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Biological Transformation of Selenium in Soil-Plant Systems

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

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  1. 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

  2. Se Selenium • A narrow margin between nutritionally required and toxic concentrations • Essential for humans & animals • Not essential for plants

  3. Chemical Forms of Se • Se(VI), selenat • Se(IV), selenite • Se(0), elemental Se • Se(-II), selenide • e.g., Selenomethionine (SeMet); Dimethylselenide (DMSe)

  4. 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.

  5. Toxicity of Different Forms of Se to Fish T=0 Se(-II) Se(IV) Se(VI) CK T=24 hrs

  6. What are the major chemical forms of Se accumulated in soil-plant systems?

  7. 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

  8. Chemical Speciation in Environmental Samples by XAS

  9. 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

  10. Selenium pollution sources & predominant chemical forms • Industry • Oil refinery wastewater with selenite • Agriculture • Drainage water with ~90% of selenate

  11. 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

  12. Drainage Water Reuse System- Zero Discharge of Drainage Water into Environment Solar Evaporator 52 ha Halophytes Field 7.6 ha Cotton Alfalfa 192 ha Salt-tolerant Trees and Grasses Lin et al., 2002, Water Research

  13. The Halophyte Study Field Cordgrass (Spartina sp.) Pickleweed (Salicornia bigelovii)

  14. Distribution of Salicorniabigelovii Torr. in the U.S. (Data from USDA)

  15. 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?

  16. Major Steps of Se Volatilization Pathway in Plant Dimethyl selenide Methyl-SeMet Se-Methionine Se-Cysteine Selenate Selenite APSe

  17. DMSe Hypothesis: Salicornia SeMet Selenite Selenate SeMet DMSe Selenate SeMet Soil Microbes Selenate in Soil

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

  19. 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

  20. 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.

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

  22. Plants Treated With: Selenate, Selenite, or Selenomethionine Salicornia bigelovii Torr.

  23. Se Volatilization Measurements

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

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

  26. 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

  27. 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?

  28. 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.

  29. Volatilization of Se by Bacteria Isolated From the Salicornia and/or Saltgrass fields

  30. Se volatilization by soil bacterial strains isolated from the Salicornia & saltgrass fields With saltgrass With Salicornia

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

  32. 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

  33. Transport of Volatile Se in the Atmosphere Lin et al., 2000, JEQ

  34. Phytoextraction Phytovolatilization Phyto- stabilization Phyto- degradation Rhizodegradation Phytoremediation Approaches:

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

  36. 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

  37. 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.

  38. Growing Canola in the San Joaquin Valley, California

  39. Se-laden Drainage Sediment & Phytoremediation Studies Bañuelos & Lin, 2004, Ecotoxicology & Environ. Safety

  40. Se Volatilization in Drainage Sediment # of measurements (n) varied from 3 to 11 in each month.

  41. Phytoremediation: A Perspective of Ecosystem Ecology Salicornia is a staple food for the endangered salt marsh harvest mouse.

  42. 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) ACKNOWLEDGEMENTS

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