Soluble Mn(III)–L complexes are abundant in oxygenated waters and stabilized by humic ligands

Dissolved Mn (dMn_T) is thought to be dominated by metastable Mn(II) in the presence of oxygen, as the stable form is insoluble Mn(IV). We show, for the first time, that Mn(III) is also stable as a soluble species in the oxygenated water column, when stabilized by organic ligands as Mn(III)–L complexes. We measured Mn(III)–L complexes in the oxygenated waters of a coastal fjord and a hemipelagic system where they make up to 86% of the dMn_T. Although Mn(III) forms similar complexes to Fe(III), unlike most of the analogous Fe(III)–L complexes, the Mn(III)–L complexes are not colloidal, as they pass through both 0.20 μm and 0.02 μm filters. Depending on the kinetic stability of the Mn(III) complexes and the microbial community of a given system, these Mn(III)–L complexes are capable of donating or accepting electrons and may therefore serve as both reductants or oxidants, can be biologically available, and can thus participate in a multitude of redox reactions and biogeochemical processes. Furthermore, sample acidification experiments revealed that Mn(III) binding to humic ligands is responsible for up to 100% of this complexation, which can influence the formation of other metal complexes including Fe(III) and thus impact nutrient availability and uptake. Hence, humic ligands may play a greater role in dissolved Mn transport from coastal areas to the ocean than previously thought.