The stability of microbial communities has been suggested to correlate not only to the number of species occupying a niche but also to the functional redundancy found in the same niche [32, 33]. Under this concept, microbial communities should be more flexible if the same function prevails under varying environmental conditions and is supported by enzymes adapted to the different conditions the microbial community is exposed to. In this study, a reductive dechlorination activity was linked to an adapted and specialized bacterial community that presented, after suitable stimulation, a high functional redundancy. According to the three diversity indexes (species richness, Simpson and Shannon index) and the Pareto Lorenz curve representation, the bacterial community is characterized by a good richness and few dominant species. Following the interpretation reported by Marzorati and colleagues , the structure of the studied microbial community (the 20% of the OTUs represent the 65-70% of the total abundance of clones) can be considered typical of a specialized community (as an enriched microbial community should be). The 16S rRNA gene clone library showed that the 1,2-DCA-degrading consortium was equally dominated by two different taxa related to Desulfitobacterium dichloroeliminans strain DCA1 and Dehalobacter sp. WL., previously shown to be involved in the 1,2-DCA reductive dechlorination. In other studies different bacteria within the same microbial community were shown to exhibit specific activities towards different chlorinated congeners . Coherently with our results, Grostern and colleagues  described the co-growth of more microorganisms competing for the same chlorinated compound. As suggested by the authors, the co-growth probably occurs due to the presence of slightly different niches for each bacteria that avoid their direct competition. We hypothesize that the differences in niche partitioning in our culture are due to the presence of a suitable and not limiting growth substrate and electron donor. This enrichment conditions enhanced the growth of those portion of the microbial community that might function as a flexible reservoir of various degraders of halogenated compounds.
The third most abundant OTU was related to Desulfitobacterium metallireducens, identified for the first time by Finneran and colleagues  in an uranium-contaminated aquifer. Desulfitobacterium metallireducens was described as an anaerobic bacterium able to couple its growth to the reduction of metals, humic acids and chlorinated compounds (trichloroethylene or tetrachloroethylene) using lactate as electron donor . However its involvement in 1,2-DCA dehalogenation was not demonstrated. The co-growth in dechlorinating enrichment cultures of bacteria not directly related to the dehalogenating activity but with an indirect role in the process is commonly reported in literature [37, 38, 26]. This is supported also by the identification of sequences related to Lactobacillus sp. and Zymophilus sp., species known to be able to produce lactate and other organic acids useful as electron donors for the reductive dechlorination process. The structure and composition of the microbial community suggested that the positive response to the treatment is possibly led by a 'complementarity effect' rather than a 'selection effect' .
A further sign of the 'complementarity effect' was the identification of possible functional redundancy related to 1,2-DCA reductive dechlorination. The RDs gene clone library showed four different RDs enzymes, all conserving signature residues possibly linked to 1,2-DCA dehalogenation [25, 26]. Two main groups of RDs (cluster I and III) characterized the consortium and it was possible to connect the presence of these functional genes to the two most abundant OTUs identified in the 16S rRNA gene library. Sequences belonging to the group I were associated to Desulfitobacterium dichloroeliminans strain DCA1, that showed the same amino acid signatures in the two reductive dehalogenases in its genome . Sequences of the group III most likely belonged to Dehalobacter. In fact, taking into account that i) rdhA1 of Dehalobacter sp. WL was found in a culture amended with only 1,2-DCA; ii) its abundance was shown to be correlated with both Dehalobacter growth and 1,2-DCA dechlorination activity; and that iii) Dehalobacter sp. WL rdhA1 transcription occurred upon exposure to 1,2-DCA, Grostern and colleagues  concluded that rdhA1 represents a putative Dehalobacter 1,2-DCA RD gene. At the moment, regarding the RDs sequences of the groups II and IV, we have no definitive proof or literature data supporting their involvement in any specific dehalogenation process.
Finally, the occurrence of Dehalobacter sp. WL rdhA2 and rdhA3 in similar abundances in a 1,1,2-TCA-amended culture was correlated to a second Dehalobacter strain that uses 1,1,2-TCA and, to a lesser extent, 1,2-DCA . Our findings could support this hypothesis, with strain WL rdhA3 that clusters together rdhA1 in group III (Figure 2) and rdhA2 completely separated from any other cluster and thus possibly not involved in 1,2-DCA degradation.