Production of extracellular superoxide radical in microorganisms and its environmental implications: A review.

Extracellular superoxide radical (O) is ubiquitous in microbial environments and has significant implications for pollutant transformation. Microbial extracellular O can be produced through multiple pathways, including electron leakage from the respiratory electron transport chain (ETC), NADPH oxidation by the transmembrane NADPH oxidase (NOX), and extracellular reactions. Extracellular O significantly influences the geochemical processes of various substances, including toxic metals and refractory organic pollutants. On one hand, extracellular O can react with variable-valence metals and detoxify certain highly toxic metals, such as As(III), Cr(VI), and Hg(II). On the other hand, extracellular O can directly or indirectly (via Bio-Fenton) degrade many organic pollutants, including a variety of emerging contaminants. In this work, we summarize the production mechanisms of microbial extracellular O, review its roles in the transformation of environmental pollutants, and discuss the potential applications, limiting factors, and future research directions in this field.