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Multiple impacts of humic-rich dissolved organic carbon on methylmercury accumulation in heterotrophic pelagic food webs
Umeå University, Faculty of Science and Technology, Department of Chemistry.
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(English)Manuscript (preprint) (Other academic)
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.

National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-166498OAI: oai:DiVA.org:umu-166498DiVA, id: diva2:1379204
Available from: 2019-12-16 Created: 2019-12-16 Last updated: 2019-12-19
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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Supervisors
Available from: 2019-12-20 Created: 2019-12-16 Last updated: 2019-12-18Bibliographically approved

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