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availability in sediments would assist in determining the long-term toxicity at selenium-impacted lakes and wetlands. 3. BIOTRANSFORMATIONS OF SELENIUM Section 2 discussed some of the microbially mediated redox reactions of selenium. This section discusses specific transformations within organisms and the mechanisms of toxicity. The complexity of selenium biochemistry is illustrated by recent research that shows how selenium deficiency affects reproduction in mammals. The investigators note that proteins often lead "multiple lives" depending upon their environment, but it is rare for a protein to act as a soluble enzyme under some circumstances and as an insoluble structural component in others. Apparently, a selenium-containing protein in sperm does behave in this fashion, which explains why selenium deficiency causes infertility. Apparently sperm with low selenium break in the middle (Ursini et al. 1999). Selenium is similar to sulfur biochemically, and cells do not discriminate between the two when carrying out protein synthesis. Substitution of selenium for sulfur disrupts the normal chemical bonding, which results in improperly formed and dysfunctional proteins and enzymes. Selenium is also necessary for proper formation and functioning of glutathione peroxidase which is a major cellular antioxidant enzyme. The enzyme protects cell membranes from damage of lysis due to lipid peroxidiation (Lemly 1998). Primary producers transform selenium into selenoamino acids that are incorporated into selenoproteins (Brix et al. 1997). The latter investigators demonstrate that microbial communities accumulate selenium and that uptake of selenite is favored over selenate. Further, these investigators note that microbial uptake explains the high concentrations of selenium in midges collected from seleniferous locations. It has also been demonstrated that aquatic microphytes volatilize selenium in the form of alkylselenides (Fan and Higashi 1997). For instance, a laboratory study shows that a filamentous cyanophyte mat cultured from waters of a large agricultural drainage basin in California volatilized selenium from selenite. Nearly 100% selenium removal occurs in the basin, and it was speculated that this is the reason. Selenite was incorporated into proteins in the form of selenomethionine at selenium concentrations of 5000 µg/L or higher in the laboratory. This reaction, however, has not been verified in the field (Fan et al. 1998). Although the structures of several selenium-containing metabolites have been identified (Fan et al. 1988), in general, little is known about specific selenium biotransformations. For this reason, research has often been conducted. with selenomethionine. One study, however, found only ultra- trace levels of this compound in the primary producers surveyed, suggesting that it may not always be a good. surrogate for biological exposure (Fan and Higashi 1997). Another complexity with respect to field research is that other ions affect selenium uptake. For example, research shows that silicate and. phosphate enhance the uptake of selenium by Chlainydomonas reinhardtii by a factor of three to five (Riedel et al. 1997). Other commonly occurring ions tested show little effect. Another study indicates that Mo, Hg, Cr, and As can inhibit microbial volatilization of selenium (Karlson and Frankenberger 1988). Data showing the