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Bacterial demethylation of methylmercury in the South Florida Ecosystem

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Frequently-anticipated questions:

What does this data set describe?

Title:
Bacterial demethylation of methylmercury in the South Florida Ecosystem
Abstract:
Methylmercury (MeHg) degradation was investigated along an eutrophication gradient in the Florida Everglades by quantifying 14CH4 and 14CO2 production after incubation of anaerobic sediments with 14C-MeHg. Degradation rate constants (k) were consistently <=0.1 per day, and decreased with sediment depth. Higher k values were observed when shorter incubation times and lower MeHg amendment levels were used, and k increased two-fold as in-situ MeHg concentrations were approached. The average floc layer k was 0.046 +/- 0.023/ d (n=17) for 1-2 day incubations. In-situ degradation rates were estimated to be 0.02 to 0.5 ng MeHg/g dry sed/d, increasing from eutrophied to pristine areas. Nitrate-respiring bacteria did not demethylate MeHg, and NO3- addition partially inhibited degradation in some cases. MeHg degradation rates were not affected by PO4-3 addition. 14CO2 production in all samples indicated that oxidative demethylation (OD) was an important degradation mechanism. OD occurred over five orders of magnitude of applied MeHg concentration, with lowest limits (1-18 ng MeHg/g dry sediment) in the range of in-situ MeHg levels. Sulfate reducers and methanogens were the primary agents of anaerobic OD, although it is suggested that methanogens dominate degradation at in-situ MeHg concentrations. Specific pathways of OD by these two microbial groups are proposed.
Supplemental_Information: This project ended in 1999.
  1. How should this data set be cited?

    Mark Marvin-DiPasquale Ronald Oremland, 2005, Bacterial demethylation of methylmercury in the South Florida Ecosystem.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -80.8
    East_Bounding_Coordinate: -80.2
    North_Bounding_Coordinate: 26.7
    South_Bounding_Coordinate: 25.2

  3. What does it look like?

    <http://sofia.usgs.gov/exchange/marvin-dipasquale/locationmmd.html> (GIF)
    location of sites for methylmercury degradation measurements

  4. Does the data set describe conditions during a particular time period?

    Calendar_Date: Jun-1996
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: spreadsheet

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

    2. What coordinate system is used to represent geographic features?

  7. How does the data set describe geographic features?

    Entity_and_Attribute_Overview:
    Data for each site includes: site name, date; sediment depth interval (cm); incubation time (days); C14-MeHg injected (ng/cc wet sed); MeHg degradation first order rate constant (k); and MeHg -degradation potential rate (ng/cc wet sed/d)
    Entity_and_Attribute_Detail_Citation: USGS personnel

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    Mark Marvin-DiPasquale
    U.S. Geological Survey
    Project manager
    345 Middlefield Road,

    Building 15, McKelvey Building
    Menlo Park, CA 94025
    USA

    650 329 4442 (voice)
    650 329-4463 (FAX)
    mmarvin@usgs.gov

Why was the data set created?

The objective of this research is to provide ecosystem managers with MeHg degradation rate data from a number of study sites that represent a diversity of hydrologic and nutrient regimes common to the Everglades, and to forge a better understanding of the microbial and geochemical controls regulating MeHg degradation in this system.

How was the data set created?

  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    Date: Unknown (process 1 of 1)
    Three years of field and laboratory studies research were completed. The specific areas of investigation are outlined below.

    1) Field Measurements: Sediment cores were sectioned into three to five discrete 2 to 4 cm horizons, within hours of sample collection. Homogenized sub-samples from each horizon were transferred into crimp sealed serum vials, purged with N2 gas, and injected with radiolabeled 14CH3Hg+. After incubating anaerobically for hours to days, incubations were arrested with either acid or base, depending on the method of 14C endproduct detection. End-products (14CH4 and 14CO2 ) were measured via gas proportional counting (prior to December 1996) or a CH4 combustion / CO2 trapping method (after December 1996). Degradation rate constants are calculated from the fraction of MeHg degraded per incubation time. The relative amounts of CH4 and/or CO2 produced provides an indication as to the relative importance of MC and/or OD. Degradation rates were assessed with respect to both sediment depth and site location.

    2) Nutrients and Microbial Inhibitors: The affect of nutrients (NO3-, PO4-3, NH4+) and SO4-2 on the degradation of 14CH3Hg+ was assessed by amending parallel sets of incubation samples with these substrates and processing as described above. Likewise, specific microbial inhibitors of both SRB and MPB were used to determine the relative contribution of these microbial groups to MeHg degradation.

    3) The Fate of Hg: Preliminary experiments were conducted exploring the fate of Hg resulting from MeHg degradation. Vapor phase Hg(0) was collected on gold traps by flushing the head-space of samples amended and incubated with MeHg. The concentration of Hg(0) was assayed by cold vapor atomic fluorescence spectroscopy. Our initial (unpublished) results suggest that very little (> 0.01% ) of liberated Hg+2 produced as a result of MeHg degradation is reduced to volatile Hg(0).

    4) Kinetic Studies: The dependence of MeHg degradation rates on MeHg amendment concentration was explored over a wide range (1-2000 ng MeH/g dry sed). The low end of this range approached natural in-situ concentrations (0.1-10 ng MeHg/g dry sed). These low levels were achieved by increasing our standard sample size from 3 to 80 cc of sediment, using a custom synthesized high-specific activity 14C-MeHg radiotracer, and employing the sensitive CH4 combustion / CO2 trapping method for 14C end-product quantification.

    Person who carried out this activity:

    Mark Marvin-DiPasquale
    U.S. Geological Survey
    Project manager
    345 Middlefield Road,

    Building 15, McKelvey Building
    Menlo Park, CA 94025
    USA

    650 329 4442 (voice)
    650 329-4463 (FAX)
    mmarvin@usgs.gov

  3. What similar or related data should the user be aware of?

    Marvin-DiPasquale, Mark C. Oremland, Ronald S., 1998, Bacterial methylmercury degradation in Florida Everglades peat sediment: Environmental Science and Technology v. 32, n. 17, American Chemical Society, Washington, DC.

    Online Links:

    Other_Citation_Details:
    All users may view the abstract of this article. If you are a subscriber, you may view the article. If you are not a subscriber, you may purchase and download the PDF version of the article.
    Marvin-DiPasquale, Mark C. Agee, Jennifer; McGowan, Ch, 2000, Methyl-mercury degradation pathways: a comparison among three mercury-impacted ecosystems: Environmental Science and Technology v. 34 , n. 23, American Chemical Society, Washington, DC.

    Online Links:

    Other_Citation_Details:
    All users may view the abstract of this article. If you are a subscriber, you may view the article. If you are not a subscriber, you may purchase and download the PDF version of the article.

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    not available

  5. How consistent are the relationships among the observations, including topology?

    not applicable

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints: none
Use_Constraints: none

  1. Who distributes the data set? (Distributor 1 of 1)

    Heather S.Henkel
    U.S. Geological Survey
    600 Fourth St. South
    St. Petersburg, FL 33701
    USA

    727 803-8747 ext 3028 (voice)
    727 803-2030 (FAX)
    hhenkel@usgs.gov

  2. What's the catalog number I need to order this data set?

    methylmercury degradation rates

  3. What legal disclaimers am I supposed to read?

    The data have no explicit or implied guarantees.

  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 03-Aug-2009
Metadata author:
Heather Henkel
U.S. Geological Survey
600 Fourth Street South
St. Petersburg, FL 33701
USA

727 803-8747 ext 3028 (voice)
727 803-2030 (FAX)
sofia-metadata@usgs.gov

Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

This page is <http://sofia.usgs.gov/metadata/sflwww/mmarvin.faq.html>

U.S. Department of the Interior, U.S. Geological Survey
Comments and suggestions? Contact: Heather Henkel - Webmaster
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