Mercury Bioavailability Positively Correlated to the Mercury-Sulfur Ratio of Dissolved Organic Matter

Mercury (Hg) assimilation by microbes in aquatic environments is an important step in bioaccumulation and bioconcentration in the food web. The bioavailability of Hg is in turn strongly influenced by dissolved organic matter (DOM), the bioavailability of which is dependent upon its molecular structure and elemental composition. The bacterial bioreporter, Escherichia coli HMS174 (pRB28/27), was used in an experimental study to assess the bioavailability of Hg in the presence of 3 different DOM samples: Suwanee River NOM, primarily from a terrestrial blackwater source; Upper Mississippi River NOM, mainly a terrestrial-derived DOM; and Pony Lake fulvic acids isolated from an entirely microbial source. The patterns of mercury bioavailability were assessed in three different Hg and dissolved organic carbon (DOC) concentrations and statistically analyzed using linear regression and correlations to the DOM characteristics, including elemental composition, molecular structure, and optical properties through fluorescence 3D excitation emission matrices. Results indicated Hg bioavailability was most strongly correlated to the sulfur content of DOM, more specifically, the Hg:S ratios between 90 and 50 ng Hg mg-1 S (r = -0.94, p = 0.22). As the relative sulfur content of DOM increased, so did Hg bioavailability, which may be due to the strong affinity Hg2+ has to sulfides. Under natural conditions in some environments, crystallized nanoparticles of HgS form, leaving Hg2+ no longer available for binding to DOM and nanoparticles of this compound available to be taken up by microbes, thereby increasing Hg bioavailability.