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

Abstract

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. 1.0 Introduction: Manganese (Mn) is ubiquitous in the global ocean, where it is an essential trace nutrient in the electron transfer processes of several metallo-enzymes-notably in photosystem II for photosynthetic organisms. In its solid oxidized form, it serves as an electron acceptor in the bacterial decomposition of sedimentary organic matter and acts as an important scavenger for 1 many trace elements and radionuclides. The chemical speciation of Mn in marine systems governs its diverse functions and is ultimately controlled by the redox conditions of the environment and the microbial community. The favorable oxidation state of Mn in oxic waters is Mn(IV), and thus Mn in oxygenated waters is primarily bound to oxygen as solid Mn(III/IV)oxides (MnO x). However, soluble Mn(II) is metastable in oxygenated waters because the oxidation of Mn(H 2 O) 6 2+ to MnO x by O 2 in an one electron transfer is thermodynamically unfavorable and the two electron transfer is kinetically slow (Luther 2010), unless facilitated by microbes, surface catalysis or superoxide-promotion (Stumm and Morgan, 1996). In the last decade, soluble manganese (dMn T) speciation has been re-evaluated to include soluble Mn(III) bound to ligands (Mn(III)-L complexes) in low oxygen environments. Mn(III)-L complexes have been measured in the suboxic porewaters of the St. Lawrence Estuary (Madison et al., 2013), the suboxic waters of the Black Sea (Yakushev et al., 2007, 2009; Trouwborst et al., 2006) and the Baltic Sea (Dellwig et al., 2012) as well as the anoxic bottom waters of the Chesapeake Bay (Trouwborst et al., 2006; Oldham et al., 2015). Because Mn(III)-L complexe...

The Spatial and Temporal Variability of Mn Speciation in the Coastal Northwest Atlantic Ocean

“…Here, we show that Mn(III)‐L complexes comprise the bulk of the total Mn (solid + soluble) at four biogeochemically distinct coastal stations, making up to 100% of the total dissolved Mn, and always significantly in excess of Mn oxide concentrations (aside from one bottom depth at Station 4). Previous water column measurements of Mn in the Northwest Atlantic have focused on open ocean measurements of total Mn (e.g., Yeats and Bewers, 1985; Statham and Burton, 1986), estuarine measurements of total Mn (Sundby et al, ; Yeats et al, ) and Mn speciation (Oldham et al, ; Oldham, Mucci, et al, ; Oldham, Tebo, et al, ), or measurements in estuarine tributaries for total Mn (Boyle et al, ; Eastman & Church, ; Graham et al, ; Moore et al, ; Sholkovitz, ) and Mn speciation (Oldham, Miller, et al, ). This study provides Mn(III)‐L measurements and profiles along continental shelves and is one of only a handful of studies determining Mn speciation profiles in seawater.…”

“…The leucoberbelin blue (LBB) assay for Mn oxides (denoted MnOx hereafter) was previously adapted from Altmann () to examine coastal water column sites (Oldham et al, ; Oldham et al, ; Oldham et al, ; Oldham et al, ). In this assay, the filter is amended with 3 ml of 20‐μM LBB dye solution (LBB, Sigma‐Aldrich).…”

“…For soluble Mn speciation analysis, an established spectrophotometric porphyrin addition method was employed (Madison et al, ; Oldham, Mucci, et al, ), which uses the ligand T(4‐CP)P (or α, β, γ, δ‐tetrakis(4‐carboxyphenyl)porphine, to 2.33 × 10 −7 M final sample concentration). In this method, cadmium chloride (CdCl 2 ; to 2.4 × 10 −7 M) is added to form a complex with the porphyrin, in the presence of an imidazole tetraborate buffer (pH = 8.2).…”

Microbial manganese(III) reduction fuelled by anaerobic acetate oxidation

“…In accordance, our results show that acetate oxidation can be linked to Mn 31 reduction at millimolar levels (pure cultures) as well as with environmentally relevant concentrations Microbial manganese(III) reduction 3479 (enrichment cultures). Widespread detection of soluble Mn 31 suggest that siderophore-or humic-stabilized Mn 31 in sediments (Madison et al, 2011;2013) and water columns (Trouwborst et al, 2006;Dellwig et al, 2012;Schnetger et al, 2012;Oldham et al, 2017) plays an important but overlooked role as an energy-generating electron acceptor to heterotrophs (Hansel, 2017). An oxygen tolerant TCA cycle is a suitable biochemical pathway used by facultative anaerobes for the transfer of electrons from acetate to dissolved Mn 31 (Fig.…”

“…Soluble Mn 3+ may form abiotically by Mn(IV) reductive dissolution of Mn(IV) oxides by siderophore‐like ligands (Duckworth and Sposito, ) or during microbial Mn 2+ oxidation (Tebo et al ., ; Webb et al ., ) or Mn(IV) reduction (Hui et al ., ). The resulting Mn can persist in solution stabilized by organic (Oldham et al ., ) or inorganic (Yakushev et al ., ) ligands. Although Mn 3+ is soluble, electron transport and protein secretion pathways involved in extracellular metal reduction are required for electron transfer to Mn 3+ (Szeinbaum et al ., ).…”