Microbe-driven generation of reactive oxygen species for contaminant degradation

Reactive oxygen species (ROS), including superoxide (e.g., •O 2 − ), peroxides (e.g., H 2 O 2 ), and free radicals (e.g., •OH), are of considerable interest in mediating contaminant transformations due to their extremely high reactivity. In the natural surface water environment, the photodependent process has traditionally been considered as the dominant source of ROS. However, more recently it was found that the activities of microbes play essential roles in ROS production, especially in dark environments. Herein, ROS production driven by microbes for contaminant degradation was summarized. The ability of microbes including fungi, phytoplankton, and heterotrophic bacteria which were capable of extracellular ROS production was compared under different environmental conditions. Furthermore, the abundant •OH production in soil, sediments, and surface waters that contained oxic–anoxic boundaries were enumerated. In addition, the production of •OH via oxidation of microbially reduced iron-containing minerals was also systematically discussed in this review. The microbe-driven ROS generation processes improved our understanding of naturally occurring ROS species and the oxidative degradation of contaminants in dark environments.