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Formation, uptake and bioaccumulation of methylmercury in coastal seas: a baltic sea case study
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Kemiska institutionen.
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Umeå: Umeå University , 2019. , s. 49
Nyckelord [en]
Mercury, methylmercury, bioaccumulation, mesososm, isotope tracers, methylation, demethylation, stability constant, kinetic model, coastal sea, ICPMS, LC-MS/MS
Nationell ämneskategori
Kemi
Identifikatorer
URN: urn:nbn:se:umu:diva-166499ISBN: 978-91-7855-177-4 (digital)OAI: oai:DiVA.org:umu-166499DiVA, id: diva2:1379211
Disputation
2020-01-24, KBE301, Lila hörsalen, Linnaeus väg 6, Umeå, 09:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2019-12-20 Skapad: 2019-12-16 Senast uppdaterad: 2019-12-18Bibliografiskt granskad
Delarbeten
1. Organic matter drives high interannual variability in methylmercury concentrations in a subarctic coastal sea
Öppna denna publikation i ny flik eller fönster >>Organic matter drives high interannual variability in methylmercury concentrations in a subarctic coastal sea
2017 (Engelska)Ingår i: Environmental Pollution, ISSN 0269-7491, E-ISSN 1873-6424, Vol. 229, s. 531-538Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Levels of neurotoxic methylmercury (MeHg) in phytoplankton are strongly associated with water MeHg concentrations. Because uptake by phytoplankton is the first and largest step of bioaccumulation in aquatic food webs many studies have investigated factors driving seasonal changes in water MeHg concentrations. Organic matter (OM) is widely accepted as an important driver of MeHg production and uptake by phytoplankton but is also known for strong interannual variability in concentration and composition within systems. In this study, we explore the role of OM on spatial and interannual variability of MeHg in a subarctic coastal sea, the northern Baltic Sea. Using MeHg (2014: 80 ± 25 fM; 2015: <LOD; 2016: 21 ± 9 fM) and OM measurements during late summer/early fall, we find that dissolved organic carbon (DOC) and humic matter content explain 60% of MeHg variability. We find that while labile DOC increases MeHg levels in the water, humic content reduces it. We propose that the positive association between MeHg and labile DOC shows that labile DOC is a proxy for OM remineralization rate in nearshore and offshore waters. This is consistent with other studies finding that in situ MeHg production in the water column occurs during OM remineralization. The negative association between water humic content and MeHg concentration is most likely due to humic matter decreasing inorganic mercury (HgII) bioavailability to methylating microbes. With these relationships, we develop a statistical model and use it to calculate MeHg concentrations in late summer nearshore and offshore waters between 2006 and 2016 using measured values for water DOC and humic matter content. We find that MeHg concentrations can vary by up to an order of magnitude between years, highlighting the importance of considering interannual variability in water column MeHg concentrations when interpreting both short and long term MeHg trends in biota.

Ort, förlag, år, upplaga, sidor
Elsevier, 2017
Nyckelord
Methylmercury, Labile dissolved organic matter, Humic matter, Hg methylation, Interannual variability
Nationell ämneskategori
Kemi
Identifikatorer
urn:nbn:se:umu:diva-139654 (URN)10.1016/j.envpol.2017.06.008 (DOI)000410010200056 ()28646796 (PubMedID)
Tillgänglig från: 2017-09-20 Skapad: 2017-09-20 Senast uppdaterad: 2019-12-16Bibliografiskt granskad
2. Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea
Öppna denna publikation i ny flik eller fönster >>Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea
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2018 (Engelska)Ingår i: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 32, nr 10, s. 1498-1513Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
AMER GEOPHYSICAL UNION, 2018
Nyckelord
mercury, methylmercury, anoxia, hypoxia, methylation, redoxcline
Nationell ämneskategori
Oceanografi, hydrologi och vattenresurser Geokemi Miljövetenskap
Identifikatorer
urn:nbn:se:umu:diva-154977 (URN)10.1029/2018GB005942 (DOI)000450063500006 ()
Tillgänglig från: 2019-01-07 Skapad: 2019-01-07 Senast uppdaterad: 2019-12-16Bibliografiskt granskad
3. Uptake Kinetics of Methymerucy in a Freshwater Alga Exposed to Methylmercury Complexes with Environmentally Relevant Thiols
Öppna denna publikation i ny flik eller fönster >>Uptake Kinetics of Methymerucy in a Freshwater Alga Exposed to Methylmercury Complexes with Environmentally Relevant Thiols
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2019 (Engelska)Ingår i: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 53, nr 23, s. 13757-13766Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
American Chemical Society (ACS), 2019
Nationell ämneskategori
Annan kemi
Identifikatorer
urn:nbn:se:umu:diva-166177 (URN)10.1021/acs.est.9b05164 (DOI)2-s2.0-85075442114 (Scopus ID)
Tillgänglig från: 2019-12-16 Skapad: 2019-12-16 Senast uppdaterad: 2019-12-20Bibliografiskt granskad
4. Multiple impacts of humic-rich dissolved organic carbon on methylmercury accumulation in heterotrophic pelagic food webs
Öppna denna publikation i ny flik eller fönster >>Multiple impacts of humic-rich dissolved organic carbon on methylmercury accumulation in heterotrophic pelagic food webs
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(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Abstract [en]

In this study we investigate how humic-rich dissolved organic carbon (DOC) impacts the bioaccumulation of methylmercury (MeHg) in coastal pelagic lower food webs. Mesocosm scale ecosystems with four levels of humic-rich DOC (ranging from 4 mg L-1 – no added DOC - up to 8.0 mg L-1) were run for 36 days and the bioaccumulation factor (BAF) of MeHg was determined in four seston size fractions at the end of the experiment. The pelagic food web was highly heterotrophic in all treatments with 72%−88% bacteria production of the total basal production. Increases in humic-rich DOC in coastal areas, manifested in our study as a shift towards more heterotrophic systems, can impact the bioaccumulation of MeHg through multiple processes. We found an increase in dissolved MeHg concentration with increased DOC loading, indicating a shift in partitioning from the particulate to the dissolved phase. However, a separate experiment showed that the presence of the humic-rich DOC lowered the bioavailability of dissolved MeHg with 40% across our study range, supporting previous results that the amount of MeHg incorporated at the base of the food web is not simply determined by the total concentration of dissolved MeHg. Our determined BAFs from the experiment supported this conclusion and we also calculated a corrected BAF’ value that took into account the change in MeHg bioavailability to isolate the impact on biomagnification processes. The range in MeHg log BAF’ values was 4.5−4.7 and 5.8−6.0 for 20-50 μm and >200 μm seston size fractions, respectively. Higher BAF’ values were observed in the highest DOC treatment and in our reference for some seston size classes. The high BAF’ at the highest DOC level may be explained by the large fraction of bacteria production observed for this treatment. The high BAF in the reference system could be explained by a slightly larger proportion of energy going through the autotrophic food web causing a higher trophic transfer efficiency of MeHg at the low trophic levels in the web. Taken together our results demonstrate a complex impact of humic DOC and pelagic food web heterotrophy on MeHg bioaccumulation in seston with partly counteracting processes. A detailed understanding of the factors controlling these multiple processes are important for accurate predictions of the net impact on MeHg bioaccumulation in coastal food webs following different environmental change scenarios.

Nationell ämneskategori
Kemi
Identifikatorer
urn:nbn:se:umu:diva-166498 (URN)
Tillgänglig från: 2019-12-16 Skapad: 2019-12-16 Senast uppdaterad: 2019-12-19

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