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projects >
bacterial demethylation of methylmercury
in the south florida ecosystem >
1999 proposal
IDENTIFYING INFORMATION
Project chief: Ronald S. Oremland
Program(s) (list all programs to which this work plan is being
submitted):
BACKGROUND NARRATIVES
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.
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
WORK PLAN
Oct, 1998 ó March, 1999: Analysis and data reduction of field periphyton
demethylation experiments.
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
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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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology This page is: http://sofia.usgs.gov/proposals/1999/bactdemp99.html Comments and suggestions? Contact: Heather Henkel - Webmaster Last updated: 11 October, 2002 @ 09:31 PM (KP) |