Mercury Cycling and Sequestration CFR 521 Marian Hanson February 15, 2012
Mercury Cycling and
Sequestration
CFR 521
Marian Hanson
February 15, 2012
What is Mercury?
• Mercury exists in three forms: elemental mercury,
inorganic mercury compounds, and organic mercury
compounds (primarily methyl mercury).
• All forms of mercury are quite toxic.
• It is widespread in the soil and water
• Threatening to human and environmental health
• Methyl mercury is a lethal pollutant found in rivers and
lakes.
Natural Sources of Mercury
• Volcanic eruptions
• Geothermal vents
• Naturally enriched substrates
• Fires
• Biological processes
Anthropogenic Sources of Mercury
• Burning of fossil fuels
• Mining (gold, coal, silver)
• Various industrial activities
Mercury Cycling in the Environment
• Once in the atmosphere elemental Hg oxidizes into
ionic Hg which is easier to deposit into the
environment.
• Anaerobic bacteria in wetlands convert inorganic
Hg2 to toxic methyl mercury.
Annual Emissions
• 4800-8300 ton per year globally
• In U.S. 48 tons of mercury annually
• Legal restrictions have reduced emissions
• Hg contamination still a concern as previous
contaminants can move around.
• Cost of remediation per pound of mercury is in the tens
of thousands using current technology
• Need alternate remediation approaches
Environmental Effects to Food Chain
= bioaccumulation
Mercury Concentrations of Puget Sound Fish
Puget Sound (hash-marked bars) & from the U.S. Food & Drug Administration’s
survey of U.S. fish species 1990-2004 (solid bars) (WSDH 2006).
Elemental Mercury • Acute exposure to high levels of elemental mercury in humans
results in CNS effects (tremors, irritability, insomnia, memory loss,
neuromuscular changes, headaches, reduction in cognitive function,
slowed sensory and motor nerve function.
• Acute inhalation has resulted in kidney effects ranging from mild to
acute renal failure.
• Gastrointestinal and respiratory effects (chest pains, cough and
pulmonary function impairment)
• Sources include thermometers, barometers, pressure-sensing
devices, batteries, lamps, industrial processes, lubrication oils, and
dental amalgams (EPA 2000).
Inorganic Hg Health Concerns
• Inorganic forms are usually less harmful than organic forms, partly
because they bind strongly to soil components that reduce their
availability and absorption.
• Acute exposure to inorganic mercury may result in nausea, vomiting,
and severe abdominal pain.
• Chronic exposure to inorganic mercury can cause kidney damage.
• Sources: Inorganic mercury was used in the past in laxatives, skin-
lightening creams and soaps, and latex paint (until 1991). Most
agricultural and pharmaceutical uses of inorganic mercury have been
discontinued in the United States, but mercuric chloride is still used as
a disinfectant and pesticide (EPA 2000).
Methyl Mercury (Organic Hg) Health
Concerns
• Organic Hg is a potent neurotoxin with over 90% absorption into the blood stream from the intestinal track (Ruiz and Daniell 2009).
• Acute exposure of humans to very high levels of methyl mercury results in CNS effects such as blindness, deafness, and impaired level of consciousness.
• Chronic exposure to methyl mercury in humans affects the CNS with paresthesia, blurred vision, malaise, speech difficulties, and narrowing of vision.
• Infants born to women who ingest high levels of methyl mercury exhibit mental retardation, ataxia, narrowing of the vision, blindness, and cerebral palsy.
• Methyl mercury has no industrial uses; it is formed in the environment from methylation of inorganic mercury ions (EPA 2000).
Phytoremediation of inorganic
contaminants
• Inorganics can be altered but cannot be degraded
• Phytostabilization – reduce mobility and bioavailability
• Phytoaccumulation – accumulate metals in plant
biomass
• Phytoextraction – in harvestable plant tissues
• Phytovolatilizaton – evapotranspired as a gas
Biotechnology for Inorganic
Phytoremediation
• Focus on plant tolerance and accumulation
• Genes that transport metal
• Genes that facilitate chelator production
• Genes that facilitate conversion to volatile forms
Mercury cycling and sequestration
in salt marshes sediments: An
ecosystem service provided by
Juncus maritimus and Scirpus
maritimus
B. Marques, A.I. Lillebo, E. Pereira, A.C.
Duarte (2011)
Research Project Objectives
• Since many salt marshes seem to be species-specific, it is important to address the services provided by halophytes with different life cycles.
• Characterize the annual life cycle of these 2 species in a salt marsh.
• Characterize the rhizosediment chemical environment
• Determine the concentrations of Hg in the belowground biomass and rhizosediment
• Evaluate how decomposition rates may affect the dynamics of Hg accumulation in the belowground parts of these 2 halophyte species
• Discuss sequestration of Hg in these salt marsh sediments as an ecosystem service provided by the 2 species
Salt Marsh Testing Environment
• Low energy, dynamic systems
• Vegetation depends on presence of mudflats
• Vegetation important to settling of suspended matter
• Small number of highly productive marsh species
• Some of the most productive ecosystems in the world
• Salt marsh ecosystems provide multiple services
• Salt marsh plants promote autoremediation through
metals rhizofiltration, phytostabilization, or
phytoaccumulation
• Marshes are species-specific
Test Site: Ria de Aveiro, Portugal
static.panoramio.com
Juncus maritimus
Annual plant found in
Europe, West Africa, and
North Asia
Field Guide to the Common Wetland Plants of Western Washington & Northwestern Oregon
Scirpus maritimus (seacoast bulrush)
Perennial plant found in European and N. American marshes
Research Methods
• Field procedures and samples – above ground plant material, detritus,
and sediments core samples of 2 depths (0-5 cm and 5-15 cm) were
sampled separately within squares. For each plant and for each sediment
depth layer 3 field replicates were obtained. Above and below ground plant
material were separated as was the below ground plant materials from
sediments (Marques et al. 2011).
• Litterbag decomposition experiment – random plants of both species
were collected monthly from April to September (180) days and monitored
for below ground decomposition (Marques et al. 2011).
• Analytical procedure – Halophytes and rhizosediment homogenized sub-
samples total Hg was determined (Marques et al. 2011).
Mercury Cycling & Sequestration in Sediments
Representing sequestration in sediments of the historically Hg contaminated
salt marsh colonized by J. maritimus & S. maritimus Fig 6 (Marques et al. 2011)
Annual variation of pH and Eh values in J. maritimus & S. maritimus rhizosediment Fig 1 (Marques et al. 2011)
Comparison of J. maritimus and S. maritimus
Aboveground and belowground biomass in an Hg-contaminated salt marsh Fig. 2 (Marques et al. 2011)
Biomass Annual Production and
Turnover Rates
Biomass production
(g DW m2 y1) Aboveground 1166 1100
Belowground 107 93
Total 1273 1193
Biomass turnover rates Aboveground 0.56 0.99
Belowground 0.53 0.33
Juncus maritimus Scirpus maritimus
Table 2 (Marques et al. 2011)
Mercury concentrations (1 yr. period)
In rhizosediment and in belowground biomass of:
(A) Juncus maritimus, (B) Scirpus maritimus
(1) Top 0-5 cm. depth, (2) 5-15 cm. depth layer Fig 4 (Marques et al. 2011)
Biomass Remaining and Hg Content
Juncus maritimus
Scirpus maritimus
Fig 5 (Marques et al. 2011)
Conclusions
• Decomposition rates of belowground biomass affected
the dynamics of Hg exchanged between the plants and
the rhizosediment.
• J. maritimus seems better at phytostabilization and
phytoaccumulation of Hg.
• S. maritimus colonized areas – Hg is more extensively
exchanged between belowground biomass and the
rhizosediment.
• These studies are important to enhance salt marsh
autoremediation capacity.
Other Research
• Study at the same marsh showed the annual bioaccumulation of Hg
in aboveground tissues of Halimione portulacoides was much lower
than in belowground parts and that J. maritimus belowground
biomass accumulated more than 98% of total Hg.
• Also, Almeida et al., 2006 concluded that J. maritimus and S.
maritimus aboveground senescent plant tissues should not be a
significant source of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn to the
Douro marsh located 60 km north from Ria (Marques et al. 2011).
• 2 bacterial genes merA and merB were used to engineer Hg
transformation and remediation system in plants.
• Plant species such as A. thaliana, tobacco, yellow poplar,
cottonwood and rice with merA were resistant to at least 10 times
greater concentrations of Hg(II)
Future Research • Mercury detoxification and complete volatization
• Enhanced Hg uptake into roots, transport to shoot, and
sequestration into aboveground tissues for later harvest
• Hg detoxification systems in chloroplasts or ER may
offer high levels of Hg tolerance and detoxification
• Use different gene combinations to enhance uptake,
translocation, chelation or detoxification and release of
Hg0 into the atmosphere.
• Plants to consider may include plants with large
biomass, rapid growth rate, wider climatic tolerance, and
expression of multiple genes in different cellular
compartments
• Field trials for plants that were successful in the lab tests
• Use of native plants
Glossary
• Ataxia – loss of muscle coordination
• Bioaccumulation – accumulation of a harmful substance (such as
Mercury in the food chain)
• Cerebral palsy – condition of lack of motor control caused by brain
damage around birth
• Chelate – a chemical compound in which metallic and nonmetallic
(organic) atoms are combined
• Halophytes – plant capable of growing in salty soil
• Malaise – feeling ill in a general, non-specific way
• Paresthesia – tingling or burning of the skin
• Perennial plant – comes back every year
• Rhizofiltration – rhizosphere accumulation of metal contaminants
through plant’s absorption, concentration and precipitation
• Sequestration – chemical process of binding a metallic ion
References
Cooke, Sarah Spear (Ed.). 1997. A Field Guide to the Common Wetland Plants of Western
Washington & Northwestern Oregon. Seattle Audubon Society, Washington Native Plant Society,
417 pp.
Dhankher, O.P., E.A.H. Pilon-Smits, R.B. Meagher, S. Doty. 2012. “Biotechnological approaches for
phytoremediation.” Elsevier Inc., pp. 309-323. DOI: 10.1016/B978-0-12-381466-1.00020-1
Kozloff, Eugene N. 1993. Seashore Life of the Northern Pacific Coast. University of Washington
Press, Seattle, p. 339.
Marques, B., A.I. Lillebo, E. Pereira, A.C. Duarte. 2011. “Mercury cycling and sequestration in salt
marshes sediments: An ecosystem service provided by Juncus maritimus and Scirpus maritimus.”
Environmental Pollution, 159:1869-1876. Available at www.elsevier.com/locate/envpol
Ruiz, O.N. and H. Daniell. 2009. “Genetic engineering to enhance mercury phytoremediation,” in
Current Opinion in Biotechnology 20:213-219. Available online at www.sciencedirect.com
U.S. Environmental Protection Agency. 2000. “Mercury Compounds.”
http://www.epa.gov/ttnatw01/hlthef/mercury.html
Wash. State Dept. of Health. 2006. “Human Health Evaluation of Contaminants in Puget Sound Fish.”
Available at www.doh.wa.gov/ehp/oehas/fish/psampreport_10-06.pdf
www.maltawildplants.com (Portugal image)
www.static.panoramio.com (Juncus maritimus image)
Union Bay Nature Area