Methylmercury formation in an ice-covered fjord system with redox stratified water column

Oxygen deficiency is increasing in coastal zones and the global ocean, potentially promoting methylmercury (MeHg) formation. Intensive work has been done to understand the factors controlling the methylation of Hg(II) to MeHg in open seas and oceans, but these processes are understudied in fjord systems which also undergo oxygen decreases. We hypothesized that in oxygen-deficient fjords, MeHg formation is influenced by (i) the Hg(II) bioavailability, as mainly controlled by Hg(II) complexes with sulfide and, (ii) the presence of microorganisms able to do Hg methylation (i.e., carrying and expressing hgcAB genes). We studied a fjord in northern Norway with a 40 m deep redox stratified water column to better understand the underlying factors/processes driving MeHg concentrations in zones affected by oxygen deficiency. Along the 40 m depth water column, O₂ concentrations decreased from 440 µM to non-detectable, and H₂S concentrations increased with depth from non-detectable to >600 µM concentrations. Total Hg and MeHg concentrations, methylation (k_m) /demethylation (k_d) rate constants and hgcAB gene abundance were determined at several depths along the vertical redox gradient. MeHg concentrations varied from 0.1 to 5.1 pM with a peak at 10 m depth, after the redox transition zone (6 – 10 m). The MeHg production, determined as the Hg(II) methylation rate constant (k_m), followed a similar vertical pattern with negligible k_m in the oxic zone and an increase with depth up to a maximum of ≈0.0001 h^-1 under anoxic condition. Statistical analysis showed a positive relationship between k_m and the concentration of dissolved Hg(II)-sulfide complexes revealing the importance of these compounds in regulating the Hg(II) availability for MeHg formation in this environment. Metagenomic analysis detected hgcAB genes in samples with low oxygen concentrations, highlighting the possible presence of Hg methylators in this ecosystem. Coupling the chemical and microbial analyses, both Hg(II) availability and the presence and abundance of hgcAB-carrying microorganisms suggest the in-situ MeHg formation in this redox stratified fjord system. The spread of oxygen-deficient coastal zones, e.g. due to global warming and nutrient inputs, is expected to increase Hg(II) bioavailability and expand the niches for Hg methylators. Both these consequences are expected to promote higher MeHg concentrations in coastal areas.