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Complete anaerobic dechlorination of chlorinated solvents such as trichloroethene (TCE) is essential for bioremediation of chloroethene-contaminated sites. We studied the influence of sulfate on microbial dechlorination of TCE to ethene both under transient and steady-state conditions, encompassing the range of hydrogen (H₂) levels commonly found at contaminated sites. The results show that sulfate at a concentration of 2.5mM limits microbial dechlorination by a mixed anaerobic culture by reducing the rate under steady-state hydrogen supply (a few nM H₂), implying a H₂ limited dechlorination. Conversely, sulfate did not affect dechlorination when rapid fermentation of lactate resulted in transient buildup of H₂ to levels around two orders of magnitude higher compared to steady-state conditions. This has important implications both for optimizing culture conditions for dehalogenating microorganisms and for the …