Diversity and evolution of 1,2-DCA Reductive Dehalogenases in polluted marine and freshwater environments

Halogenated organic compounds are extensively used in a wide range of industrial applications and have caused, as a consequence, serious environmental contaminations. Among them, 1,2-dichloroethane (1,2-DCA) is one of the most important quantitatively, being used as an intermediate for polyvinyl chloride (PVC) production. Microorganisms capable of deriving energy for their growth from the reduction of halogenated compounds are widespread in natural ecosystems. Organohalide-respiring bacteria belong to several phyla, including Proteobacteria, Firmicutes and Chloroflexi. The metabolic process known as reductive dehalogenation has been demonstrated to be a key microbial metabolism for groundwater bioremediation. Since dehalogenase homologous genes have been retrieved both in contaminated groundwater and seawater sites, aim of this work was to study and compare the phylogenetic and functional diversity of the reductive dechlorinating microbiota of freshwater and marine environments with different contamination backgrounds. Microcosms, inoculated with water and sediments from contaminated aquifers and coastal sites, were set up and spiked with known amounts of 1,2-DCA as the only electron acceptor and different sources of electron donors. A series of subsequent culture transfers, where an aliquot of the microbial consortia were inoculated in new microcosms containing fresh media, 1,2-DCA and the proper electron donor, were established with the aim of selecting the best adapted bacterial dehalogenating consortium. Bacterial phylogenetic diversity was monitored by 16S rRNA gene-based PCR-Denaturing Gradient Gel Electrophoresis (DGGE) and barcoded pyrosequencing, while the functional diversity was described by DGGE fingerprinting on dehalogenase encoding genes. The results indicated that both groundwater and seawater contaminated samples have dehalogenating potentials for remediation, showing complete degradation of 1,2-DCA in anaerobic microcosms and the enrichment of different dehalogenase-homologous encoding genes characterized by sequence domains specifically associated to 1,2-DCA metabolism. Despite a conserved 1,2-DCA reductive dechlorinating potential, the different environments and contamination context lead to the selection of specific communities with functional and phylogenetic diversity adapted to the peculiar geo-chemical settings.