Fig. 1. Illustration of the experimental design.

Starting from the top, the illustration shows the complexing agents used to synthesize the different Hg^II tracers utilized in this study. To prepare Hg^II-POM, Hg^II-DOM, Hg^II-HOC_sediment and Hg^II-LOC_sediment, the isotopically enriched Hg^II(aq) tracer was equilibrated for 24 h (gray field) with the complexing agent. For the case of HgS(s) particles, β-HgS(s)_micro was formed when S^2- was added to the Hg^II(aq) tracer while β-HgS(s)_nano was formed when DOM followed by S^2- were added to the Hg^II (aq) tracer. Both particles were aged for 3 days (pink field). The synthesized tracers were then added to four different sediment slurries and incubated for 0 h and 12 h. Finally, the methylation rate constant (k_m) was calculated using the formula shown in the bottom of the illustration. Dashed lines represent control experiments where k_m of Hg^II(aq) was quantified in presence of POM, DOM or β-HgS(s)_nano (by the simultaneous addition of Hg^II(aq) with POM, DOM or β-HgS(s)_nano to sediment slurries). If methylation was not different from the k_m of Hg^II(aq) when added without DOM, POM or β-HgS(s)_nano, we conclude that the complexing agent did not impact the rate of methylation by e.g. altering the activity of Hg^II methylating bacteria.