Evidence of molybdenum association with particulate organic matter under sulfidic conditions

Abstract

The geochemical behavior of molybdenum (Mo) in the oceans is closely linked to the presence of sulfide species in anoxic environments, where Fe availability may play a key role in the Mo scavenging. Here, we show that Mo(VI) is reduced in the presence of particulate organic matter (represented by sulfate-reducing bacteria). Molybdenum was immobilized at the surface of both living cells and dead/lysed cells, but not in cell-free control experiments. Experiments were carried out at four different Mo concentrations (0.1 to 2 mm) to yield cell-associated Mo precipitates with little or no Fe, consisting of mainly Mo(IV)-sulfide compounds with molecular structures similar to Mo enzymes and to those found in natural euxinic sediments. Therefore, we propose that Mo removal in natural sulfidic waters can proceed via a non-Fe-assisted pathway that requires particulate organic matter (dead or living sulfate-reducing bacteria). This pathway has implications for global marine Mo cycling and the current use of Mo-based proxies for paleo-environmental investigations.

Molybdenum speciation and burial pathway in weakly sulfidic environments: Insights from XAFS

“…The spectral similarity of natural samples to APS2 supports earlier suggestions that sedimentary Mo exists in multiple phases (Chappaz et al, 2014;Helz et al, 1996) and that Mo is strongly associated with organic matter (Algeo and Lyons, 2006;Ardakani et al, 2016;Dahl et al, 2017;Glass et al, 2013;Helz et al, 1996;McManus et al, 2006). (For example, catecholate moieties associated with tannins in leaf litter have been shown to bind MoO 4 2in soils, preventing its leaching (Wichard et al, 2009).…”

“…In modern marine systems, above a threshold sulfide concentration of ~11 µM (as H 2 S (aq) ), a -geochemical switch‖ is flipped, and complete conversion to tetrathiomolybdate (MoS 4 2-) occurs (Erickson and Helz, 2000;Helz et al, 1996). Fixation in sediments has previously been hypothesized to occur in association with iron-sulfur phases (Bostick et al, 2003;Helz et al, 1996), although recent work has proposed multiple co-occurring pathways involving Mo reduction and/or Mo association with Fe phases, organic matter, and/or polysulfide (Algeo and Lyons, 2006;Ardakani et al, 2016;Chappaz et al, 2014;Dahl et al, 2013;Dahl et al, 2017), highlighting substantial remaining knowledge gaps. Mo in the open ocean have additionally led to the idea that MoO 4 2is an unreactive species.…”

Associations between redox‐sensitive trace metals and microbial communities in a Proterozoic ocean analogue

“…This article is protected by copyright. All rights reserved 2006; Vorlicek, Chappaz, Groskreutz, Young, & Lyons, 2015), buried with Fe(II)-S phases (Helz, Erikson, & Vorlicek, 2019), and/or buried with organic matter independent of Fe or S (Dahl et al, 2017). As a result, a strong covariation between Mo and C org has been demonstrated in euxinic basins and euxinic black shales (where 'euxinia' is defined by persistent hydrogen sulfide in the water column; Lyons, Anbar, Severmann, Scott, & Gill, 2009;Helz & Vorlicek, 2019).…”

A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

“…The sensitivity of sedimentary Mo content to redox fluctuations is due to the conversion of the relatively inert molybdate ion (MoO 2− 4 ) in seawater to a series of particlereactive thiomolybdates (MoO x S 4−x ) under exposure to hydrogen sulfide (H 2 S). Where the concentration of H 2 S aq exceeds ∼ 11 µM, the so-called sulfide switch is activated and a quantitative conversion to tetrathiomolybdate MoS 2− 4 may occur (Helz et al, 1996;Erickson and Helz, 2000), triggering effective fixation of Mo in association with Fe-S phases (Helz et al, 1996(Helz et al, , 2011O'Connor et al, 2015) and organic matter (Helz et al, 1996;Algeo and Lyons, 2006;Dahl et al, 2017). In addition, Mo is reduced from oxidation state (VI) to (IV) during burial in sediments (Dahl et al, 2013).…”