Developing a predictive understanding of Hg transformation in ecosystems requires knowledge of exchange and feedback processes at Critical Interfaces. Task 1 involves ecosystem features influencing mercury transformation.

Task 1: Ecosystem Features Influencing Mercury Transformation

Overarching Objectives

The general objectives of Task 1 are to (1) identify ecosystem compartments and hydrobiogeochemical conditions that govern net methylmercury concentration in EFPC and (2) understand the extent to which groundwater–surface water exchange drives mercury transformations in EFPC. These objectives are addressed through a set of hypotheses-driven field and laboratory investigations. Additionally, the SFA program is developing a process-rich numerical model to integrate past results and challenge current understanding of watershed processes occurring through space and time. The model will be used in an iterative fashion with experiments to help inform the design of experiments and subsequently to refine the model based on experimental results.

Questions to be Addressed

  • What is the role of EFPC periphyton biofilms in Hg transformations?
  • Which Hg-methylating microbial groups dominate in different EFPC ecosystem compartments? How do these groups interact across compartment interfaces?
  • What changes occur in Hg-methylating microbial community along the length of the EFPC? Can point-scale measurements of Hg methylation/stream metabolism inform reach-scale estimates of the same process?

Hypotheses to Test

  • Convergence of chemically distinct groundwater and surface water in saturated sediments under and around the creek create dynamic gradients with locally higher microbial community diversity and rates of Hg methylation.
  • SRB are the dominant microbes responsible for Hg methylation, and their abundance and activity are highest at the groundwater–surface water interface.
  • In-stream production of MeHg generated in biofilms colonizing bed substrates is the dominant constant source of MeHg in EFPC on an annual basis. The high percentage of MeHg in creek periphyton is due to its production in the biofilm, not just bioconcentration from the water column.
  • Net MeHg concentration at the reach scale is significantly correlated to net primary productivity within that reach.

Low-Order Stream Systems. Streams are ranked based on a hierarchical network of channels within a watershed. Low-order streams (i.e., first- through fourth-order streams) are located in the headwater areas and typically convey small volumes of water. Such streams play a dominant role in the flow, biogeochemistry, and water quality of downstream higher-order reaches.

Recent Accomplishments

During 2014-15, researchers within Task 1 made significant progress toward milestones and published two papers. One paper, in collaboration with Task 2, described the interactions between dissolved organic matter (DOM) and methylmercury and the mechanisms by which these interactions can promote photo-demethylation of methylmercury in creek water. In collaboration with scientists at Harvard University and the Woods Hole Oceanographic Institute, SFA researchers also report on interfacial biogeochemical reactions between bacteria and the surfaces of sulfide minerals they colonize. Using a combination of field deployments, laboratory experiments, and high-resolution surface spectroscopy, we show that commonly occurring chemotrophic bacteria of the genus Thiobacillus enhance metacinnabar (β-HgS) solubility under circumneutral pH, resulting in the subsequent reduction of the released Hg(II) to Hg(0). These results demonstrate a novel pathway for Hg(0) formation in aquatic environments.

Role of Periphyton in EFPC Mercury Cycling

Dissolved methylmercury concentration in EFPC has shown diel variability over multiple sampling events, and this variability appears to be correlated with the daily photocycle. These observations, coupled with the results of storm event sampling, imply that key controls on net mercury methylation occur within the stream or on the stream bed and include factors such as small-scale temperature changes in the water column and the photosynthetic activity of stream biofilms.

We began initial assessments of the role of stream periphyton biofilms in methylmercury generation. Periphyton biofilms are complex assemblages comprising algae, bacteria, fungi, diatoms, extracellular polymers, invertebrates, detritus, and mineral particles. Due to the diversity of their components, these biofilms can be important in the biogeochemical cycling of many elements. Moreover, their structure and activity can promote the development of the biogeochemical conditions associated with mercury methylation (e.g., iron-reducing, sulfate-reducing, and fermentative conditions). Periphyton samples collected monthly along the length of the creek are analyzed for total mercury and methylmercury content to assess spatial and seasonal effects. The methylmercury content of the biofilms is higher than that in the surrounding creek sediment and increases with distance downstream. Biofilm samples grown on artificial substrates deployed in the creek have methylmercury content similar to natural biofilms.

Samples of intact periphyton biofilms grown in the field and returned to the lab are used in mercury methylation and methylmercury demethylation assays. These assays use enriched stable isotopes of mercury to distinguish de novo activity from the ambient mercury and methylmercury present in the samples. Initial experiments demonstrate that both mercury methylation and methylmercury demethylation occur in the biofilms, but the estimated rate of methylation outpaced the rate of demethylation by a factor of ~2.5 (see figure). During our February 2015 tests, methylmercury produced in the biofilms could account for 30% to 40% of the methylmercury flux, as estimated at our downstream monitoring station (EFK 5.4). Periphyton biofilms thus appear to play a central role in net methylmercury generation in EFPC.

Redox Zonation Within Periphyton Biofilms

We acquired a potentiostat to conduct electrochemical measurements within the periphyton biofilms. Using the correct combination of microelectrodes and micromanipulators allows us to directly quantify important redox couples at high spatial resolution, including dissolved oxygen, manganese, Fe(II), S(II), and iron sulfide. The development of iron- and sulfate-reducing conditions is particularly important because such conditions are necessary for mercury-methylating microorganisms. With this new equipment, we demonstrated that over a distance of ~2 mm, conditions changed from aerobic above the biofilm to sulfate-reducing at its base (see figure). These findings are thus consistent with the methylmercury-generation results mentioned above.

Shallow Groundwater

Groundwater sampled from shallow wells at two sites has a distinct chemical signature from the surface water, as evidenced by lower pH (1–1.5 units), absence of dissolved oxygen and nitrate, lower sulfate concentration, and elevated concentrations of Fe(II) and sulfide. The lower pH is likely due to organic acid production from fermentative microorganisms, and the other parameters are indicative of active anaerobic microbial activity. Dissolved methylmercury concentration in the groundwater is comparable to or up to 10 times greater than the surface water, suggesting that the groundwater may be negatively impacting surface water quality. Further studies including collaboration with Task 2 and 3 are planned.

(a) Methylmercury demethylation (black) and mercury methylation (red) in periphyton biofilms. Lines represent pseudo first-order decay and production curves fit to the data by adjusting the value of the rate constant. (b) Voltammograms collected from intact periphyton samples demonstrate a steep gradient in geochemical conditions from aerobic to sulfate-reducing over a distance of less than 2 mm.

Creek Response to Changes in Flow Management Practices

Since the mid-1990s, the flow in EFPC had been managed via the addition of lake water at the rate of 5 million gallons per day at the headwaters of the creek (this constituted ~65% of baseflow as measured at the point where EFPC exits Y-12). By order of the Tennessee Department of Environment and Conservation, this practice was ceased on April 30, 2014. We took this opportunity to begin a sampling campaign to document creek response to the decreased flow. Notable among the changes are a decrease in total suspended solids (TSS) and an increase in the fraction of total mercury and dissolved methylmercury (< 0.2 μm). On average, dissolved methylmercury concentration in the creek was 1.6 times greater in EFPC during summer 2014 relative to conditions during flow management. To the extent that dissolved methylmercury is more bioavailable than particle-associated methylmercury, this change in flow management and the consequent changes in mercury and methylmercury speciation could have important implications for mercury bioaccumulation in EFPC.

The ORNL Mercury SFA is sponsored by the Subsurface Biogeochemical Research (SBR) program within the U.S. Department of Energy's Office of Biological and Environmental Research.