Tracking functional guilds: "Dehalococcoides" spp. in European river basins contaminated with hexachlorobenzene.

Hexachlorobenzene (HCB) has been widely used in chemical manufacturing processes and as a pesticide. Due to its resistance to biological degradation, HCB has mainly accumulated in freshwater bodies and agricultural soils. "Dehalococcoides" spp., anaerobic dechlorinating bacteria that are capable of degrading HCB, were previously isolated from river sediments. Yet there is limited knowledge about the abundance, diversity, and activity of this genus in the environment. This study focused on the molecular analysis of the composition and abundance of active Dehalococcoides spp. in HCB-contaminated European river basins. 16S rRNA-based real-time quantitative PCR and denaturing gradient gel electrophoresis in combination with multivariate statistics were applied. Moreover, a functional gene array was used to determine reductive dehalogenase (rdh) gene diversity. Spatial and temporal fluctuations were observed not only in the abundance of Dehalococcoides spp. but also in the composition of the populations and rdh gene diversity. Multivariate statistics revealed that Dehalococcoides sp. abundance is primarily affected by spatial differences, whereas species composition is under the influence of several environmental parameters, such as seasonal changes, total organic carbon and/or nitrogen content, and HCB contamination. This study provides new insight into the natural occurrence and dynamics of active Dehalococcoides spp. in HCB-contaminated river basins.