Distribution of desferrioxamine-B-extractable soluble manganese(III) and particulate MnO ₂ in the St. Lawrence Estuary, Canada

Soluble organically-complexed manganese(III) (Mn(III)-L) is an intermediate species in the manganese cycle. The reactivity of the complex(es) depends on the abundance and nature of the ligand(s), as well as its physicochemical environment. Currently, the strength of Mn(III)-L complexes is assessed by their ability to react either directly with a porphyrin complex, using the kinetics of its competitive ligand exchange reactions, or after being reduced to manganese(II). We present a new three-step technique that quantifies part of the Mn(III)-L pool which may represent a bioavailable and reactive fraction of the complexes. This technique relies on 1) ligand exchange of the dissolved complexes in a filtered field sample with the siderophore desferrioxamine-B (DFOB), 2) chromatographic separation and concentration of manganese(III)-DFOB from the aqueous matrix, and 3) quantification of the naturally ligated manganese(III) that was outcompeted by DFOB (Mn(III)-L _DFOB ) at low concentrations by flow injection UV-Vis spectrophotometry. The formation of an extractable Mn(III)-L _DFOB complex provides an operationally-defined benchmark by which to assess biological interactions and better understand the cycling of manganese(III) in the environment. The technique, with a detection limit of 0.09 nM, was applied to water samples collected in the St. Lawrence Estuary (SLE) and adjacent Saguenay Fjord in Canada. Mn(III)-L _DFOB was ubiquitous at our study sites, but its concentration was low relative to total dissolved manganese (dMn _T ) and particulate MnO ₂ . Spatial variations of the dMn _T speciation within the Saguenay Fjord suggest that Mn(III)-L forms from the reduction of MnO ₂ . Likewise, variations of the dMn _T speciation along a dissolved oxygen gradient in the SLE leads us to believe that Mn(III)-L _DFOB likely represents a fraction of the total Mn(III)-L that cycles more readily in estuarine and marine systems.