Biogeochemical controls on mercury methylation in the Allequash Creek wetland.

We measured mercury methylation potentials and a suite of related biogeochemical parameters in sediment cores and porewater from two geochemically distinct sites in the Allequash Creek wetland, northern Wisconsin, USA. We found a high degree of spatial variability in the methylation rate potentials but no significant differences between the two sites. We identified the primary geochemical factors controlling net methylmercury production at this site to be acid-volatile sulfide, dissolved organic carbon, total dissolved iron, and porewater iron(II).

Importance of hypolimnetic cycling in aging of "new" mercury in a northern temperate lake.

The aging of "new" mercury (Hg) was investigated in Experimental Lake 658 as part of the Mercury Experiment To Assess Atmospheric Loading In Canada and the United States (METAALICUS). Mercury enriched in (202)Hg was added to the epilimnion over a three-year period to simulate direct atmospheric deposition. We evaluated the aging of newly added mercury (HgLake) in the water column using chemical methods and experiments to examine differences in phase partitioning and transport compared to the ambient pool, HgAmb.

Relationships between surface-bound and internalized copper and cadmium and toxicity in Chlamydomonas reinhardtii.

In the present study, the adsorption and uptake of copper (Cu) and cadmium (Cd) in Chlamydomonas reinhardtii were examined to establish fundamental toxicity relationships to glutathione and cell-growth endpoints. Establishing these fundamental relationships of metal accumulation and toxicity metrics is necessary to subsequently implement an algal biotic ligand model. The glutathione response was similar to the response measured from growth endpoints for both internal and adsorbed Cu, indicating that glutathione may be a useful biomarker of toxicity.

Induction of reactive oxygen species in chlamydomonas reinhardtii in response to contrasting trace metal exposures.

The toxicity of metals to organisms is, in-part, related to the formation of reactive oxygen species (ROS) in cells and subsequent oxidative stress. ROS are by-products of normal respiration and photosynthesis processes in organisms, but environmental factors, like metal exposure, can stimulate excess production. Metals involved in several different mechanisms such as Haber-Weiss cycling and Fenton-type reactions can produce ROS.

Differential effects of copper and cadmium exposure on toxicity endpoints and gene expression in Chlamydomonas reinhardtii.

The toxicity of cadmium to aquatic organisms is well known, but the mechanisms of toxicity are not as clearly understood. In the present study, Cd bioassay experiments incorporating both traditional endpoints and novel thiol-based endpoints were conducted with Chlamydomonas reinhardtii. The results were compared with results from previous bioassay experiments to probe the apparent contrasting biochemical mechanisms of toxicity of copper and cadmium as expressed in cellular glutathione and the glutathione cycle.

Strong colloidal and dissolved organic ligands binding copper and zinc in rivers.

The speciation or physicochemical form of copper and zinc in freshwater plays an important role in reactivity, bioavailability, and toxicity. Strong metal-binding ligands, which determine speciation, were detected by voltammetric methods, both anodic stripping voltammetry (ASV) and competitive ligand equilibration adsorptive stripping voltammetry (CLE-AdSV); the latter technique can detect nanomolar levels of extremely strong (log K' > 13) ligands. Through careful field site selection and the investigation of ultrafiltration permeate samples, natural organic ligands were measured with limited interferences of colloidal inorganic iron- and aluminum-based trace metal-binding phases.

Analysis of glutathione endpoints for measuring copper stress in Chlamydomonas reinhardtii.

Glutathione (GSH) is the most abundant nonprotein thiol in eukaryotic cells and it protects cells by functioning as an antioxidant and a metal-binding ligand. Because glutathione readily undergoes oxidation-reduction reactions to combat oxidative stress, intracellular ratios of the reduced (GSH) to the oxidized (GSSG) forms of glutathione may serve as an important biomarker of exposure and effect of trace metals in eukaryotic cells. We compared sensitivity of glutathione ratios in the freshwater alga Chlamydomonas reinhardtii to the traditional endpoints of cell growth rates and chlorophyll a following exposure to Cu for periods of 6 and 24 h.

Speciation of aqueous methylmercury influences uptake by a freshwater alga (Selenastrum capricornutum).

Uptake of methylmercury (MeHg) by the alga Selenastrum capricornutum was measured in freshwater batch culture bioassays. The concentration of MeHg in the alga increased rapidly (within 15 min), reached a maximum by 6 h, and then declined because of growth dilution. The alga's rapid growth rate (doubling time, approximately 10 h) contributed to the importance of growth dilution.

Subsurface sources of methyl mercury to Lake Superior from a wetland-forested watershed.

Identification of sites of methyl mercury (MeHg) production is critical to predicting long-term fate of bioaccumulative Hg in the aquatic environment. During baseflow, when groundwater sources dominate, we observed consistently elevated levels of MeHg (0.1-0.

Influences of iron, manganese, and dissolved organic carbon on the hypolimnetic cycling of amended mercury.

The biogeochemical cycling of iron, manganese, sulfide, and dissolved organic carbon were investigated to provide information on the transport and removal processes that control the bioavailability of isotopic mercury amended to a lake. Lake profiles showed a similar trend of hypolimnetic enrichment of Fe, Mn, DOC, sulfide, and the lake spike ((202)Hg, purity 90.8%) and ambient of pools of total mercury (HgT) and methylmercury (MeHg).

Factors affecting the presence of dissolved glutathione in estuarine waters.

We investigated factors influencing the presence of the thiol glutathione (GSH) in estuarine waters. Our study addressed thiol phase-association, the biological release from algal cultures, and the role of copper in both thiol release and preservation. Our measurements in three diverse estuaries in the continental United States (San Diego Bay, Cape Fear Estuary, and Norfolk Estuary) show that dissolved GSH, present at sub-nanomolar levels, is preferentially partitioned into the ultra-filtrate fraction (<1 kDa) in comparison with dissolved organic carbon (DOC).

Physical and kinetic speciation of copper and zinc in three geochemically contrasting marine estuaries.

The physical and kinetic speciation of Cu and Zn in three impacted marine estuaries was examined. Contrasts in sources of metal-binding ligands, solution chemistry, and hydrologic forcing between and withinthethree study systems (Cape Fear River Estuary, North Carolina; Norfolk-Hampton Roads-Elizabeth River, Virginia; San Diego Bay, California) were exploited to enhance our understanding of Cu and Zn speciation. Trace metal-optimized tangential-flow ultrafiltration at 1 kDa nominal molecular weight limit (NMWL) was used to fractionate <0.

Determination of dissolved thiols using solid-phase extraction and liquid chromatographic determination of fluorescently derivatized thiolic compounds.

A method employing solid-phase extraction coupled with HPLC separation of thiol-monobromobimane (mBBr) derivatives was developed and optimized to quantify dissolved thiols at concentrations as low as 0.1 nM for glutathione (GSH) and gamma-glutamylcysteine (gammaEC) in natural waters. The reducing reagent, tri-n-butylphosphine (TBP), is needed for complete derivatization.

Factors affecting enhanced mercury bioaccumulation in inland lakes of Isle Royale National Park, USA.

We investigated factors causing mercury (Hg) concentrations in northern pike to exceed the consumption advisory level (>500 ng/g) in some inland lakes of Isle Royale National Park. Using Hg-clean techniques, we collected water, zooplankton, macro invertebrates, and fishes in 1998 and 1999 from one advisory lake, Sargent Lake, for analysis of total mercury (Hg(T)) and methylmercury (MeHg). For comparison, samples were also collected from a non-advisory lake, Lake Richie.