| Abstract | This study investigated the effect of toxic chemicals on oxygen utilization rate (OUR) of nitrifying mixed cultures namely ammonia oxidizing mixed culture (AOMC) and nitrite oxidizing mixed culture (NOMC) while having the future goal to develop a biosensor using nitrifiers for toxicity monitoring. Mixed nitrifying culture was used instead of purified nitrifying culture because of the easiness of the culture development. Nitrifying mixed cultures were enriched from domestic wastewater treatment processes microbial seed, with the addition of ammonia or nitrite as nitrogen source together with inorganic substrates or mineral solutions. After enrichment, the cultures were then subjected to the toxicity tests where toxic chemicals exposed to them in the existence of ammonia or nitrite and oxygen. Toxic chemicals considered in this study were; cadmium, nickel, copper, zinc, phenol, 2-CP, 3-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,5-DCP, 2,6-DCP and 3,4-DCP. The endpoint of the toxicity test was determined in terms of the reduction or inhibition of oxygen utilization rate (OUR). In the toxicity test to observe the effect of exposure times, OUR was measured during three different exposure periods of 0-15, 25-40 and 50-65 min. In the toxicity tests with heavy metals, cadmium and nickel appear to have stronger inhibitory effect on AOMC than copper and zinc, where copper has less sensitive to AOMC than other heavy metals. The variability of toxic response of AOMC to heavy metals was exhibited at low heavy metal concentrations (1.0, 2.5, 5.0 mg/L) and at rapid response time (0-15 min), either %inhibition increased or reduced. However, at higher heavy metal concentrations (10, 20 mg/L), the reduction of OUR was clearly detected and it was found that to have more reliable response, longer exposure time is recommendable for the detection of these heavy metals. As the lowest EC50 value of the toxicity tests with cadmium and nickel were determined at the longest exposure time of 50-65 min. The similar study on heavy metals was also performed with NOMC and it was found that NOW is not effective to detect heavy metals in comparison to AOMC. Basically, the reason for this is that the maximum OUR of NOMC was about ten times less than that of AOMC. The toxic response of AOMC to phenolic compounds was investigated in the same way and phenolic compounds remarked the stronger inhibitory effect and less variability than the tests with heavy metals. As more than 10% of OUR reduction was detected at the presence of 0.25 mg/L within 15 min response time. Generally, the sensitivity of AOMC to phenolic compounds was improved by extending of exposure times to 25-40 and 50-65 min. In other words, the longer exposure times increased %inhibition in comparison to the tests at 0-15 min especially to low phenolic compound concentrations (0.25, 0.50, 1.0 mg/L). However, the highest %inhibition was observed at 25-40 min not at 50-65 min. Furthermore, at the higher concentrations (2.5 and 5.0 mg/L) of phenol and monochlorophenols (2-CP, 3-CP, 4-CP), the extended exposure times did not have important effect on the response pattern of AOMC. The different observation was only for 2,4-DCP and 2,6-DCP where the highest %inhibition was achieved at the longest exposure time of 50-65 min. The most sensitive AOMC that gave EC50 for Cd, Ni, Cu, Zn, phenol, 2-CP, 3-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,5-DCP, 2,6-DCP and 3,4-DCP in this study to be 6.9-8.5 mg/L, 2.42.5 mg/L, >20 mg/L, 3.7-6.0 mg/L, 1.4-1.6 mg/L, 0.5 mg/L, 0.2 mg/L, 0.5-0.7 mg/L, 0.2 mg/L., 1.5-1.6 mg/L, 0.2-0.3 mg/L, 3.0-3.1 mg/L and 1.5-1.7 mg/L, respectively. The results from this study illustrate that AOMC can be applied for the toxicity detection as purified ammonia oxidizer (Blum and Speece, 1991; Inui et al, 2002). Though the application of AOMC to the monitoring of heavy metals is not as easy but AOMC is very sensitive to phenolic compounds and its sensitivity is high enough for the detection of phenol and chlorophenols at the level of effluent standards which value of phenolic compounds concentration in between 1-5 mg/L, for example, in Thailand and in Japan. These findings show the possibility to develop biosensor using AOMC for the monitoring of phenolic compounds as a tool for environmental protection due to its simplicity, short response time and high sensitivity |