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
Se
Selenium
• A narrow margin between nutritionally
required and toxic concentrations
– Essential for humans & animals
– Not essential for plants
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
• 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
T=0
Se(-II)
Se(IV)
Se(VI)
CK
T=24 hrs
What are the major chemical
forms of Se accumulated in soilplant 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
Normalized Absorbance
2.5
STANDARDS
Se(VI)
Se(IV)
Se(0)
SeMet
2.0
1.5
1.0
Sediment (0-2 cm)
0.5
0.0
12650
12660
12670
Energy (eV)
12680
12690
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
• 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
East
West
Drainage Water Reuse System
- Zero Discharge of Drainage Water into Environment
Solar Evaporator
52 ha
7.6 ha
Halophytes Field
Cotton
Alfalfa
192 ha
Salt-tolerant Trees
and Grasses
Lin et al., 2002, Water Research
The Halophyte
Study Field
Cordgrass (Spartina sp.)
Pickleweed (Salicornia bigelovii)
e
at
iv
N
gr
as
s
le
x
rip
At
ss
Sa
ltg
ra
ss
gr
a
or
d
C
rn
ia
Sa
lic
o
-2
µg Se m d
-1
Maximum Rates of Se Volatilization
500
400
300
200
100
0
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
Dimethyl selenide
Methyl-SeMet
Se-Methionine
Selenate
APSe
Selenite
Se-Cysteine
DMSe
Hypothesis:
SeMet
Selenite
Salicornia
Selenate
SeMet
Selenate
DMSe
SeMet
Soil Microbes
Selenate in Soil
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
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?
Plants Treated With:
Selenate,
Selenite, or
Selenomethionine
Salicornia bigelovii Torr.
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)
SeMet
Se(0)
Se volatilization by soil bacterial strains
isolated Maximum
from theRates
Salicornia
saltgrass fields
of Se &Volatilization
500
300
200
100
ra
tg
Sa
l
ico
rn
ia
ss
0
Sa
l
g Se m-2 d-1
400
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 Isolated
From the Salicornia and/or Saltgrass fields
g volatile Se per 500 ml culture per day
Se volatilization by soil bacterial strains
isolated from the Salicornia & saltgrass fields
300
Common
Species-specific
250
200
150
100
50
0
c
Ba
s
illu
bt
su
ilis
s
Ba
lu
cil
lo
ha
cis
s
an
r
u
nt
a
s
d
ns
e
i
c
a
hr
lu
cil
a
B
a
ell
e
Sh
n
wa
fa
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e
Ps
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na
o
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t
pu
ida
Finding:
• Shewanella putrefaciens, a Salicorniaassociated 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
Transport of Volatile Se in the Atmosphere
Lin et al., 2000, JEQ
Phytoremediation Approaches:
Phytoextraction
Phytostabilization
Rhizodegradation
Phytovolatilization
Phytodegradation
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 seleniumrich 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
100
Bare Soil
In a Brassica Field
-2
g Se m d-1
80
60
40
20
0
May
Jun
Jul
Aug
Sep
Oct
Nov
Time (2002-03)
Dec
Jan
Feb
# 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.
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)