Aspects of bioavailability of mercury for methylation in pure cultures of Desulfobulbus propionicus (1pr3).

We have previously hypothesized that sulfide inhibits Hg methylation by decreasing its bioavailability to sulfate-reducing bacteria (SRB), the important methylators of Hg in natural sediments. With a view to designing a bioassay to test this hypothesis, we investigated a number of aspects of Hg methylation by the SRB Desulfobulbus propionicus, including (i) the relationship between cell density and methylmercury (MeHg) production, (ii) the time course of Hg methylation relative to growth stage, (iii) changes in the bioavailability of an added inorganic Hg (Hg(I)) spike over time, and (iv) the dependence of methylation on the concentration of dissolved Hg(I) present in the culture. We then tested the effect of sulfide on MeHg production by this microorganism. These experiments demonstrated that under conditions of equal bioavailability, per-cell MeHg production was constant through log-phase culture growth. However, the methylation rate of a new Hg spike dramatically decreased after the first 5 h. This result was seen whether methylation rate was expressed as a fraction of the total added Hg or the filtered Hg(I) concentration, which suggests that Hg bioavailability decreased through both changes in Hg complexation and formation of solid phases. At low sulfide concentration, MeHg production was linearly related to the concentration of filtered Hg(I). The methylation of filtered Hg(I) decreased about fourfold as sulfide concentration was increased from 10(-6) to 10(-3) M. This decline is consistent with a decrease in the bioavailability of Hg(I), possibly due to a decline in the dissolved neutral complex, HgS(0).