292 HOLM AND COX 



38.2 mg; K2HPO4, 1.1 mg; MgClj , 5.7 mg; MgS04 • 7H2O, 1.9 mg; 

 CaCl2 • 2H2O, 4.4 mg; NaHCOj, 150 mg; and micronutrient stock, 

 10 ml. The medium was sterilized by filtration, and 100-ml ediquots 

 were added to sterilized 250-ml Erlenmeyer flasks. 



Appropriate amounts of sodium arsenate or cacodylic acid were 

 added to the medium before filter sterilization. Reagent grade 

 chemicals were used to prepare the growth medium. Mixed nitrifying 

 populations (containing both Nitrosomonas and Nitrobacter types) 

 from local garden soil were used as inocula for the systems. Although 

 the functional populations were not identified, oxidation of am- 

 monia to nitrate indicated that both Nitrosomonas and Nitrobacter 

 were present. 



Ammonia and nitrite concentrations in each flask were measured 

 at selected intervals during the incubation period using a Technicon 

 Autoanalyzer unit (AAII). Ammonia was measured using the auto- 

 mated colorimetric phenate method (Technicon Corp., 1971). Nitrite 

 concentrations were measured using the automated cadmium reduc- 

 tion method (Environmental Protection Agency, 1974). 



Experimental Design 



Enrichment of nitrifying populations was completed by adding 

 1 g of garden soil to 250-ml Erlenmeyer flasks containing 100 ml of 

 the growth medium. The flasks were incubated in the dark at 25° C at 

 a shaking rate of 125 rpm. At 2- to 3-day intervals, samples were 

 taken, diluted, and analyzed for ammonia and nitrite. After 

 populations had oxidized all the ammonia to nitrate, 1-ml inocula 

 were transferred into fresh media, and the sampling cycle was 

 repeated. Inocula from the second and third transfers were used for 

 the arsenic studies. 



The reproducibility of the nitrification system was measured in 

 seven replicate flasks, each receiving a 5-ml inoculum from a culture 

 transferred twice previously. Biological activity was determined 

 within each flask by measuring the oxidation of ammonia to nitrate 

 over a 20-day incubation period. 



The impact of both sodium arsenate and cacodylic acid (at 

 arsenic concentrations of 0, 0.1, 1, 10, 100, and 1000 mg/liter) was 

 measured on mixed cultures of nitrifiers in aqueous systems 

 containing 10 mg/liter nitrogen as ammonia. Duplicate flasks were 

 set up for each treatment, and nitrification rates were determined in 

 each flask by measuring levels of ammonia and nitrite over a 24-day 

 period. 



The hours required for the oxidation of ammonia by Nitro- 

 somonas and nitrite by Nitrobacter to a nitrogen concentration of 1 



