Assessment of\nIncubation Experiments Using Isotopically\nEnriched Mercury Compounds in Seawater including Dissolved Gaseous\nMercury

The main drivers of mercury (Hg) compound distribution\nin seawater\nare poorly understood, calling for novel spatial and seasonal observations\nof potential transformations. However, scientific progress is hindered\nby a lack of intercomparability among incubation studies and the infrequent\ninclusion of dissolved gaseous mercury species (DGM = elemental Hg\n(Hg(0)) + dimethyl Hg (DMHg)) despite their importance in the biogeochemical\ncycle of Hg. We perform a comprehensive quality assessment on our\nproposed incubation protocol at near ambient concentrations (∼10\n× ambient background) including the formation of DGM on three\ndistinct coastal seawaters and discuss intercomparability with previous\nexperimental approaches. We establish an excellent mass balance for\ntracer isotopes both excluding DGM (<sup>199</sup>Hg = 99.5% and <sup>201</sup>Hg = 100.4%, median) and including DGM (<sup>199</sup>Hg\n= 100.3% and <sup>201</sup>Hg = 101.7%, median). We find a good median\nrelative standard deviation for experimental triplicates (<sup>199</sup>Hg(II) ∼ 1.7%, MM<sup>201</sup>Hg ∼ 1.5%, <sup>199</sup>DGM ∼ 10%, <sup>201</sup>Hg(II) ∼ 13%, and <sup>201</sup>DGM ∼ 22%), enabling the accurate determination of methylation,\ndemethylation, and reduction rate constants at femtomolar concentration\nlevels. We observed Hg(0) formation from MMHg, potentially indicating\nreductive demethylation. This study highlights the practicability\nand importance of incorporating DGM species (here, Hg(0), eventually\nDMHg) in future incubation studies.