Biological Transformation of Selenium in Soil-Plant

Systems

Z-Q Lin1 and Gary Bañuelos2

1Environ. Sci. Program & Dept. of Biol. Sci.Southern Illinois Univ., Edwardsville

2USDA-ARS, Water Management Research Lab

Selenium

• A narrow margin between nutritionally required and toxic concentrations– Essential for humans & animals– Not essential for plants

Se

• Se(VI), selenat

• Se(IV), selenite

• Se(0), elemental Se

• Se(-II), selenide– e.g., Selenomethionine (SeMet);

Dimethylselenide (DMSe)

Chemical Forms of Se

• Toxicities of different chemical forms– Toxic to fish: SeMet > selenite > selenate– Elemental Se is not toxic because it is not

water soluble.– DMSe, a volatile Se compound, is less toxic

to rats, compared with inorganic Se.

Toxicity of Se

Toxicity of Different Forms of Se to Fish

Se(-II) Se(IV) Se(VI) CK

T=0

T=24 hrs

What are the major chemical forms of Se accumulated in

soil-plant systems?

Chemical Speciation of Se by X-ray absorption spectroscopy (XAS)

• XAS is one of the most advanced techniques that currently available for chemical speciation of Se and other environmentally important trace elements.

– Element specific– Direct determination & non-destructive– Various complex environmental substrates

Chemical Speciation in

Environmental Samples by XAS

Se X- ray Absorption Spectra

Energy (eV)

12650 12660 12670 12680 12690

Nor

mal

ized

Abso

rban

ce

0.0

0.5

1.0

1.5

2.0

2.5

Se(VI)Se(IV)Se(0)SeMet

Sediment (0-2 cm)

STANDARDS

Factors Affecting Se Transformation in Soil-Plant

Systems • Sulfate

– Chemical similarity between selenate & sulfate

• Redox potential– Anaerobic microbial reduction of selenate

• pH– Enhanced Se methylation in alkaline soils

• Organic matter– Adsorption of selenite

• Soil moisture– Se bioavailability

• Plants & associated microbial communities in soil– Root exudates

• Industry– Oil refinery wastewater with selenite

• Agriculture– Drainage water with ~90% of selenate

Selenium pollution sources & predominant chemical forms

The San Joaquin Valley:– One of the most productive agriculture areas– Subsurface tile-drainage contains Se & other salts.

Soils contain high levels of Se

WestEast

Drainage Water Reuse System- Zero Discharge of Drainage Water into Environment

Solar Evaporator

Halophytes Field

Salt-tolerant Treesand Grasses

192 ha

52 ha 7.6 ha

Alfalfa

Cotton

Lin et al., 2002, Water Research

The Halophyte Study Field

Cordgrass (Spartina sp.)

Pickleweed (Salicornia bigelovii)

Maximum Rates of Se Volatilization

Salico

rnia

Cordg

rass

Saltgr

ass

Atriple

x

Native

gra

ss

µg

Se

m-2

d-1

0

100

200

300

400

500

Distribution of Salicornia bigelovii Torr. in the U.S.(Data from USDA)

Why Does Salicornia Volatilize More Se Than Other Species ?

• Unique physiological processes in Salicornia?

• Volatilization by microbes associated with Salicornia?

• Interaction of Salicornia and microbes in soil?

Major Steps of Se Volatilization Pathway

in Plant

Selenate APSe Se-Cysteine

Se-Methionine

Dimethyl selenide

Selenite

Methyl-SeMet

Selenate in Soil

SeMet

Sel

enat

eS

elen

ate

SeleniteSeMet

SeMet

Soil Microbes

Salicornia

DMSe

Hypothesis:DMSe

Question 1:

• Does Salicornia have an enhanced capacity of reducing selenate into organoselenium (SeMet) compounds?

Lee & Lin et al. 2001. Planta

• Salicornia was supplied with selenate or selenite.

• Experiments were conducted under sterile vs non-sterile conditions.

• Se speciation in Salicornia shoots & roots was determined by XAS.

Findings:

– Compared with other species, Salicornia has an enhanced capacity to reduce selenate into organic forms.• Salicornia reduced >65% of selenate or

selenite into SeMet in tissues.

• Chemical reduction of selenate without the presence of microbes.

Question 2:

• Will chemical forms of selenium (e.g., selenate, selenite, SeMet) affect rates of Se volatilization in the soil-Salicornia system?

Salicornia bigelovii Torr.

Plants Treated With:Selenate,Selenite, or

Selenomethionine

Se Volatilization Measurements

Finding:

The soil-Salicornia system volatilized organic SeMet ~20 times faster than inorganic selenate (or ~15 times with selenite).

Chemcial Forms of Se in Top Soil with Salicornia

Se(VI)

Se(IV)

Se(0)SeMet

Se volatilization by soil bacterial strains isolated from the Salicornia & saltgrass

fields

76% of the total Se accumulated in Salicornia roots were SeMet-like organic compounds, while saltgrass

accumulated 48% of SeMet-like compounds

Salicornia root Saltgrass root

Question 3:

• What is the role of soil microbes in Se volatilization?– Is there a special microbial community

associated with Salicornia?– Are there any microbes that have an

enhanced capacity to volatilize Se?

Soil Microbial Study

• Soil samples were collected from the Salicornia and saltgrass fields.

• Cultureable bacteria were studied only.

• 5 identical bacterial strains were isolated and identified by 16 S rDNA technique.

Volatilization of Se by Bacteria IsolatedFrom the Salicornia and/or Saltgrass fields

Se volatilization by soil bacterial strains isolated from the Salicornia & saltgrass

fields

With

Salic

ornia

With sa

ltgrass

Finding:

• Shewanella putrefaciens, a Salicornia-associated bacterial strain, volatilized more Se than any others.

Volatilization of Se into the Air

• An environmentally-important pathway of Se removal– Volatile Se compound, DMSe, is less toxic– Se removed from polluted ecosystem– Less hazardous waste

Lin et al., 2000, JEQ

Transport of Volatile Se in the Atmosphere

Phytoremediation Approaches:

Phyto-stabilization Phyto-

degradation

Rhizodegradation

PhytoextractionPhytovolatilization

Salicornia:A succulent, crunchy, and tasty vegetable; known as samphire, sea beans, or sea asparagus.

Selenium accumulation in Canola & Broccoli

• Canola:– In soil: ~ 2.5 ppm

• Extractable soil Se: ~0.5 ppm• In irrigation water: ~ 0.25 ppm

– In seed: ~ 1 ppm• Canola oil: ~ 1 ppm• Seed by-products: ~ 1 ppm

– Dried leaves: 2-5 ppm

• Broccoli:– In florets: ~ 4 ppm

Dr. Gary Bañuelos evaluates canola plants grown for cleaning selenium-rich soils. In studies on livestock, he is testing the

potential use of high-selenium canola forage as feed.

Growing Canola in the San Joaquin Valley, California

Se-laden Drainage Sediment & Phytoremediation Studies

Bañuelos & Lin, 2004, Ecotoxicology & Environ. Safety

Se Volatilization in Drainage Sediment

# of measurements (n) varied from 3 to 11in each month.

Phytoremediation: A Perspectiveof Ecosystem Ecology

Salicornia is a staple food for the endangered salt marsh harvest mouse.

ACKNOWLEDGEMENTS

• Graduate Students, Ramya Nadella, Bikram Shrestha, Shawn Lipe, SIU Edwardsville

• Irvin Arroyo, USDA-ARS, Water Management Research Lab

• Drs. N. Terry, A. Tagmount, H. Mohamed, A. Lee, UC Berkeley

• A. Illes, B. “moose” Peterson, H. Castle for the adapted illustration & photos

• California State Agricultural Research Initiative Grant (to Bañuelos)

• The Joint Interagency (DOE/NSF/EPA/ONR) Phytoremediation Research Program– U.S. DOE, Grant No. DE-FG02-03ER63621 (to Lin)