Linked Redox Precipitation of Sulfur and Seleniumunder Anaerobic Conditions by Sulfate-Reducing BacterialBiofilms

Abstract

A biofilm-forming strain of sulfate-reducing bacteria (SRB), isolated from a naturally occurring mixed biofilm and identified by 16S rDNA analysis as a strain of Desulfomicrobium norvegicum, rapidly removed 200 M selenite from solution during growth on lactate and sulfate. Elemental selenium and elemental sulfur were precipitated outside SRB cells. Precipitation occurred by an abiotic reaction with bacterially generated sulfide. This appears to be a generalized ability among SRB, arising from dissimilatory sulfide biogenesis, and can take place under low redox conditions and in the dark. The reaction represents a new means for the deposition of elemental sulfur by SRB under such conditions. A combination of transmission electron microscopy, environmental scanning electron microscopy, and cryostage field emission scanning electron microscopy were used to reveal the hydrated nature of SRB biofilms and to investigate the location of deposited sulfur-selenium in relation to biofilm elements. When pregrown SRB biofilms were exposed to a selenite-containing medium, nanometer-sized selenium-sulfur granules were precipitated within the biofilm matrix. Selenite was therefore shown to pass through the biofilm matrix before reacting with bacterially generated sulfide. This constitutes an efficient method for the removal of toxic concentrations of selenite from solution. Implications for environmental cycling and the fate of sulfur and selenium are discussed, and a general model for the potential action of SRB in selenium transformations is presented.Sulfate-reducing bacteria (SRB) are a phylogenetically and physiologically diverse group of bacteria, characterized by their common capacity to conserve energy for growth by linking the oxidation of various substrates to the dissimilatory reduction of sulfate (S 6ϩ ) to sulfide (S 2Ϫ ). As such, SRB comprise a functional group within a sulfuretum, linking broad-scale cycling between sulfate and sulfide by ecological communities of SRB and sulfide-oxidizing bacteria (12, 28). Biological reoxidation of reduced sulfur species typically occurs at oxic-anoxic transition zones and is attributed largely to phototrophs and chemolithotrophs. Small-scale cycling through elemental sulfur (S 0 ) also occurs and is generally attributed to syntrophic associations of sulfide oxidizers and sulfur reducers (3,17).The biological cycling of selenium is receiving increasing attention, due not only to the biological importance of selenium as an essential trace element but also to the potential for selenium pollution to cause significant ecological damage (42). Selenium is a group 16 metalloid element possessing several stable oxidation states. Under oxic conditions, selenium is present mostly as the oxyanions selenite (SeO 3 2Ϫ , Se 4ϩ oxidation state) and selenate (SeO 4 2Ϫ , Se 6ϩ oxidation state), whereas under anoxic conditions, selenide (Se 2Ϫ ) and elemental selenium (Se 0 ) appear predominant (5, 6). Selenium is incorporated by organisms through selenide, is important in some enzyme systems,...

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