Mercury methylation and sulfate reduction rates in mangrove sediments, Rio de Janeiro, Brazil: The role of different microorganism consortia.

Recent studies have shown Hg methylation in mangrove sediments, however, little is known about the different microorganism consortia involved. We investigated the participation of prokaryotes in general, iron-reducing bacteria-IRB, sulfate-reducing bacteria-SRB, methanogens and fungi in Hg methylation and sulfate reduction rates (SRR) in mangrove sediments using iron amendments for IRB and specific inhibitors for the other microorganisms. Sediment samples were collected from two mangrove zones, tidal flat and mangrove forest (named root sediments). Samples were incubated with 203Hg or 35SO42- and Me203Hg/35Sulfur were measured by liquid scintillation. Methylmercury (MeHg) formation was significantly reduced when SRB (87.7%), prokaryotes (76%) and methanogens (36.5%) were inhibited in root sediments, but only SRB (51.6%) and prokaryotes (57.3%) in tidal flat. However, in the tidal flat, inhibition of methanogens doubled Hg methylation (104.5%). All inhibitors (except fungicide) significantly reduced SRR in both zones. In iron amended tidal flat samples, Hg methylation increased 56.5% at 100 µg g-1 and decreased at 500 and 1000 µg g-1 (57.8 and 82%). In the roots region, however, MeHg formation gradually decreased in response to Fe amendments from 100 µg g-1 (37.7%) to 1000 µg g-1 (93%). SRR decreased in all iron amendments. This first simultaneous evaluation of Hg methylation and sulfate-reduction and of the effect of iron and inhibitors on both processes suggest that SRB are important Hg methylators in mangrove sediments. However, it also suggests that SRB activity could not explain all MeHg formation. This implies the direct or indirect participation of other microorganisms such as IRB and methanogens and a complex relationship among these groups.

Impacts of crab bioturbation and local pollution on sulfate reduction, Hg distribution and methylation in mangrove sediments, Rio de Janeiro, Brazil.

Mercury (Hg) and methylmercury (MeHg) are highly toxic and poorly studied in mangroves. Burrowing Uca crabs change sediment topography and biogeochemistry and thus may affect Hg distribution and MeHg formation. We studied added (203)Hg distribution, Me(203)Hg formation and sulfate reduction rates (SRR) in sediment aquariums containing Uca leptodactyla; and analyzed profiles of Me(203)Hg formation and SRR in sediment cores from two mangroves with distinct environmental impacts. MeHg formation and SRR were higher in the top (≤6cm) sediment and there was no significant difference in Hg methylation in more or less impacted mangroves. In aquariums, crab bioturbation favored Hg retention in the sediment. In the treatment without crabs, Hg volatilization and water Hg concentrations were higher. Hg methylation was higher in bioturbated aquariums but SRR were similar in both treatments. These findings suggest that bioturbating activity favors Hg retention in sediment but also promotes MeHg formation near the surface.

Mercury methylation in sediments of a Brazilian mangrove under different vegetation covers and salinities.

The presence and formation of methylmercury (MMHg), a highly toxic form of Hg, in mangrove ecosystems is poorly studied. Therefore the aim of this study was to evaluate mercury methylation potentials in sediment, litter and root samples (Avicennia shaueriana and Spartina alterniflora) from different regions of a mangrove ecosystem, as well as the influence of salinity on methylation. Sediment was sampled under different depths and in mangrove regions with different plant covers and salinities. All samples were incubated with (203)Hg and MM(203)Hg was extracted and measured by liquid scintillation. MMHg was formed in all samples and sites tested including plant roots and litter. Higher Hg methylation was found in the superficial fraction of sediments (0.47-7.82%). Infralittoral sandy sediment had low MMHg formation (0.44-1.61%). Sediment under Rhizophora mangle had lower MMHg formation (0.018-2.23%) than under A. shaueriana (0.2-4.63%) and Laguncularia racemosa (0.08-7.82). MMHg formation in sediment tended to increase with salinity but the differences were not significant. Therefore, MMHg formation occurs in different sites of mangrove ecosystems and may be an important threat that requires further study.

Mercury distribution, methylation and volatilization in microcosms with and without the sea anemone Bunodosoma caissarum.

Mercury (Hg) has a complex biogeochemical cycle in aquatic environments. Its most toxic form, methylmercury (MeHg), is produced by microorganisms. This study investigated how the sea anemone Bunodosoma caissarum affects Hg distribution, methylation and volatilization in laboratory model systems. (203)Hg was added to microcosms and its distribution in seawater, specimens and air was periodically measured by gamma spectrometry. MeHg was measured by liquid scintillation. After the uptake period, specimens had a bioconcentration factor of 70 and in microcosms with and without B. caissarum, respectively 0.05% and 0.32% of the initial spike was found as MeHg. After depuration, MeHg in specimens ranged from 0.2% to 2.4% of total Hg. Microcosms with B. caissarum had higher Hg volatilization (58%) than controls (17%), possibly due to Hg(2+) reduction mediated by microorganisms associated with its tissues and mucus secretions. Marine organisms and their associated microbiota may play a role in Hg and MeHg cycling.

Mercury methylation in mesocosms with and without the aquatic macrophyte Eichhornia crassipes (mart.) Solms.

Mercury is a toxic pollutant and spreads to several compartments in the environment. Previous in-vitro studies showed that roots of aquatic macrophytes are sites of methylmercury formation, performed mainly by sulfate-reducing bacteria (SRB). The objective of this study was to observe MMHg formation and distribution among filtered water (0.2µm), suspended and settled particles and macrophyte roots during seventeen days, in (203)Hg- spiked mesocosms with and without live Eichhornia crassipes whole plants and a SRB inhibitor. Root samples were also incubated in-vitro for comparison of MM(203)Hg formation under in-vitro and in-vivo conditions. To evaluate the effect of SRB inhibition by sodium molybdate on total heterotrophic activity, the latter was measured by (3)H-leucine uptake. Inhibition of Hg methylation by sodium molybdate decreased with time in mesocosms. MMHg averaged 10, 12.4 and 0.23 percent of total (203)Hg present in filtered water, suspended particles and roots respectively. In vitro MMHg formation in roots averaged 5.54 percent of total added (203)Hg, with a clearer SRB inhibition effect than in mesocosms. Though significant, MMHg formation in roots from in-vivo mesocosms was one order of magnitude lower than previously found in in-vitro incubations of roots alone.