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Biological Transformation of Selenium in Soil-Plant Systems. Z-Q Lin 1 and Gary Bañuelos 2 1 Environ. Sci. Program & Dept. of Biol. Sci. Southern Illinois Univ., Edwardsville 2 USDA-ARS, Water Management Research Lab. Se. Selenium.

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biological transformation of selenium in soil plant systems

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



  • A narrow margin between nutritionally required and toxic concentrations
    • Essential for humans & animals
    • Not essential for plants
chemical forms of se
Chemical Forms of Se
  • Se(VI), selenat
  • Se(IV), selenite
  • Se(0), elemental Se
  • Se(-II), selenide
    • e.g., Selenomethionine (SeMet); Dimethylselenide (DMSe)
toxicity of se
Toxicity 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 Different Forms of Se to Fish


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

T=24 hrs

chemical speciation of se by x ray absorption spectroscopy xas
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
factors affecting se transformation in soil plant systems
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
selenium pollution sources predominant chemical forms
Selenium pollution sources & predominant chemical forms
  • Industry
    • Oil refinery wastewater with selenite
  • Agriculture
    • Drainage water with ~90% of selenate
The San Joaquin Valley:
  • One of the most productive agriculture areas
  • Subsurface tile-drainage contains Se & other salts.

Soils contain high levels of Se



drainage water reuse system zero discharge of drainage water into environment
Drainage Water Reuse System- Zero Discharge of Drainage Water into Environment

Solar Evaporator

52 ha

Halophytes Field

7.6 ha



192 ha

Salt-tolerant Trees

and Grasses

Lin et al., 2002, Water Research

the halophyte study field
The Halophyte Study Field

Cordgrass (Spartina sp.)

Pickleweed (Salicornia bigelovii)

why does salicornia volatilize more se than other species
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
Major Steps of Se Volatilization Pathway in Plant

Dimethyl selenide


















Soil Microbes

Selenate in Soil

question 1
Question 1:
  • Does Salicornia have an enhanced capacity of reducing selenate into organoselenium (SeMet) compounds?
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.

Lee & Lin et al. 2001. Planta

  • 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
Question 2:
  • Will chemical forms of selenium (e.g., selenate, selenite, SeMet) affect rates of Se volatilization in the soil-Salicornia system?

Plants Treated With:


Selenite, or


Salicornia bigelovii Torr.

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

76% of the total Se accumulated in Salicornia roots were SeMet-like organic compounds, while saltgrass accumulated 48% of SeMet-like compounds

Saltgrass root

Salicornia root

question 3
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 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 Isolated

From the Salicornia and/or Saltgrass fields

se volatilization by soil bacterial strains isolated from the salicornia saltgrass fields34
Se volatilization by soil bacterial strains isolated from the Salicornia & saltgrass fields

With saltgrass

With Salicornia

  • Shewanella putrefaciens, a Salicornia-associated bacterial strain, volatilized more Se than any others.
volatilization of se into the air
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








Phytoremediation Approaches:

salicornia a succulent crunchy and tasty vegetable known as samphire sea beans or sea asparagus
Salicornia:A succulent, crunchy, and tasty vegetable; known as samphire, sea beans, or sea asparagus.
selenium accumulation in canola broccoli
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.

se laden drainage sediment phytoremediation studies
Se-laden Drainage Sediment & Phytoremediation Studies

Bañuelos & Lin, 2004, Ecotoxicology & Environ. Safety

se volatilization in drainage sediment
Se Volatilization in Drainage Sediment

# of measurements (n) varied from 3 to 11

in each month.


Phytoremediation: A Perspective

of Ecosystem Ecology

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

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)