Biological and photochemical production of dissolved gaseous mercury in a boreal lake

Abstract

We used in situ experiments and measured depth profiles of dissolved gaseous mercury (DGM) to investigate the relative contribution of photochemical versus biological processes on the production of DGM in an oligomesotrophic lake of the Canadian Shield. At the surface, DGM production was mainly photomediated, with reduction rates being twice as high in the wetland than in the lake. In the water column, the distribution of DGM concentrations was not strictly related to light but followed a multimodal distribution, with peaks encountered below the epilimnion at depths receiving Ͻ5% of the incident light. Those peaks were recorded in the middle and at the bottom of the metalimnion during the ice-free season, as well as under ice cover and at the bottom of an anoxic hypolimnion. Rather than being a consequence of the bacterial mercuric reductase activity, metalimnetic DGM peaks were associated with the intensity and duration of phytoplankton blooms. In situ incubation experiments also showed that DGM production ceased when samples were kept in the dark, filtered, or when an inhibitor of photosynthesis was added. Our results illustrate the important role of phytoplankton on Hg redox dynamics in the water column of lakes.

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[87] The fate of mercury deposited to the Arctic In temperate environments some bacteria carry genes that make them resistant to Hg II and MeHg toxicity because they convert these Hg compounds into the volatile and less toxic Hg 0 . [88] Algae are also capable of catalysing Hg 0 production [43,58,59,89] and algae have been found in Arctic snow. [90] However, their contribution to the Hg redox cycle in snow is unknown.…”

“…At depth, other processes likely to be associated with microbial activity dominate. [57][58][59] The relative importance of photochemical v. biological processes in controlling the reduction rate of Hg II in Arctic freshwaters remains to be elucidated. The rate is controlled by: the intensity of solar radiation, particularly the UV-B (280-320 nm) and UV-A (320-400 nm) wavebands, and the concentration of available photo-reducible Hg II complexes.…”

Mercury reduction and complexation by natural organic matter in anoxic environments

“…It is known to form exceptionally strong complexes with the oxidized mercuric species, Hg(II), due to its coordination with reduced sulfur (−S) or thiol (−SH) functional groups in DOM at relatively high DOM:Hg(II) ratios (11,(18)(19)(20)(21). Such complexation has been shown to limit Hg(II) availability for bacterial methylation (9, 22, 23); however, facilitated uptake and methylation are also reported, especially when Hg(II) is complexed with small molecular-weight thiol compounds such as cysteine (5,24).Although a large body of literature is now available on the interactions of oxidized Hg(II) species with DOM, reactions between reduced gaseous Hg(0) and DOM have rarely been examined in natural sediments and water where dissolved Hg(0) is also observed (16,17,(25)(26)(27)(28)(29)(30)(31). Hg(0) has a solubility of ∼56 μg/L in water (32).…”

“…Although a large body of literature is now available on the interactions of oxidized Hg(II) species with DOM, reactions between reduced gaseous Hg(0) and DOM have rarely been examined in natural sediments and water where dissolved Hg(0) is also observed (16,17,(25)(26)(27)(28)(29)(30)(31). Hg(0) has a solubility of ∼56 μg/L in water (32).…”

Changes in the non-protein thiol pool and production of dissolved gaseous mercury in the marine diatom Thalassiosira weissflogii under mercury exposure

“…Field and laboratory studies suggest that, besides bacteria, phytoplankton can play an important role in the processes of the formation of DGM, through an indirect contribution due to the release of biogenic organic matter involved in the photochemical reactions of DGM production, as well as through a cellular direct reduction (Mason et al, 1995;Devars et al, 2000;Lanzillotta et al, 2004;Poulain et al, 2004).…”