Simultaneous determination of mercury methylation and demethylation capacities of various sulfate‐reducing bacteria using species‐specific isotopic tracers

Abstract

The use of species-specific isotopic tracers for inorganic and methyl mercury has allowed the simultaneous determination of the methylation and demethylation potentials of pure culture of isolated sulfate-reducing (SR) bacterial strains using low Hg species concentration levels (7 µg/L (199)Hg(II), 1 µg/L Me(201)Hg). A major advantage of the method reported here is that it can be used to follow simultaneously both the degradation of the species added but also the formation of their degradation products and thus the determination during the same incubation of the specific methylation/demethylation yields and rate constants. Methylation/demethylation capacities and extents have been found to differ between the tested strains and the tested conditions. The methylating/demethylating capacities of bacteria appear to be strain specific. All the methylating strains were found to demethylate methylmercury (MeHg). The active mechanism responsible for Hg methylation appears directly dependent on the bacterial activity but is not dependent on the metabolism used by the tested bacteria (sulfate reduction, fermentation, or nitrate respiration). The results provide confirmation that SR strains contribute to MeHg demethylation under anoxic conditions, leading to Hg(II) as the end product, consistent with the oxidative degradation pathway. Kinetic experiments have allowed specific transformation rate constants to be addressed for the two reversible processes and the reactivity of each isotopic tracer to be compared. The differential reactivity highlighted the different steps involved in the two apparent processes (i.e., uptake plus internal transformation of mercury species). Methylation appears as the slowest process, mainly controlled by the assimilation of Hg(II), whereas demethylation is faster and not dependent on the MeHg concentration.

Detailed Assessment of the Kinetics of Hg-Cell Association, Hg Methylation, and Methylmercury Degradation in Several Desulfovibrio Species

“…These differences likely reflect interacting dependencies on Hg concentration, cell density and/or growth stage, temperature, and medium chemistry (4,11,20,21,38,50). Such variability complicates efforts to make comparisons across organisms or studies.…”

“…(21, 37), all belonging to the Deltaproteobacteria. Many field studies, using selective microbial stimulants (1, 10, 26, 44, 57), inhibitors (1, 16, 24, 26, 59), and biogeochemical correlates (6,39,40,45), have buttressed the paradigm of SRB and FeRB as the dominant Hg methylators in natural aquatic systems (16,24,59), though recent studies have hypothesized that methanogens may be significant in some systems (31).Only a subset of SRB and FeRB are capable of Hg methylation (11,23,37,50), but why this is the case remains unclear. Early work by Choi and Bartha (13) suggested that Hg methylation was a "metabolic mistake" of SRB utilizing the acetyl coenzyme A (acetyl-CoA) pathway for carbon metabolism.…”

“…Early investigations, prior to the advent of modern methylmercury (MeHg) analyses, reported a wide variety of aerobic and anaerobic Gram-positive and Gram-negative bacteria (30,49,55,58) and fungi (55) to be capable of MeHg production. However, subsequent studies with pure cultures have conclusively demonstrated a role only for sulfate-reducing bacteria (SRB) (4,8,11,13,20,23,38,50) and iron-reducing bacteria (FeRB; principally Geobacter spp.) (21, 37), all belonging to the Deltaproteobacteria.…”

“…Only a subset of SRB and FeRB are capable of Hg methylation (11,23,37,50), but why this is the case remains unclear. Early work by Choi and Bartha (13) suggested that Hg methylation was a "metabolic mistake" of SRB utilizing the acetyl coenzyme A (acetyl-CoA) pathway for carbon metabolism.…”

Identification of mercury methylation product by tert-butyl compounds in aqueous solution under light irradiation

“…Microorganisms, such as sulfate-reducing bacteria and ion-reducing bacteria (Bridou et al, 2011;Fleming et al, 2006), are the major microbial contributors to MeHg production in the environment. However, the abiotic methylation of Hg, which has been validated as an important pathway for MeHg production (Hammerschmidt et al, 2007), has not been studied thoroughly.…”