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«“√ “√ √“™∫— ≥±‘ µ¬ ∂“π ªï ∑’Ë Û ©∫— ∫∑’Ë Ò ¡.§.-¡’ .§. ÚıÙ¯ 67 Somsak Damronglerd, et al. direct contact between the sulfonated azo dyes and bacterial cell membrane. Since hydroquinones are readily oxidized by sulfonated azo dyes, this hydroquinones only needs to be present at substoichiometric concentrations to be an effective electron carrier as long as these azo dyes are abundant in the wastewaters. Although the effectiveAQDS dosage levels were low, continuous dosing implies continuous expenses related to procurement of the chemical as well as continuous discharge of this recalcitrant sulfonated azo compounds. Therefore, it is desirable to immobilize the redox mediator in the bioreactor for treatment azo dye-containing wastewater conti- nuously. For this propose, various reactor configurations were employed to demonstrate that, though the use of redox mediator such as AQDS, direct contact between azo dye and microbial cells is not required, which allows microbial activity to decoupled in space and time from azo dyes reduction process. For example, a system of two separated columns which one used for redox mediator reduction and the another one use for azo dye reduction has been set up for stimulation of azo dye reduction by Burkholderia cepacia (30). Alternatively, the laboratory-scale upflow anaerobic sludge bed (UASB) containing activated carbon as an immobilized quinoid redox mediator in the sludge bed has been tested for its accelerating effect on anaerobic reduction of a recalcitrant azo dye (54). The fact that exogenous extra- cellular molecules (such as humic substances) can participate in electron transfer to extracellular environmental contaminants indi- cates that they may make a significant contribution to biotransformation of such xenobiotics in many environ- ments (7, 10, 11). Whether microbially produced extracellular molecules have a similar role remains an important question. Another possibi- lity for the reduction of extracellular quinoid redox mediator suchAQDS, which does not require the transport of both redox mediators and azo dyes though the cell membrane, has been suggested for humate-respiring bacterium, Shewanella putrefaciens (34). Non-proteinaceous small com- pound that has characteristics similar to a quinone and can be excreted into the medium is involved in electron transfer to AQDS and humic acid by this strain (34). Moreover, a deriva- tive of 1,4-dihydroxy-2-naphthoate (DHNA), precursor of menaquinone, is responsible for the carbon tetra- chloride transformation activity observed in Shewanella oneidensis MR-1 after aerobic growth (55). Menaquinone (MK) is the only common link for the different electron transfer routes in B. subtilis, and it is tempting to propose that MK itself can be the component whose reduction-oxida- tion is controlled by energization (34). Because the rather high reduc- tion rate of sulfonated azo dyes in the absence of any exogenous redox mediators found in whole cells experiment (Fig. 2 and Table 3), we cannot eliminate the possibility that some unknown enzymatic activities are involved in the anaerobic reduction of sulfonated azo dyes by whole cells of strain A5. We are currently attempting to explore the other extracellular electron trans- ferring mechanisms of strain A5 which are involved for the reduction of theAQDS to clarify thismechanism. ACKNOWLEDGEMENTS This work was funded by the Thailand Research Fund. We gratefully acknowledge Dystar Thai Ltd., for providing the dyes used in this study. REFERENCES 1. Anliker, R. 1979. Ecotoxicology of dyestuffs: a joint effort by industry. Ecotox. Environ. Safety 3 : 59-74. 2. Bechtold, T., E. Burtscher, A. Turcanu, and O. Bobleter. 1994. The reduction of vat dye by indirect electrolysis, J. Soc. Dyers Color. 110 : 14-19. 3. Bergsma, J., R. Strijker, J.Y. Alkema, H.G. Seijen, and W.N. Konings. 1981 NADH dehydrogenase and NADH oxidation in membrane vesicle from Bacillus subtilis . Eur. J. Biochem. 120 : 599-606. 4. Bertsova, Y.V., A.V. Bogachev, and V.P. Skulachev. 1998. Two NADH : ubiquinone oxidoreductases of Azotobacter inelandii and their role in the respiratory protection. Biochem. Biophys. Acta. 1363 : 125-133. 5. Bjorklof, K., V. Zickermann, and M. Finel. 2000. Purification of the 45 kDa, membrane bound NADH dehydrogenase of Escherichia coli (NDH-2) and analysis of its interaction with ubiquinone analogues. FEBS Lett. 467 : 105-110. 6. Bradford, M.M. 1976. A rapid and sensitive method for the quantitation of