Effects of growth conditions
on production of methyl selenides
in cultures of Rhodobacter sphaeroides

V. Van Fleet-Stalder1, H. Gürleyük1, R. Bachofen2, and T.G. Chasteen1*
1Department of Chemistry, Sam Houston State University, Huntsville, Texas, 77341-2117
2Institute for Plant Biology and Microbiology, University of Zürich, Switzerland

Journal of Industrial Microbiology and Biotechnology, 1997, 19, 98-103. JIMB abstract


Rhodobacter sphaeroides 2.4.1 exposed to selenate or selenite produced volatile selenium compounds. Total amounts of dimethyl selenide, dimethyl diselenide, dimethyl sulfide and dimethyl disulfide in culture medium and headspace were determined. The highest selenate volatilization occurred in the late stationary phase of growth. However, cultures deprived of light in the stationary phase of growth produced much less of the volatile organo-selenium compounds. Lower culture pHs increased the rate of selenium volatilization. Low sulfate concentration limited biomass production and selenium volatilization; high sulfate concentrations had an enhancing effect on the release of organo-selenium compounds. Cultures of R.Êsphaeroides reacted very differently to amendments with increasing amounts of selenate and selenite. Only small amounts of selenite were volatilized; meanwhile high amounts of methylated selenides were found in selenate poisoned cultures. Keywords: Rhodobacter sphaeroides, selenate, selenite, reduction, methylation.

In a comparison of samples incubated in the light or in the dark, this figure compares the production of dimethyl selenide (CH3SeCH3), dimethyl selenenyl sulfide (CH3SeSCH3), and dimethyl diselenide (CH3SeSeCH3) in cultures of Rhodobacter sphaeroides 2.4.1 grown on a minimal medium at about 28 degrees C. Clearly the samples grown in light produced more organoselenides even though, compared to the dark samples, their cell populations were not significantly different. This strongly suggest that reduction and methylation of selenate in these cultures was favored by light driven processes.

The effects on bacterial growth of a range of culture pH was investigated in this second figure. While biomass production appears to maximize at a pH of about 6 to 6.5 depending on whether or not selenate has been added, dimethyl selenide production was not a simple function of pH.

Three days after adding 1 mM selenate to the following cultures, the effect of different carbon sources were investigated. In general, the higher the redox potential of a carbon source (more oxidized), the higher the concentration of the organoselenides determined in the bacterial headspace of cultures grown on that substrate.

Finally, the variation of amounts of selenate and selenite added to this microorganism grown on a single carbon source (20 mM succinate) showed interesting effects on biomass production and headspace production of organoselenides determined after 7 days of photosynthetic growth. Selenate at levels of 6 mM and more led to a decrease in biomass production in poisoned cultures (Figure 4A). The highest amounts of dimethyl selenide were found in the headspace above cultures amended with 6 to 10 mM selenate; the highest yield of 0.16% (selenium contained in dimethyl selenide and dimethyl diselenide) occurred when the cultures were amended with 6 mM selenate.

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