Molybdenum reduction in a sulfidic lake: Evidence from X-ray absorption fine-structure spectroscopy and implications for the Mo paleoproxy

Dahl, Tais W.; Chappaz, Anthony; Fitts, Jeffrey P.; Lyons, Timothy W.

doi:10.1016/j.gca.2012.10.058

Cited by 123 publications

(74 citation statements)

References 61 publications

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“…In the sulfide-rich OAE oceans (20), bioavailable molybdate would have been scavenged into sedimentary sulfides (21-23) and organic matter (24) via formation of particle reactive thiomolybdates (21-23) and reduced Mo-polysulfide species (25,26). This would have lowered the concentration of Mo in seawater significantly (17,(27)(28)(29)(30)(31).…”

Section: Resultsmentioning

confidence: 99%

Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia

Zhang¹,

Sigman²,

Morel³

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

176

172

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Biological nitrogen fixation constitutes the main input of fixed nitrogen to Earth's ecosystems, and its isotope effect is a key parameter in isotope-based interpretations of the N cycle. The nitrogen isotopic composition (δ 15 N) of newly fixed N is currently believed to be ∼-1‰, based on measurements of organic matter from diazotrophs using molybdenum (Mo)-nitrogenases. We show that the vanadium (V)-and iron (Fe)-only "alternative" nitrogenases produce fixed N with significantly lower δ 15 N (-6 to -7‰). An important contribution of alternative nitrogenases to N 2 fixation provides a simple explanation for the anomalously low δ 15 N (<-2‰) in sediments from the Cretaceous Oceanic Anoxic Events and the Archean Eon. A significant role for the alternative nitrogenases over Mo-nitrogenase is also consistent with evidence of Mo scarcity during these geologic periods, suggesting an additional dimension to the coupling between the global cycles of trace elements and nitrogen. Mo is ∼+2‰ (3-6)], which is thought to be the most abundant in nature. As a result, the δ 15 N of newly fixed N is ∼-1‰ (i.e., ∼+2‰ lower than the δ 15 N of dissolved N 2 substrate, +0.7‰, Fig. 1A).In addition to Mo-nitrogenase (the most common form of the enzyme), diazotrophs can possess two other nitrogenase isozymes (7). These so-called "alternative" nitrogenases differ chiefly from Mo-nitrogenases in that V or Fe replaces Mo in the active site. They also contain an additional protein subunit and exhibit slower kinetics compared with the Mo-nitrogenase (8). Such differences could result in distinct isotope effects, a possibility supported by Rowell et al. (9), who reported small but significant variations in biomass δ 15 N from growth of wild-type diazotrophs possessing all three isozymes in media containing Fe and amendments of Mo, V, or neither metal. However, the use of multiple nitrogenase isozymes in the wild type (10) precludes the direct association between biomass δ 15 N and the isotope effect of a particular isozyme. Here we (i) measured directly the isotope effects for Mo-, V-, and Fe-only nitrogenases using diazotroph mutant strains that could express only a single nitrogenase isozyme, (ii) determined the impact of metal limitation on alternative nitrogenase use and N isotope fractionation in wild-type bacteria, and (iii) provide several examples of how these results on N isotope fractionation may change our understanding of the N cycle in the past. Results and DiscussionIsotope Fractionation During Nitrogen Fixation by Mo-, V-, and Fe-only Nitrogenases. We measured the in vivo isotope effect associated with each type of nitrogenase in two phylogenetically and metabolically distinct diazotrophic bacteria, Rhodopseudomonas palustris and Azotobacter vinelandii. R. palustris is an alpha-proteobacterium. It fixes N 2 anaerobically and was grown under anaerobic and photoheterotrophic conditions. A. vinelandii is a gamma-proteobacterium. It fixes N 2 aerobically and was grown under aerobic chemoheterotrophic conditions. All three nitroge...

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Section: Resultsmentioning

confidence: 99%

Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia

Zhang¹,

Sigman²,

Morel³

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

176

172

View full text Add to dashboard Cite

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“…Accordingly, in the Stoer Group, the Mo is a trace component of pyrite, which precipitated early, before compaction of the host shale, due to microbial reduction of sulphate in the lake waters 36 . Mo in modern euxinic stratified lake waters is similarly sequestered rapidly into iron sulphides 40,41 . A broad correlation between Mo content and S content ( Supplementary Fig.…”

Section: Resultsmentioning

confidence: 99%

High Molybdenum availability for evolution in a Mesoproterozoic lacustrine environment

et al. 2015

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Trace metal data for Proterozoic marine euxinic sediments imply that the expansion of nitrogen-fixing cyanobacteria and diversification of eukaryotes were delayed while the availability of bioessential metals such as molybdenum in the ocean was limited. However, there is increasing recognition that the Mesoproterozoic evolution of nitrogen fixation and eukaryotic life may have been promoted in marginal marine and terrestrial environments, including lakes, rather than in the deep ocean. Molybdenum availability is critical to life in lakes, just as it is in the oceans. It is, therefore, important to assess molybdenum availability to the lacustrine environment in the Mesoproterozoic. Here we show that the flux of molybdenum to a Mesoproterozoic lake was 1 to 2 orders of magnitude greater than typical fluxes in the modern and ancient marine environment. Thus, there was no barrier to availability to prevent evolution in the terrestrial environment, in contrast to the nutrient-limited Mesoproterozoic oceans.

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“…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). Under non-euxinic conditions, where H 2 S is only found in pore waters but not in bottom waters, the initial sedimentation of water column Mo often occurs through adsorption to solidphase manganese (Mn) oxides at the sediment-water interface (Scott and Lyons, 2012;Scholz et al, 2013;Noordmann et al, 2015).…”

Section: Molybdenum Contentmentioning

confidence: 99%

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

et al. 2018

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Abstract. The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous paleoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that although multicentennial climate variability has modulated the depositional conditions and delivery of organic matter (OM) to the basin the modern aggravation of coastal hypoxia is unprecedented and, in addition to gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. Alongside the anthropogenic nutrient input, the progressive deoxygenation since the beginning of the 1900s was fueled by the combined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s. We find no evidence of similar anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.

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Molybdenum reduction in a sulfidic lake: Evidence from X-ray absorption fine-structure spectroscopy and implications for the Mo paleoproxy

Cited by 123 publications

References 61 publications

Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia

Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia

High Molybdenum availability for evolution in a Mesoproterozoic lacustrine environment

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

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