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