3.2. Degradation of TCE with ammonium
AOB have been shown to degrade TCE through aerobic cometabolism with ammonium as the primary energy source (Arciero et al., 1989; Ely et al., 1997). The results discussed here demonstrate that the population of ammonium-oxidizing organisms closely associated with C. comosa roots obtained from a local wetland can also be stimulated to degrade TCE. At 29 lg L_1 TCE, a relatively sustained TCE degradation was observed with either 20 mg L_1 or 30 mg L_1 NH4 + –N for four weeks (cycles AMT 1-4; Fig. 1). Microbial TCE oxidation remained largely unaffected in three subsequent weeks (cycles AMT 5-7) even after TCE concentration increased to 46 lg L_1 (with NH4 + –N at 30 mg L_1). However,
as TCE concentration was increased to 64 lg L_1 in four subsequent weeks (cycles AMT 8-11; Fig. 1), a gradual microbial deactivation may have set in, and the AOB activity nearly ceased in cycle AMT 11. Microbial deactivation in cycles AMT 8-11 was also evident as gradually declining ammonium degradation, oxygen consumption and NOx production as TCE concentration increased to 69 lg L_1 (cycles AMT 8-11; Figs. 1 and 2). A modest decrease in O2 consumption in cycle AMT 11 may suggest some heterotrophic activity still persisting at that time (Figs. S2 and S5, in SM).The pH decline in control reactors was presumably caused by H+production upon heterotophic oxidation of root-derived DOC in the reactors (Fig. S5 in SM). In comparison, the pH decline in live reactors was greater than in control reactors through the cycles due to combined H+ contributions from ammonia (Eq. S1 in SM) and DOC oxidation. However, at 69 lg L_1 TCE (cycle AMT 11) the decline in pH in the live and control reactors were nearly identical arguably because of inhibition in metabolic AMO oxidation in live reactors,and only heterotrophic activity continued. Further, the changes cycle AMT 10 as it was after cycle AMT 7. Even though the AMO enzyme had been inactivated by TCE oxidation before cycle AMT 10 ended, the AMO gene required to synthesize more enzyme was present in abundance.