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Uptake Kinetics of Methymerucy in a Freshwater Alga Exposed to Methylmercury Complexes with Environmentally Relevant Thiols
Umeå University, Faculty of Science and Technology, Department of Chemistry. (Erik Björn's research group)
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.ORCID iD: 0000-0002-9277-4534
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2019 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 53, no 23, p. 13757-13766Article in journal (Refereed) Published
Abstract [en]

Cellular uptake of dissolved methylmercury (MeHg) by phytoplankton is the most important point of entry for MeHg into aquatic food webs. However, the process is not fully understood. In this study we investigated the influence of chemical speciation on rate constants for MeHg accumulation by the freshwater green microalga Selenastrum capricornutum. We used six MeHg–thiol complexes with moderate but important structural differences commonly found in the environment. Rate constants for MeHg interactions with cells were determined for the MeHg–thiol treatments and a control assay containing the thermodynamically less stable MeHgOH complex. We found both elevated amounts of MeHg associated with whole cells and higher MeHg association rate constants in the control compared to the thiol treatments. Furthermore, the association rate constants were lower when algae were exposed to MeHg complexes with thiols of larger size and more “branched” chemical structure compared to complexes with simpler structure. The results further demonstrated that the thermodynamic stability and chemical structure of MeHg complexes in the medium is an important controlling factor for the rate of MeHg interactions with the cell surface, but not for the MeHg exchange rate across the membrane. Our results are in line with uptake mechanisms involving formation of MeHg complexes with cell surface ligands prior to internalization.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2019. Vol. 53, no 23, p. 13757-13766
National Category
Other Chemistry Topics
Identifiers
URN: urn:nbn:se:umu:diva-166177DOI: 10.1021/acs.est.9b05164Scopus ID: 2-s2.0-85075442114OAI: oai:DiVA.org:umu-166177DiVA, id: diva2:1378886
Available from: 2019-12-16 Created: 2019-12-16 Last updated: 2019-12-20Bibliographically 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)
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Available from: 2019-12-20 Created: 2019-12-16 Last updated: 2019-12-18Bibliographically approved

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Skrobonja, AleksandraGojkovic, ZivanWestlund, Per-OlofFunk, ChristianeBjörn, Erik

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