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SummaryThe fate and persistence of chlorinated organics in the environment have been a concern for the past 50 years. Industrialization and extensive agricultural activities have led to the accumulation of these pollutants in the environment, while their adverse impact on various ecosystems and human health also became evident. This review provides an update on the current knowledge of specialized anaerobic bacteria, namely ‘Dehalococcoides’ spp., which are dedicated to the transformation of various chlorinated organic compounds via reductive dechlorination. Advances in microbiology and molecular techniques shed light into the diversity and functioning of Dehalococcoides spp. in several different locations. Recent genome sequencing projects revealed a large number of genes that are potentially involved in reductive dechlorination. Molecular approaches towards analysis of diversity and expression especially of reductive dehalogenase‐encoding genes are providing a growing body of knowledge on biodegradative pathways active in defined pure and mixed cultures as well as directly in the environment. Moreover, several successful field cases of bioremediation strengthen the notion of dedicated degraders such as Dehalococcoides spp. as key players in the restoration of contaminated environments.

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.Halogenated organic compounds are among the most widespread environmental pollutants. Although these compounds were previously believed to be only anthropogenic, a large number of them, including aliphatic, aromatic, and heterocyclic derivatives, are introduced into the environment via biogenic and geogenic sources (9, 21). Hexachlorobenzene (HCB) is believed to be persistent in the environment (22) due to its chemical stability and its resistance to biodegradation. HCB is a hydrophobic and bioaccumulative compound and is listed in the EC Directive (15) as a "priority hazardous substance." At the peak production of HCB in the early 1980s, thousands of tons were produced to be used as fungicides, wood preservatives, and porosity control agents or in the manufacturing of dyes. The use of HCB is no longer allowed in most countries because of its toxicity and carcinogenicity toward fish and mammals. Nevertheless, it is still being released into the environment as a by-product of various chemical processes, as a result of incomplete combustion, or from old landfills (4, 6, 7). HCB contamination has been reported in different environments. Compared to rivers in sparsely populated regions, lakes, and the sea (32, 42), significantly larger amounts of HCB could be found in river water in agricultural areas and in densely populated or highly industrialized areas. HCB concentrations were shown to positively correlate with organic matter content of sediments and soils, and European soils were observed to have the highest HCB concentrations globally (38). Several authors reported o...

Recently, S-metric spaces are introduced as a generalization of metric spaces. In this paper, we consider the relationships between of an S-metric space and a metric space, and give an example of an S-metric which does not generate a metric. Then, we introduce new contractive mappings on S-metric spaces and investigate relationships among them by counterexamples. In addition, we obtain new fixed point theorems on S-metric spaces.

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