Factors influencing biodegradation of benzoate by denitrifying bacterial enrichment cultures

The recent requirement to remove nitrogen from wastewaters has led to the design and construction of wastewater treatment processes in which the bacteria are cycled (either spatially or temporally) between anoxic and aerobic zones. Although such processes are mainly used for the treatment of municipal wastewaters, they are being applied with increasing frequency to industrial wastewaters containing aromatic compounds. Based on information in the literature, it is clear that denitrifying bacteria that are alternately exposed to aerobic and anoxic conditions during biodegradation of nonaromatic compounds maintain the systems required to pass electrons from the electron transport chain (ETC) to both oxygen and the various nitrogen oxides. Both the synthesis and the activity of the enzymes involved in the aerobic and anoxic ETCs in those systems are regulated by the concentrations of oxygen and nitrogen oxides present. However, little is known about the regulation of the enzymes involved in the biodegradation of aromatic compounds under alternating aerobic/anoxic conditions. Therefore, the stability of the anoxic and aerobic benzoate-degrading ability of mixed microbial cultures under endogenous conditions and the stability and inducibility of their ability in fed-batch reactors (FBRs) were investigated when the cultures were exposed to the opposite redox condition. The results showed that the stability and inducibility of their benzoate-degrading ability were different for the different bacteria in the mixed microbial communities, suggesting that those characteristics depend strongly upon the nature of the bacteria in the communities.; The effect of oxygen on the anoxic biodegradation of benzoate was investigated by adding different amounts of oxygen to a denitrifying mixed microbial culture in a chemostat. The results suggested that the addition of oxygen adversely affected the activity and/or synthesis of the downstream denitrifying enzymes (i.e., nitric oxide reductase and/or nitrous oxide reductase), possibly leading to the release of nitric and nitrous oxide in the gas phase. In addition, mixed microbial cultures growing under microaerobic-denitrifying conditions (DO concentration <0.05 mg/L) were able to degrade benzoate via the anaerobic pathway while utilizing the added oxygen as a terminal electron acceptor. Finally, the stability and inducibility of the benzoate-degrading ability of the mixed microbial cultures from experimental chemostats receiving different oxygen inputs were tested in aerobic and anoxic FBRs. The results suggested that appropriate oxygen inputs into anoxic reactors can help mixed microbial cultures to recover and/or induce the anoxic biodegradative capability for benzoate more easily when they are exposed to alternating aerobic/anoxic environments, thereby shortening the time lags associated with readaptation to anoxic conditions.