AbstractSelenium (Se) is a contaminant of agricultural drainage water in the San Joaquin Valley, California and the cause of major toxicity problems among the wildlife. The objective of this study was to investigate the stimulatory properties of proteins and their components with respect to biological Se dissipation. Laboratory studies showed that Se biomethylation is protein‐peptidelimited rather than nitrogen‐, amino acid‐ or carbon‐limited. Crude casein and its components, ∂‐, β‐, and κ‐caseins and peptides (1 g carbon C L−1pond water) were equally stimulatory producing a>50‐fold enhancement in dimethylselenide (DMSe) yield. Dephosphorylation of casein resulted in a 57 reduction in Se volatilization. Dialysis studies indicated that the active ingredient of casein is likely to be a peptide or peptide mixture. Casein did not appear to increase Se methylation through any physicochemical characteristics such as a change in pH or chelation of pond water ions. Alternative protein sources including cottonseed meal, cheese whey and yeast sludge dramatically increased methylation (29‐fold, 300‐fold and 41‐fold, respectively) over unamended samples, with over 10 of the Se inventory converted into DMSe in 17 d. A single casein amendment (0.2 g CL−1) to water columns in the field caused a 38 Se loss from the initial inventory of a San Joaquin Valley evaporation pond in 142 d. Under optimum conditions, protein‐mediated deseleinfication of agricultural drainage water is a promising and novel bioremediation technique t
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