298 



HOLM AND COX 



Fig. 4 Impact of the organic arsenical, cacodylic acid, on Nitro- 

 bacter. •, no arsenic; Z\ 100 mg As/liter; and A, 1000 mg As/liter. 



molecular oxygen are required to oxidize 1 mg of ammonia nitrogen 

 to nitrate nitrogen (Young, 1969). 



Our results show that arsenate inhibits the second step of 

 nitrification to a greater degree than it does the first stage. This was 

 reflected in a delay in the oxidation of nitrite at low concentrations 

 of arsenate; at high levels no nitrite was oxidized within 25 days. The 

 mechanism of this inhibition was not studied. Button and Dunker 

 (1971), however, showed that arsenate interrupts phosphate metabo- 

 lism in some microorganisms. Torstensson (1974) reported that 

 MCPA, 2,4, 5-T, and Linuron inhibit Nitrobacter and cause an 

 accumulation of nitrite. 



Nitrite in the environment is undesirable because of its toxicity 

 to biota (National Academy of Sciences, 1973) and its possible 

 implication in the formation of carcinogenic nitrosamines (Lijinsky 



