University of California, Riverside

Environmental Sciences

Research Interests--William Frankenberger

Microbial Production of Phytohormones in Soils

For the last two decades of my career, the main focus of my research has been on precursor-dependent plant hormone production of microbial origin. An exogenous source of phytohormones promotes a more vigorous root system within the plant resulting in yield enhancement. We can alter plant growth by physiological manipulation, strategically inoculating "living" hormone producers. With autoradiography, I have demonstrated that plants do take up exogenous sources of plant hormones within the root zone. The application of precursors to the rhizosphere provides a delayed continuous source of phytohormones to the roots. This work allows for more consistent crop yield in greenhouse and field studies. One landmark publication of my work was published in Applied Environmental Microbiology (54:2728-2732) demonstrating the ecological significance of microbially-produced ethylene. By using an ethylene bioassay, the classic "triple"response in etiolated pea seedlings, I demonstrated that ethylene of microbial origin could induce reduction in stem elongation, swelling of the hypocotyl and change in direction of growth (horizontal). My research is highlighted in my two books, Phytohormones in Soils: Microbial Production and Function (1995) and Ethylene: Agricultural Sources and Applications (2002).

Bioremediation of Selenium

Another primary area of my research is devoted to the biogeochemistry of selenium (Se) in the environment. Microorganisms play a major role in the geochemical cycles on earth. Microbial transformations including oxidation, reduction, methylation, and demethylation reactions affect the solubility, toxicity, sorption, volatility and specific gravity of metals and metalloids. Selenium accumulation in evaporation ponds throughout central California has killed many birds and waterfowl and is blamed in causing deformities in offspring. Remediation techniques are being investigated to deal with this problem. My laboratory has investigated volatilization of Se as a means to dissipate Se vapor from seleniferous soils, sediments and water. The global atmospheric flux of Se from natural sources is estimated at 9.3 x 109 g/yr. Ninety percent of the natural emission is thought to be of biogenic processes, approximately 1/3 of them being continental and 2/3 marine. On the continent, approximately 2/3 of the biological emissions is thought to be produced by microorganisms and 1/3 by higher plants. I have developed this remedial strategy based on molecular and biochemical studies to optimize gaseous Se production. Our studies involve enzymes kinetics of Se75 alkylation and interactions of transmethyl donors, cofactors, heavy metals, redox reactions, selenium species and osmotic tension. As a result of these extensive studies, I have patented a bioremediation approach to stimulate this natural process.

In addition, I have studied the reduction of Se oxyanions (selenate and selenite) in aquatic systems to elemental Se (ppt). Elemental selenium [Se(0)] is considered as an unavailable form of Se because of its insolubility. Formation of Se(0) in aquatic systems generally involves a two-step reduction process from Se(VI) to Se(IV) and then to colloidal Se(0). Se(0) can directly attach on the surface of the sediment where it is formed. Colloidal aggregation and flocculation or attachment to inorganic and organic particles in water, can lead to settling and removal of Se(0) from the water column. I have edited 2 books on the environmental fate of selenium including Selenium in the Environment (1994) and Environmental Chemistry of Selenium (1998).

Bioremediation of Perchlorate

My most recent area of study involves the environmental fate of perchlorate. Perchlorate (ClO4-) is a strong oxidizing agent that has been used industrially as a component of solid propellant in rockets and missiles, as well as in explosives and various pyrotechnics. Perchlorate contamination is a widespread problem. It has been found in water supplies in at least 22 states, including Inland water basins serving Rialto, Redlands and Riverside, as well as in Colorado River water used to irrigate crops in the Imperial Valley. Perchlorate contamination of ground water has been estimated to potentially affect the drinking water supplies of at least 12 million people in the United States. I have developed treatment technologies for perchlorate involving microbial-mediated remediation. My publications involve in-depth studies on the biochemistry of perchlorate transformations including kinetics and inhibition as well as molecular analysis of perchlorate reductase.

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University of California, Riverside
900 University Ave.
Riverside, CA 92521
Tel: (951) 827-1012

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Environmental Sciences
Geology 2460

Tel: (951) 827-5116
Fax: (951) 827-4652