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

We have previously hypothesized that sulfide inhibits Hg methylation by dec reasing its bioavailability to sulfate-reducing bacteria (SRB), the importa nt methylators of Hg in natural sediments. With a view to designing a bioas say to test this hypothesis, we investigated a number of aspects of Hg meth ylation by the SRB Desulfobulbus propionicus, including (i) the relationshi p between cell density and methylmercury (MeHg) production, (ii) the time c ourse of Hg methylation relative to growth stage, (iii) changes in the bioa vailability of an added inorganic Hg (Hg-I) spike over time, and (iv) the d ependence of methylation on the concentration of dissolved Hg-I present in the culture. We then tested the effect of sulfide on MeHg production by thi s microorganism. These experiments demonstrated that under conditions of eq ual bioavailability, per-cell MeHg production was constant through log-phas e culture growth. However, the methylation rate of a new Hg spike dramatica lly 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 b oth changes in Hg complexation and formation of solid phases. At low sulfid e 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 decli ne is consistent with a decrease in the bioavailability of Hg-I, possibly d ue to a decline in the dissolved neutral complex, HgS0.