Extracellular haem peroxidases mediate Mn(II) oxidation in a marine Roseobacter bacterium via superoxide production

Summary Manganese ( M n) oxides are among the strongest sorbents and oxidants in environmental systems. A number of biotic and abiotic pathways induce the oxidation of M n( II ) to M n oxides. Here, we use a combination of proteomic analyses and activity assays, to identify the enzyme(s) responsible for extracellular superoxide‐mediated M n oxide formation by a bacterium within the ubiquitous Roseobacter clade. We show that animal haem peroxidases ( AHPs ) located on the outer membrane and within the secretome are responsible for M n( II ) oxidation. These novel peroxidases have previously been implicated in direct M n( II ) oxidation by phylogenetically diverse bacteria. Yet, we show that in this Roseobacter species, AHPs mediate M n( II ) oxidation not through a direct reaction but by producing superoxide and likely also by degrading hydrogen peroxide. These findings point to a eukaryotic‐like oscillatory oxidative‐peroxidative enzymatic cycle by these AHPs that leads to M n oxide formation by this organism. AHP expression appears unaffected by M n( II ), yet the large energetic investment required to produce and secrete these enzymes points to an as yet unknown physiological function. These findings are further evidence that bacterial peroxidases and secreted enzymes, in general, are unappreciated controls on the cycling of metals and reactive oxygen species (ROS), and by extension carbon, in natural systems.