สำนักราชบัณฑิตยสภา

197 สมศั กดิ์ ด� ำรงค์เลิ ศ และคณะ วารสารราชบัณฑิตยสถาน ปีที่ ๓๗ ฉบับที่ ๑ ม.ค.-มี.ค. ๒๕๕๕ ment may contain humic substances, which are known to accelerate reductive processes by redox mediation (7, 11, 31, 47). In the present study, humic substance is evaluated as a potential redox mediator for the reduction of sulfonated azo dyes. Humic substance is the stable organic matter ac- cumulating in soils and sediments (49). Although humic substance is generally consider to inert for microbial catabolism, it has recently been reported to play an active role in an- aerobic oxidation of a wide variety of ecologically relevant organic substrate (10, 15, 31). These studies have demonstrated that the reduction of humic substances may be important mechanisms for organic substrate oxidation in many anaerobic environments. Quinone moieties of humus have been implicated as redox active groups (47) accepting the electrons. Anthraquinone-2,6-disulfonate (AQDS) have been used as a defined model for such moieties (10, 11, 31). Most AQDS-respiring microorganisms are capable of transferring electrons to AQDS, reducing it to anthraquinone-2,6-disulfonate (AH2QDS). The role of humic analog AQDS as an electron shuttle has been demonstrated previously and thought to provide a strategy for Fe (III) reducers to access insoluble Fe (III) compounds (31). The anaerobic microbial oxidation of phenol and p -cresol in granular sludge was recently found to be coupled to the reduction of AQDS (11). The addition of humic acids or AQDS was also shown to stimulate the mineralization of the priority pollutants vinyl chloride and dichloroethene by a humus-respiring consortium un- der anaerobic conditions (7). The rates of azo dye decolorization are also enhanced in the presence of different quinoid redox mediators, especially anthraquinone-2-sulfonate. (29, 42, 59). Because the utilization of humic substance as a redox mediator should allow very unspecific reduction processes with various azo dyes, in the present study it was therefore to examined the mechanism which, humic substance stimulate anaerobic reduction of sulfonated azo dyes by Paenibacillus sp. strain A5. The loca- tion of the enzyme system which is responsible for the reduction of sulfonated azo dyes by whole cells of strain A5 in the presence of humic substance was also determined.