Fig 2. Methylation rate constant determined using different Hg^II tracers in sediments with a) high organic carbon content (HOC 1 and HOC 2) and b) low organic carbon content (LOC 1 and LOC 2).

The methylation rate constant (k_m, d^-1; ±SD, n=3) was determined by adding Hg^II to sediment slurries as an inorganic aqueous tracer (Hg^II(aq), white bars), as Hg^II bound to particulate organic matter (Hg^II-POM, green bars), as β-HgS(s) nanoparticles (β-HgS(s)_nano, gray bars), and as Hg^II bound to dissolved organic matter (Hg^II-DOM, blue bars). To test how POM and DOM added with the Hg^II-POM, β-HgS(s)_nano, and Hg^II-DOM tracers affected Hg^II methylation rates when not complexed to the Hg^II substrate (by e.g. altering the activity of Hg^II methylating bacteria), control experiments were conducted. In these control experiments, the Hg^II(aq) tracer was added to the sediment slurries simultaneously with POM (squares), β-HgS(s)_nano (circles) or DOM (triangles). For the controls, error bars show ±SD calculated from n=2. An * associated with any bar or symbol indicates the data < Limit of Detection (LOD) and this value is shown.