Reductive dechlorinating microbiomes in contaminated seawater sediments and groundwaters

Halogenated organic compounds are extensively used in industry and agriculture and are, as a consequence, common contaminants in waters and sediments. Organohalide-respiring bacteria, capable of deriving energy for growth from dehalogenation, belong to several phyla, including Proteobacteria, Firmicutes and Chloroflexi. Reductive dehalogenation has been demonstrated to be a key microbial metabolism for groundwater remediation, and dehalogenase homologous genes have been retrieved both in contaminated groundwater and seawater environments. Aim of this work was to study and compare the phylogenetic and functional diversity of the reductive dechlorinating microbiota in groundwater and seawater sediments from sites with a different contamination context. Sediment slurry microcosms have been set up and spiked with known amounts of 1,2-dichloro ethane (1,2-DCA) as electron acceptor and different sources as electron donors. A series of subsequent culture transfers, where the dehalogenating consortia were inoculated in fresh media supplemented with 1,2-DCA and the proper electron donor, were established with the aim of selecting the best adapted bacterial 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 conserved domains of the dehalogenase encoding genes. The results indicated that both groundwater and seawater examined 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.