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Bacterial demethylation of methylmercury in the South Florida Ecosystem

Project Proposal for 1999

IDENTIFYING INFORMATION
Project title: Bacterial demethylation of methylmercury in the South Florida Ecosystem
Geographic area: Florida Everglades and Everglades Nutrient Removal Areas
Project start date: October 1, 1995
Project end date: September 30, 1999

Project chief: Ronald S. Oremland
Region/Division/Team/Section: WR/WRD/NRP
Email: roremlan@usgs.gov
Phone: (650) 329-4482
Fax: (650) 329-4463
Mail address:  345 Middlefield Road, ms 480
                        Menlo Park, CA 94025

Program(s) (list all programs to which this work plan is being submitted):
INATURES (formerly ìFragile Ecosystemsî)
Program element(s)/task(s) (show percent distribution if more than one element/task). If submitting to more than one program, include element(s)/tasks from each program here:
Task 3.5: Mercury Cycling in the Everglades and the Everglades Nutrient Removal Areas (renewal).

BACKGROUND NARRATIVES
Project summary: A brief (<100 words) summary written to be understood by a nonspecialist. State the issues to be addressed, the objectives of the project, significance to scientific questions and land-management and policy issues, and benefit to the Nation, all in terms that do not require technical translation. This paragraph will be used to describe this project in the Program and Division science plans.
 

Bacterial methylation of mercury occurs widely within the anoxic sediments and periphyton mats of the South Florida Ecosystem (SFE), however the highly toxic methylmercury (MeHg) formed is immediately subject to a bacterial degradation reaction (demethylation). The net difference between methylation and demethylation controls the amount of MeHg available for incorporation into food chains. This study will determine the rates of demethylation and the mechanisms by which it occurs.

Project objectives and strategy: More detailed/technical than the project summary, describing BRIEFLY the technical goals and approaches to be used over the LIFETIME of the project.

The biota of SFE contain high body burdens of MeHg. The MeHg is formed primarily by the methylation of Hg (II) carried out by anaerobic bacteria (e.g., sulfate-reducers) by their biochemical methylation reactions which can involve methylcobalamins (Choi et al., 1994). However, at the level of the microbial ecosystem, the MeHg formed is immediately subject to the demethylation reaction carried out by these and other types of bacteria (Oremland et al., 1991; 1994; Marvin DiPasquale and Oremland, 1998). In some cases, the same bacterium can carry out both the methylation and demethylation reactions (Pak and Bartha, 1998). Hence, only by making simultaneous measures of methylation and demethylation rates at ambient levels of Hg (II) and MeHg can the net production of MeHg be gauged. This project has devised the techniques necessary to make measurements of demethylation at environmentally realistic concentrations of MeHg and has detected significant activity at all the sites assayed for methylation by our colleagues. Therefore, such information is critical for the accurate formulation of mathematical models of mercury dynamics in the SFE. Such models are needed in order to achieve and implement effective managerial controls on the extent of MeHg contamination of the biota in the SFE.

We have employed 14C-MeHg to measure demethylation and have confirmed that this occurs primarily by oxidative demethylation (OD) as opposed to the organomercurial lyase pathway. We have measured activities across nutrient gradients in the SFE (i.e., north to south sampling) in surficial sediments, waters, and periphyton communities, and have conducted downcore measures of demethylation activity. This has been done over several seasons in the SFE. We have devised methods to measure demethylation using 3 orders-of-magnitude less applied 14C-MeHg than was used in our earlier work, and are now in the range of ~2 ng MeHg/cc wet sample. This value approaches the in situ concentrations of MeHg in these systems. This year we will complete our field work on assessing the extent of demethylation in various ecosystem components (eg, sediments, periphyton, waters), and how this activity varies temporally and spatially. This effort has been closely linked with other ongoing work lead by C. Gilmourís team (Hg methylation) and by D.
Krabbenhoftís team (mercury speciation). We will devote a large portion of the time and effort in FY 99 to carrying out the remaining analytical work associated with this yearís field trips, to the reduction, graphical representation, and statistical interpretation of the data generated, and to writing synthesis-level manuscripts with the goal of their publication in peer-reviewed journals.

We also plan to carry out during FY 1999 a laboratory-based program aimed at determining what is the fate of the mercury once MeHg has undergone demethylation. In other words, does it remain as Hg (II) and is therefore subject to re-methylation (or precipitation/complexation with sulfide), is it reduced to Hg (0) and therefore escapes the ecosystem via volatilization, or does only a partial reduction to Hg (I) occur? We will pursue these investigations using sediment samples archived from our Florida trips (or with freshly-collected materials when possible). We will also isolate from these sediments pure cultures of sulfate-reducers, methanogens, and other anaerobes which are capable of methylating and or demethylating mercury at rapid rates. We will follow the fate of mercury in these reactions by speciating collected samples via cold vapor atomic fluorescence spectroscopy, as well as with use of radiolabelled precursors.

Potential impacts and major products: Describe expected outcomes, both scientific and management/policy-related. What scientific questions and land-management and policy issues does this project help answer? Why is it important to Program priorities? What products will you produce to contribute to the desired outcomes?

Our field data will be integrated with those of the C. Gilmour team and the D. Krabbenhoft team to determine if the net rates of methyl mercury production (Methylation - Demethylation) can account for the observed levels of MeHg present in Everglades waters, sediments, and periphyton. This information lies at the very core of the mercury problem and will result in the identification of spatially/temporally located ìhot spotsî of net methylation. Such information is of fundamental importance when devising hydrological remediation strategies to minimize the extent of MeHg introduction into the Everglades food web.

Collaborators, clients (Names, affiliation, and roles of internal and external users of information generated by
project):
D. Krabbenhoft, USGS/WRD, Madison, WI
C. Gilmour, Academy of Natíl Sciences, Benedict, MD
L. Fink, SFWMD, Miami, FL
R. Aragu, USEPA, Athens, GA

WORK PLAN
Time line (FY 1999 to project end): List MAJOR tasks and deliverables by fiscal year and indicate key staff responsible for each.

Oct, 1998 ó March, 1999: Analysis and data reduction of field periphyton demethylation experiments.
Oct, 1998 ó March, 1999: Analysis and data reduction of field sediment experiments.
March, 1999 ó Sept 1999: Writing manuscripts based on field data.
Dec. 1998 ó Sept. 1999: Laboratory experiments with sediments and bacterial cultures to determine fate of Hg after demethylation.

FY 1999 activities: Statement of the work to be undertaken in FY 1999 and a description of the methods and procedures.

Field sample processing for 14CH4 and 14CO2 from 14C-MeHg degradation. The procedures for these analyses have been delineated in detail in previous proposals and in a publication in the journal Environ. Science & Technology which has been accepted for publication.

Analysis of incubated sediments and cultures for degradation of MeHg and formation of Hg (0), and Hg (II) using cold vapor atomic fluorescence and selective trapping.

Analysis of incubated sediments and cultures using 203Hg-labelled MeHg+ or Hg (II).

FY 1999 deliverables/products: Describe in more detail the specific deliverables/products that will result from this work in FY 1999.

2 Co-authored manuscripts (with C. Gilmour and D. Krabbenhoft) on the cycling or Hg in periphyton communities and on the net methylation of mercury across a nutrient gradient in the Everglades.

1 or more manuscripts on the mechanism(s) of demethylation and the chemical fate of mercury.
 

FY 1999 outreach: Emphasizing FY 1999, describe plans to address client requirements, decisions, and deadlines. New directions or major changes for FY 1999 (if applicable):

No changes to current plan.

ACCOMPLISHMENTS, OUTCOMES, PRODUCTS, OUTREACH

FY 1998 accomplishments and outcomes, including outreach:

-Continued seasonal measures of field MeHg demethylation across nutrient gradients in the Everglades. Examined underlying mechanisms and kinetics of demethylation.
 

FY 1998 deliverables, products completed:

- Marvin DiPasquale, M. and R.S. Oremland. Bacterial methylmercury degradation in Florida Everglades peat sediment. Environ. Sci. & Technol. (in press).

- Marvin DiPasquale, M. and R.S. Oremland. 1998. Methyl mercury degradation pathways: A comparison among three mercury impacted ecosystems. Ann. Mercury Discussion Grp, Asilomar, CA.

- Marvin DiPasquale, M. and R.S. Oremland. 1998. Bacterial methylmercury degradation in Florida Everglade sediment and periphyton. Amer. Geophys. Union spring meeting, Boston, MA.

- Ibid. 1998. Annual Workshop of the South Florida Mercury Science Program, W. Palm Beach, FL.

PROJECT SUPPORT REQUIREMENTS
Names and expertise (e.g. carbonate petrology) of key project staff (list by fiscal year for duration of project):
1 Project Chief, part time
1 Postdoctoral researcher, full-time
2 Undergraduate(technicians, part-time
1 graduate technician, part time
1 chemical oceanographer (career technical staff), part time

Other required expertise for which no individual has been identified (list by fiscal year for duration of project):

None

Major equipment/facility needs (list by fiscal year for duration of project):

No new needs other than supplies of radioisotopes, radwaste disposal fees, and lab expendables.


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