New Players and Pathways in Manganese Oxidation Along an Estuarine Gradient

Manganese (Mn), as an important and powerful redox metal, is an essential trace element for all life. The direct and indirect activity of Mn-oxidizing microorganisms leads to the formation of particulate manganese oxides (MnO_x) within the environment. Dissolved Mn has historically been assumed to be Mn(II), yet, it has recently been shown that a dominant percentage of natural dissolved Mn exists as Mn(III)-ligand (L) complexes. Despite there being many known species of bacteria and fungi that oxidize Mn(II), it is unknown whether there are microbes that alternatively or exclusively oxidize Mn(III)-L. While the oxidation of Mn(II) to MnO_xis not coupled to energy gain for microbes, the oxidation of Mn(III)-L by oxygen is energetically favorable and thus there is potential for this redox couple to fill a metabolic niche. Accordingly, the goal of this study was to characterize Mn speciation and Mn-oxidizing microorganisms along a river to estuary gradient. Similar to previous studies, both Mn(II) and Mn(III)-L were detected throughout the estuarine system, with Mn(III)-L representing the dominant dissolved pool. We cultivated 16 Mn(II)- and 9 Mn(III)-oxidizing bacteria and 4 Mn(II)-oxidizing fungi, which include novel species and a diversity of growth morphologies and Mn oxide products. Continued characterization of these isolates will provide key insight into the mechanisms of Mn oxidation along a freshwater to salinity gradient.