THE IMPACT OF CYCLIC AEROBIC/ANOXIC CONDITIONS ON BIODEGRADATION OF BENZOATE

The response of a mixed microbial culture to different cyclic aerobic and anoxic (denitrifying) conditions was studied in a chemostat with a 48 hr hydraulic residence time receiving a feed containing benzoate and pyruvate as sole carbon sources. The culture was well adapted to the cyclic conditions as evidenced by the lack of accumulation of benzoate or pyruvate in the chemostat. When the cyclic conditions were 3 hr aerobic and 9 hr anoxic, the bacteria degraded benzoate under aerobic conditions via the catechol 2,3- dioxygenase (C23DO) pathway. The quantity of enzyme remained essentially constant throughout the anoxic period but experienced a transient decrease during the initial portion of the aerobic period before returning to the level present during the anoxic period. That decrease was most likely caused by the production of H2O2 by the cells upon being returned to aerobic conditions. The quantity of benzoyl-CoA reductase (a key enzyme in the anoxic biodegradation pathway for benzoate) remained constant regardless of the redox condition. The aerobic benzoate uptake capability (AeBUC) of the microorganisms increased during the aerobic period but decreased during the anoxic period. The anoxic benzoate uptake capability (AnBUC) exhibited the opposite response. Under the second cyclic condition, 6 hr aerobic and 6 hr anoxic, protocatechuate 4,5- dioxygenase (P45DO) was the only aerobic aromatic ring cleavage enzyme expressed. P45DO activity decreased early in the aerobic cycle, which also likely resulted from H2O2 inactivation. As in the other experiment, the level of benzoyl-CoA reductase in the cells was constant throughout the entire cycle. Furthermore, AeBUC and AnBUC were affected by the cyclic conditions in much the same way as in the 3 hr/9 hr chemostat. The results suggest that the degradation of aromatic compounds will be unimpaired in systems such as biological nutrient removal systems in which the bacteria are cycled between aerobic and anoxic conditions.