This need led to a number of studies showing that donorcompeting terminal electron accepting processes affect not only the rates but also the extent of reductive AG-013736 VEGFR/PDGFR inhibitor dechlorination of chlorinated ethenes. At contaminated sites undergoing in situ bioremediation, these processes could lead to minimal biostimulation of Dehalococcoides, prolonged lag times before the onset of dechlorination, and/or incomplete dechlorination. But despite the acknowledgement of donor competition as a variable in reductive dechlorination, its importance is often dismissed at field sites on the grounds that fermentable substrates responsible for H2 production are added in excess. In this study, microcosms were biostimulated with TCE and fermentable substrates to promote growth of Dehalococcoides mccartyi but stalled at cis-DCE, irrespective of the fact that electron donor was supplied,150 times in stoichiometric excess for complete dechlorination of TCE to ethene. Our findings strongly support electron donor competition for the inability to produce VC and ethene in microcosms stalled at cis-DCE. Transfers from stalled microcosms in the absence of soil/sediment produced ethene and yielded in subsequent enrichment cultures robust growth of Dehalococcoides mccartyi and fast rates of dechlorination. Lactate and methanol were readily fermented through acetate and propionate, and methane evolution was recorded within the first weeks of incubation in these microcosms. We believe this outcome was obtained by removing or diluting other competing electron acceptors present in the soil/sediment which were supporting microbial guilds competing for the electron donor. The most abundant sequences for all three enrichment cultures belong to Clostridia. The obvious increase in relative abundance of this class containing fermenting bacteria was the result of feeding excess fermentable substrates throughout the enrichment process. For bioremediation of PCE or TCE contaminated sites, microcosm experiments have historically been utilized as indicators of indigenous microbial activity. The results of microcosm experiments help researchers and bioremediation practitioners decide whether biostimulation or bioaugmentation is the appropriate treatment for decontamination of environments polluted by chlorinated solvents. In cases where incomplete dechlorination was observed in microcosms, this has been attributed to the presence of inhibitors in the soil or the lack of Dehalococcoides mccartyi capable of complete dechlorination. Our findings clearly show that neither result from Cuzdrioara nor Carolina biostimulated microcosms could be explained by these two hypotheses. Instead, an electron donor competition is proposed, supported by our data, in which components of the soil or sediment serve as electron acceptor for competing H2oxidizing microorganisms. Our results bring experimental evidence towards a new possible explanation to “unsuccessful” microcosm experiments. If indeed microbial competition for electron donor is a major determining factor in the success of established microcosms, it will certainly be a determining factor in bioremediation as well, and adding excess electron donor to biostimulate could prove unsuccessful.