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Bioremediation and Biodegradation

The Potential of Rhizosphere Microbes Isolated from a Constructed Wetland to Biomethylate Selenium

^a Research and Development Center, The Galilee Society (affiliated with Haifa University), P.O. Box 437, Shefa-Amr 20200, Israel
^b Institute of Phytospheric Research, Research Center Juelich, 52425 Juelich, Germany

* Corresponding author (hazaizi{at}gal-soc.org)

Received for publication February 12, 2002. The potential of rhizosphere microbes isolated from common reed [Phragmites australis (Cav.) Trin. ex Steud] plants grown in a subsurface-flow constructed wetland to biomethylate selenate or selenite was studied in liquid cultures under controlled conditions. Total mean percentages of volatilized Se from half-strength Hoagland culture solutions (low C content) supplemented with selenate or selenite and inoculated with cultured rhizosphere microbes after 15 d of incubation were 7.9 and 49.1%, respectively. There was a relative best fit (r = 0.87) between total number of rhizosphere and cultured microbes and the percentage of volatilized Se in Hoagland solution after 15 d of incubation. However, when the same microbes were cultured in tryptic soybean broth (TSB) medium (high C content), the percentages of volatilized Se from selenate and selenite were 1.3 and 1.9%, respectively. The volatilization percentages of Se from selenate or selenite in culture solutions inoculated with rhizosphere suspension instead of cultured rhizosphere microbes were very low (1.2–3.0%) in both cultivation media. In all experiments, selenite was volatilized significantly (p < 0.05) in higher amounts by cultured rhizosphere microbes after 15 d of incubation compared with selenate. Dissolved biomethylated dimethylselenide (DMSe) in water samples taken from the subsurface-flow bed was determined by purging with helium. The DMSe in water samples was indirectly detected up to 2.4 µg Se L^-1, which indicates that part of the produced DMSe was dissolved in the matrix before being released into the atmosphere. Our results show that rhizosphere microbes isolated from common reed plants have a high potential of Se biomethylation and volatilization from selenate and selenite.

Abbreviations: AMVF, aquatic microcosm vegetation facility • CFU, colony forming units • DMDSe, dimethyldiselenide • DMSe, dimethylselenide • HG–AAS, hydride generation–atomic absorption spectrometry • ICP–MS, inductively coupled plasma–mass spectrometry • TSB, tryptic soybean broth