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Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea
Umeå University, Faculty of Science and Technology, Department of Chemistry.
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2018 (English)In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 32, no 10, p. 1498-1513Article in journal (Refereed) Published
Abstract [en]

Oxygen-depleted areas are spreading in coastal and offshore waters worldwide, but the implication for production and bioaccumulation of neurotoxic methylmercury (MeHg) is uncertain. We combined observations from six cruises in the Baltic Sea with speciation modeling and incubation experiments to gain insights into mercury (Hg) dynamics in oxygen depleted systems. We then developed a conceptual model describing the main drivers of Hg speciation, fluxes, and transformations in water columns with steep redox gradients. MeHg concentrations were 2-6 and 30-55 times higher in hypoxic and anoxic than in normoxic water, respectively, while only 1-3 and 1-2 times higher for total Hg (THg). We systematically detected divalent inorganic Hg (Hg-II) methylation in anoxic water but rarely in other waters. In anoxic water, high concentrations of dissolved sulfide cause formation of dissolved species of Hg-II: HgS2H(aq)- and Hg (SH)(2)(0)((aq)). This prolongs the lifetime and increases the reservoir of Hg-II readily available for methylation, driving the high MeHg concentrations in anoxic zones. In the hypoxic zone and at the hypoxic-anoxic interface, Hg concentrations, partitioning, and speciation are all highly dynamic due to processes linked to the iron and sulfur cycles. This causes a large variability in bioavailability of Hg, and thereby MeHg concentrations, in these zones. We find that zooplankton in the summertime are exposed to 2-6 times higher MeHg concentrations in hypoxic than in normoxic water. The current spread of hypoxic zones in coastal systems worldwide could thus cause an increase in the MeHg exposure of food webs.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION , 2018. Vol. 32, no 10, p. 1498-1513
Keywords [en]
mercury, methylmercury, anoxia, hypoxia, methylation, redoxcline
National Category
Oceanography, Hydrology and Water Resources Geochemistry Environmental Sciences
Identifiers
URN: urn:nbn:se:umu:diva-154977DOI: 10.1029/2018GB005942ISI: 000450063500006OAI: oai:DiVA.org:umu-154977DiVA, id: diva2:1275726
Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-12-16Bibliographically approved
In thesis
1. Formation, uptake and bioaccumulation of methylmercury in coastal seas: a baltic sea case study
Open this publication in new window or tab >>Formation, uptake and bioaccumulation of methylmercury in coastal seas: a baltic sea case study
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Methylmercury (MeHg) is a potent neurotoxin which can bioaccumulate to harmful levels in aquatic food webs. Methylmercury formation is a predominantly biotic process which involves phylogenically diverse microorganisms (e.g. iron- or sulfate-reducing bacteria). The formation of MeHg is related to the presence of organic matter (OM) which contains substrates essential for methylating microbes and reduced sulfur ligands (thiols, RSH) that form strong bonds with inorganic mercury (HgII) and affect its bioavailability. In aquatic systems, MeHg is bio-concentrated from the water column to the base of the food web and this step is crucial for MeHg levels found at higher trophic levels. Trophic transfer processes of MeHg in the food web are also of great importance. Discharge of OM in coastal areas affects light conditions needed for phytoplankton growth, and promotes heterotrophy, i.e. bacteria production. This may lead to a shift from the phytoplankton-based to the longer bacteria-based (microbial loop) food web and influence the amount of bioaccumulated MeHg in higher trophic levels. Methylmercury levels in predatory biota is thus affected by the bioavailability of HgII for methylation (studied in Paper I & II), MeHg speciation in the water column, crucial for MeHg incorporation at the base of the food web (Paper III), and the structure of the pelagic food web (Paper IV).In this thesis, it is shown that OM can act as a predictor of dissolved MeHg levels in estuarine and coastal systems. It impacts MeHg levels both by affecting HgII bioavailability (through Hg complexation with humic matter) and the activity of methylating microbes (providing metabolic electron donors) (Paper I). Moreover, elevated concentrations of particulate and dissolved HgII and MeHg, are associated with the presence of pelagic redoxclines in coastal seas. The redoxcline affects HgII speciation in the water column and its bioavailability for methylation (Paper II). It is further shown that the molecular structure of ligands in MeHg complexes affects the kinetics of MeHg uptake in phytoplankton. Rate constants for association of MeHg to the cell surface of a green algae were higher in treatments containing smaller thiol ligands of simpler structure than in treatments with larger thiols and more “branched” structure (Paper III). Finally, it is demonstrated that MeHg bioaccumulation in zooplankton can increase in systems with highly heterotrophic food webs and enhanced loadings of terrestrial OM (Paper IV). Such conditions are expected to occur in northern latitude coastal systems following climate changes.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2019. p. 49
Keywords
Mercury, methylmercury, bioaccumulation, mesososm, isotope tracers, methylation, demethylation, stability constant, kinetic model, coastal sea, ICPMS, LC-MS/MS
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-166499 (URN)978-91-7855-177-4 (ISBN)
Public defence
2020-01-24, KBE301, Lila hörsalen, Linnaeus väg 6, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2019-12-20 Created: 2019-12-16 Last updated: 2019-12-18Bibliographically approved

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Skrobonja, AleksandraLiem-Nguyen, VanBjörn, Erik

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