The role of oxygen and electron donors in indole biodegradation and mineralization

Abstract

AbstractIndole is a recalcitrant compound whose aerobic biodegradation is triggered by a mono‐oxygenation process. Generally, the mono‐oxygenation process involves intracellular electron carriers and molecular oxygen as co‐substrates; the increase of concentrations for either one can accelerate the biotransformation of refractory substances. In this study, the initial biotransformation of indole proved to be a special mono‐oxygenation reaction, neither endogenous nor exogenous electron donors have influences on indole biodegradation. However, oxygen plays a significant role in indole mono‐oxygenation. Higher dissolved oxygen led to a faster rate of indole biodegradation. As the product of indole mono‐oxygenation, unstable indoxyl indirectly turned out to exist by its tautomer 2‐oxindole. Intermediates like isatin, N‐formylanthranilic acid, and anthranilic acid were confirmed. In addition, coupling biodegradation with UV photolysis accelerated indole mineralization, and longer photolysis time gave faster TOC removal. This is because the supplement of endogenous electron donors by photolysis could enhance the catabolism of isatin and anthranilic acid, thus promoting indole mineralization. The biodegradation of these two intermediates was further verified to be initiated by donor‐requiring steps, so the photolytic products which generated labile electron donors could accelerate their oxygenations. This study provides valuable information on further controlling the process of aerobic biodegradation for indole.