The influence of clay minerals, oxides, and humic matter on the methylation and demethylation of mercury by micro‐organisms in freshwater sediments

Abstract

AbstractLaboratory experiments and analysis of field samples showed that clay minerals, Fe and Mn oxides, and humic matter (colloids) have complex and often dramatic effects on the microbial methylation and demethylation of Hg, and on other microbial activities, in lake sediments. Depending on the nature, abundance, and surface chemistry of the colloids, the source of the sediment, the nature of the microbes, and synergistic/antagonistic effects of environmental variables, the colloids either strongly inhibited or stimulated the Hg transformations, had little or no effect, or alternated in their effects. Most of the results suggest specific effects on particular kinds of microbes, and are not attributable to general inhibition or stimulation of microbial growth or to effects due to the binding of Hg by the colloids. The colloids probably alter the species composition of the microbial community and affect the course of ecological succession, upsetting the dynamic balance between methylation and demethylation and causing alternate increases and decreases in methyl mercury (CH3Hg+) levels along with changes in other indicators of microbial activity [CO2 and CH4 production and oxidationreduction potential (Eh)].The role of clays was critically dependent on surface coatings. Clays often interfered with methylation (while in some cases strongly promoting subsequent demethylation); but iron oxide (FeOOH) often promoted methylation, and FeOOH coatings on clay tended to counterbalance the negative influence of the clay. Removal of oxide coatings depressed both methylation and demethylation. Manganese oxide (MnOOH) coatings sometimes promoted methylation, but larger amounts of MnOOH (unlike FeOOH) strongly suppressed methylation. On addition of organic nutrients, oxide coatings enhanced methylation and impeded demethylation; without nutrient enrichment, the reverse tended to occur. Humic matter in solution tended to stimulate methylation; but humic coatings on clay impeded methylation and fostered demethylation. Thus, the effects of natural colloids on Hg speciation are vitally important but variable, inconsistent, and not altogether predictable.