A kinetic approach to assess the strengths of ligands bound to soluble Mn(III)

Luther, George W.; Madison, Andrew S.; Mucci, Alfonso; Sundby, Bjørn; Oldham, Véronique E.

doi:10.1016/j.marchem.2014.09.006

Cited by 51 publications

(45 citation statements)

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“…Fourthly, it has been suggested that dissolved Mn ligands keep Mn in solution (Sander and Koschinsky, 2011;Madison et al, 2013;Luther et al, 2015). That would result in less conversion to manganese oxide aggregates, and hence may solve the removal problem, but this would yield wrong Mn ox /Mn diss concentration ratios like we saw for Ox-idThreshold.…”

Section: Export Dynamicsmentioning

confidence: 77%

“…The complete process may be more complicated than described above (Boyle et al, 2005), e.g. because Mn binds to dissolved ligands such that more of it may stay in solution (Sander and Koschinsky, 2011;Madison et al, 2013;Luther et al, 2015). Manganese oxides are important scavengers of other trace metals like iron, cobalt, nickel and zinc (Yamagata and Iwashima, 1963;Murray, 1975;Means et al, 1978;Saito et al, 2004;Tonkin et al, 2004), as well as insoluble radionuclides such as thorium and protactinium (e.g Reid et al, 1979;Hayes et al, 2015;Jeandel et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

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Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

et al. 2017

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Dissolved manganese (Mn) is a biologically essential element. Moreover, its oxidised form is involved in removing itself and several other trace elements from ocean waters. Here we report the longest thus far 17 500 km length full-depth ocean section of dissolved Mn in the West Atlantic Ocean, comprising 1320 data values of high accuracy. This is the GA02 transect that is part of the GEOTRACES programme, which aims to understand trace element distributions. The goal of this study is to combine these new observations with a new, state-of-the-art, modelling to give a first assessment of the main sources and redistribution of Mn throughout the ocean. To this end, we simulate the distribution of dissolved Mn using a global-scale circulation model. This first model includes simple parameterisations to account for the sources, processes and sinks of Mn in the ocean. Oxidation and (photo)reduction, aggregation, settling, as well as biological uptake and remineralisation by plankton, are included in the model. Our model provides, together with the observations, the following insights: -The high surface concentrations of manganese are caused by the combination of photoreduction and sources to the upper ocean. The most important sources are sediments, dust, and, more locally, rivers. -Observations and model simulations suggest that surface Mn in the Atlantic Ocean moves downwards into the southward flowing North Atlantic Deep Water (NADW), but because of strong removal rates there is no elevated concentration of Mn visible any more in the NADW south of 40 • N.-The model predicts lower dissolved Mn in surface waters of the Pacific Ocean than the observed concentrations. The intense Oxygen Minimum Zone (OMZ) in subsurface waters is deemed to be a major source of dissolved Mn also mixing upwards into surface waters, but the OMZ is not well represented by the model. Improved high resolution simulation of the OMZ may solve this problem.-There is a mainly homogeneous background concentration of dissolved Mn of about 0.10 nM to 0.15 nM throughout most of the deep ocean. The model reproduces this by means of a threshold on particulate manganese oxides of 25 pM, suggesting that a minimal concentration of particulate Mn is needed before aggregation and removal become efficient.-The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vents.

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Section: Export Dynamicsmentioning

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

et al. 2017

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“…All samples were analyzed for soluble Mn speciation within two hours of collection following the addition of a soluble porphyrin ligand [α, β, γ, δ-tetrakis(4-carboxyphenyl)porphine (T(4-CP)P)] , according to the method described by Madison et al (2011). The method allows for the subsequent spectrophotometric determination of both Mn(II) and Mn(III)-L weak (Mn(III)-L weak can be outcompeted by the added porphyrin ligand, and has logK cond <13.2; Oldham et al, 2015;Luther et al, 2015) using the differential kinetics of the reaction between Mn and the added porphyrin which is initially complexed to Cd(II). The Mn(II) displaces the Cd(II) and reacts rapidly with the porphyrin, forming a Mn(III)-porphyrin complex as Mn(II) oxidizes in the presence of oxygen, whereas the Mn(III)-L weak undergoes a ligand substitution reaction with the porphyrin to form the Mn(III)-porphyrin complex (logK cond <13.2, Oldham et al, 2015;.…”

Section: Dissolved Mn Speciationmentioning

confidence: 99%

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

Oldham

Mucci

Tebo

et al. 2017

Geochimica et Cosmochimica Acta

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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...

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“…In fact, these Mn(III)‐L complexes have recently been shown to be a dominant component of dissolved Mn within estuaries and ocean basins (Trouwborst et al, ; Dellwig et al, ; Yakushev et al, ; Madison et al, , ; Oldham et al, , 2017a,b,c). Additionally, Mn(III) has been shown to bind to the same ligands as Fe(III) (Duckworth & Sposito, , ; Parker et al, ) with stability constants similar to or exceeding those of analogous Fe(III)‐L complexes (Luther et al, ). Thus, Mn(III)‐L complexes are now considered to be an important part of the Mn pool and may be a particularly reactive fraction as they can both donate and accept electrons.…”

Section: Introductionmentioning

confidence: 99%

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

2020

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Manganese (Mn) is distributed widely throughout the global ocean, where it cycles between three oxidation states that each play important biogeochemical roles. The speciation of Mn in seawater was previously operationally defined on filtration, with soluble Mn presumed to be Mn(II) and solid-phase Mn as Mn(III/IV) oxides. Recent findings of abundant soluble Mn(III) complexes (Mn(III)-L) highlights the need to reexamine the redox cycling of Mn, as these complexes can donate or accept electrons. To better understand the complex cycling of Mn in coastal waters, the distribution of Mn species at four Northwest Atlantic sites with different characteristics was examined. Diurnal influences on Mn speciation were investigated within a productive site. At all sites, Mn(III)-L complexes dominated, particularly in surface waters, and Mn oxides were low in abundance in surface waters but high in bottom waters. Despite intrasite similarities, Mn speciation was highly variable between our stations, emphasizing the diverse processes that impact Mn redox. Diel Mn measurements revealed that the cycling of Mn is also highly variable over time, even on time scales as short as hours. We observed a change of over 100 nM total Mn over 17 hrs and find that speciation changed drastically. These changes could include contributions from biological, light-mediated, and/or abiotic mechanisms but more likely point to the importance of lateral mixing at coastal sites. This exploration demonstrates the spatial and temporal variability of the Mn redox cycle and indicates that single timepoint vertical profiling is not sufficient when describing the geochemistry of dynamic coastal systems. Plain Language Summary Manganese (Mn) is an essential micronutrient in seawater and animportant player in the reactions of many other biologically relevant elements. The role of Mn in seawater is governed by its speciation: oxidation state (+2 to +4 in natural systems) and coordination environment. Manganese speciation is dominated by three species: soluble Mn(II), soluble Mn(III)-ligand complexes, and solid Mn oxides. Most studies of Mn in seawater focus only on solid versus soluble phase speciation, but this does not adequately describe the redox cycling of Mn because each species has unique chemistry in seawater. Here, we add to the limited studies that examine the speciation of Mn in seawater and find that Mn(III)-ligand complexes dominate our four coastal sites. This is important because Mn(III) can donate or accept electrons, making it particularly reactive and versatile. The complex cycle of Mn in coastal waters is dominated by each site's unique biology, the light regime, and water mixing processes. In addition, we examine the speciation of Mn over a diel cycle and find that Mn speciation changes dramatically from day to night, likely a mixing response. Thus, we find that the cycling of Mn is more variable over space and time than previously thought.

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A kinetic approach to assess the strengths of ligands bound to soluble Mn(III)

Cited by 51 publications

References 30 publications

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

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

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

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