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Abstract Title: Abundance and Expression of hgc Genes and Hg Speciation Drive Methylmercury Formation in Stratified Brackish Waters
Presenter Name: Erik Björn
Company/Institution: Umeå University
Session: Mercury in Marine Ecosystems
Day and Session: Thursday 28th July - Session Two
Start Time: 10:00 UTC
Co-Authors: Erik Björn,Eric Capo,Caiyan Feng,Andrea G. Bravo,Stefan Bertilsson,Anne Soerensen,Jarone Pinhassi,Moritz Buck,Camilla Karlsson,Jeffrey Hawkes

Abstract Information :

Microbial methylation of inorganic divalent mercury (HgII) to methylmercury (MeHg) occurs predominantly in oxygen-deficient environments and is enabled by hgcA and hgcB genes. Oxygen deficiency is spreading in coastal seas and the global ocean which potentially opens new habitats for HgII methylators. It is, however, uncertain if the molecular-level methylation processes mediated by the hgc genes are rate-limiting or enable accurate prediction of MeHg formation in nature. This knowledge gap contributes to uncertainty also in forecasting the impact on MeHg formation from environmental change processes. In this study, we investigated the relationships between hgc genes and MeHg across redox stratified water columns in the brackish central Baltic Sea. The hgc gene and transcript abundances and HgII methylation rate increased with decreasing redox potential and were highest in deep euxinic waters with high dissolved sulfide concentrations. Metagenomics and metatranscriptomics analyses pointed to sulfate reduction, fermentation and hydrogen oxidation as the three principal metabolisms among putative HgII methylators in oxygen-deficient waters of the Baltic Sea. We found significant relationships between hgcA gene or transcript abundance and MeHg formation rate or concentration. Our results further show that models with both hgcA gene or transcript abundance and HgII chemical speciation improved the prediction of MeHg formation, suggesting that hgcA and HgII availability jointly control MeHg formation in stratified pelagic waters. The unraveling of these mechanistic principles governing MeHg formation in oxygen-deficient coastal seas is an important step to refine predictive frameworks for MeHg exposure to marine food webs and humans from current and future spread of oxygen deficiency in coastal waters and the global ocean, as well as for MeHg formation in the environment in general.

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