



295 



DAYS 



Fig. 1 Impact of arsenate on Nitrosomonas. •, no arsenic; o, 10 mg 

 As/liter; A, 100 mg As/liter; and A, 1000 mg As/liter. 



this approach. First, mixed cultures growing in a selective medium 

 provide a larger genetic pool than do pure cultures. This large genetic 

 pool may allow the system to adapt to the toxicant much as 

 populations do in nature. Second, enrichment of the mixed 

 population assures the researcher uniform cultures to be used for a 

 given set of toxicity tests (Table 1). This assurance usually is not 

 present in studies of functional groups using natural samples or 

 colonized slides. Third, monitoring ecosystem function is usually 

 faster and easier than measuring specific bacterial populations. For 

 example, the study by Fliermans, Bohlool, and Schmidt (1974), 

 using fluorescent antibodies for counting nitrifiers, is very time and 

 labor consuming. 



