Role of the Bacterial Organomercury Lyase (MerB) in Controlling Methylmercury Accumulation in Mercury-Contaminated Natural Waters

Abstract

The curious phenomenon of similar levels of methylmercury (MeHg) accumulation in fish from contaminated and pristine environments may be explained by the observation that the proportion of total mercury (HgT) present as MeHg is inversely related to HgT in natural waters. We hypothesize that this "MeHg accumulation paradox" is explained by the quantitative induction of bacterial enzymes that are encoded by the mercury resistance (mer) operon, organomercury lyase (MerB), and mercuric reductase (MerA) by inorganic Hg (Hg[II]). We tested this hypothesis in two ecosystems in New Jersey: Berry's Creek in the Meadowlands (ML) and Pine Barren (PB) lakes. Across all sites, an inverse correlation (r2 = 0.80) between the concentration of HgT (ML, 113-4220 ng L(-1); PB, 0.3-5.4 ng L(-1)) and the proportion of HgT as MeHg (MeHg in ML and PB ranged from 0.08 to 1.6 and from 0.03 to 0.34 ng L(-1), respectively) was observed. The planktonic microbial community in Meadowlands surface waters exhibited adaptation to mercury, the presence of mer genes and mRNA transcripts, and high rates of reductive demethylation (k(deg) = 0.19 day(-1)). In contrast, the microbial community of PB was not adapted to mercury and demonstrated low rates of oxidative demethylation (k(deg) = 0.01 day(-1)). These results strongly support our hypothesis and show that the degradation of MeHg by mer-encoded enzymes by the water column microbiota of contaminated environments can significantly affect the amount of MeHg that is available for entry into the aquatic food web.

Mercury Resistance in Bacterial Strains Isolated from Tailing Ponds in a Gold Mining Area Near El Callao (Bolívar State, Venezuela)

“…However, further research is needed to determine more adequately the co-selection for antibiotic-resistant strains, particularly of pathogenic species to humans and animals. It has been proposed that bacterial-mediated attenuation mechanisms may result in a decrease of Hg levels in aquatic systems [3,18]. The results of the present study clearly show the existence of indigenous bacterial strains with such a potential in El Callao tailing ponds.…”

Macroalgae response to a mercury contamination gradient in a temperate coastal lagoon (Ria de Aveiro, Portugal)

“…In low contamination sites the three macroalgae present similar responses, suggesting that no specific decontamination strategy is occurring. Organic mercury results show a similar pattern (increased percentage of organic mercury in low contamination situation) to that of planktonic communities from contaminated and pristine environments (Schaefer et al, 2004). Adaptation to contamination and enhanced reductive demethylation rates may explain this feature (Schaefer et al, 2004).…”

Factors Influencing Mercury in Freshwater Surface Sediments of Northeastern North America

“…Unlike lotic and organic-rich systems, where sediment MeHg may remain bound in the sediment matrix due to sulfide complexation (Marvin-DiPasquale et al, 2000;Kamman et al, 2004), sediments of rivers, with their higher relative %MeHg values, may provide an important source of MeHg to the food web. Schaefer et al (2004) describe a similar inverse relationship between aqueous HgT and %MeHg in several waterbody types. Using experimental evidence, they showed that reduced %Hg as MeHg in higher HgT systems is the result of active demethylation and subsequent Hg reduction by bacteria that posses organomercury lyase enzymes known as ''MerB'' and MerA.''…”