Persistent Hg contamination and occurrence of Hg-methylating transcript (hgcA) downstream of a chlor-alkali plant in the Olt River (Romania)

Abstract

Chlor-alkali plants using mercury (Hg) cell technology are acute point sources of Hg pollution in the aquatic environment. While there have been recent efforts to reduce the use of Hg cells, some of the emitted Hg can be transformed to neurotoxic methylmercury (MeHg). Here, we aimed (i) to study the dispersion of Hg in four reservoirs located downstream of a chlor-alkali plant along the Olt River (Romania) and (ii) to track the activity of bacterial functional genes involved in Hg methylation. Total Hg (THg) concentrations in water and sediments decreased successively from the initial reservoir to downstream reservoirs. Suspended fine size particles and seston appeared to be responsible for the transport of THg into downstream reservoirs, while macrophytes reflected the local bioavailability of Hg. The concentration and proportion of MeHg were correlated with THg, but were not correlated with bacterial activity in sediments, while the abundance of hgcA transcript correlated with organic matter and Cl− concentration, indicating the importance of Hg bioavailability in sediments for Hg methylation. Our data clearly highlights the importance of considering Hg contamination as a legacy pollutant since there is a high risk of continued Hg accumulation in food webs long after Hg-cell phase out

Diversity of microbial communities potentially involved in mercury methylation in rice paddies surrounding typical mercury mining areas in China

“…Of course, the combining of mercury and OM is subject to many other factors, such as the degradation and mineralization of OM (Gray & Hines, 2009), the concentration and presence of inorganic mercury, and the type and activity of the SRB (Macalady, Mack, Nelson, & Scow, 2000) and sulfate ion concentration (Branfireun, Roulet, Kelly, & Rudd, 1999). These findings are in agreement with previous studies (Gu et al, 2011;Zhu, Han, & Wu, 2017) in soil samples is the net result of a dynamic equilibrium between Hg methylation, MeHg demethylation, and the reduction of Hg 2+ to Hg (Bravo et al, 2016).…”

“…These findings are in agreement with previous studies (Gu et al., ; Zhu, Han, & Wu, ), indicating that OM and pH had a high impact on MeHg production. The concentration of MeHg in soil samples is the net result of a dynamic equilibrium between Hg methylation, MeHg demethylation, and the reduction of Hg 2+ to Hg (Bravo et al., ).…”

“…The observed hgcA abundance in the three mercury mining areas did not exhibit an obvious positive correlation with MeHg concentrations, which is not consistent with a previous study (Du et al., ) that observed a significant positive correlation between hgcA abundance and MeHg concentrations. Although a correlation between the presence of the hgcA in the genome of a particular phylogenetic cluster and methylation ability has been demonstrated in the laboratory (Gilmour et al., ), no relationships between hgcA expression levels and net mercury methylation have been observed (Bravo et al., ; Goñi‐Urriza et al., ). In particular, the MeHg content in sediments resulted in the increased formation of mercury–OM complexes than did net mercury methylation, which could explain the lack of a significant correlation between hgcA gene expression levels and MeHg concentrations.…”

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Two laboratory studies later confirmed the efficiency of methanogens in converting Hg II to MMHg (Yu et al ; Gilmour et al ). By culturing and isolating Hg II ‐methylating strains or by using inhibitors of known Hg II ‐methylators such as molybdate for sulfate‐reduction and BESA for methanogenesis, Hg II ‐methylation has mainly been then attributed to the action of SRB (Devereux et al ; Pak and Bartha ; Hylander ; King et al ; Achá et al , ; Yu et al ; Bravo et al ), and in some cases to FeRB (Fleming et al ; Bravo et al , ) as well as methanogens (Hamelin et al ; Bravo et al ).…”

“…A recent study in sediments collected in eutrophic lakes showing cyanobacteria blooms in China also reported a correlation between Archae hgcA + DNA relative abundance and MMHg concentration (Lei et al ). However, several studies that tried to correlate the level of expression of hgcA mRNA with Hg II methylation rates (Goñi‐Urriza et al ; Bravo et al ; Christensen et al ) were mostly unsuccessful. For example, in pure cultures of Desulfovibrio dechloroacetivorans BerOc1, the level of expression of hgcA was not correlated with Hg II methylation rates (Goñi‐Urriza et al ).…”