Dechlorination of three tetrachlorobenzene isomers by contaminated harbor sludge-derived enrichment cultures follows thermodynamically favorable reactions.

Dechlorination patterns of three tetrachlorobenzene isomers, 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-TeCB, were studied in anoxic microcosms derived from contaminated harbor sludge. The removal of doubly, singly, and un-flanked chlorine atoms was noted in 1,2,3,4- and 1,2,3,5-TeCB fed microcosms, whereas only singly flanked chlorine was removed in 1,2,4,5-TeCB microcosms. The thermodynamically more favorable reactions were selectively followed by the enriched cultures with di- and/or mono-chlorobenzene as the main end products of the reductive dechlorination of all three isomers. Based on quantitative PCR analysis targeting 16S rRNA genes of known organohalide-respiring bacteria, the growth of Dehalococcoides was found to be associated with the reductive dechlorination of all three isomers, while growth of Dehalobacter, another known TeCB dechlorinator, was only observed in one 1,2,3,5-TeCB enriched microcosm among biological triplicates. Numbers of Desulfitobacterium and Geobacter as facultative dechlorinators were rather stable suggesting that they were not (directly) involved in the observed TeCB dechlorination. Bacterial community profiling suggested bacteria belonging to the phylum Bacteroidetes and the order Clostridiales as well as sulfate-reducing members of the class Deltaproteobacteria as putative stimulating guilds that provide electron donor and/or organic cofactors to fastidious dechlorinators. Our results provide a better understanding of thermodynamically preferred TeCB dechlorinating pathways in harbor environments and microbial guilds enriched and active in anoxic TeCB dechlorinating microcosms.

Application of biodegradation in mitigating and remediating pesticide contamination of freshwater resources: state of the art and challenges for optimization.

In recent years, the application of pesticide biodegradation in remediation of pesticide-contaminated matrices moved from remediating bulk soil to remediating and mitigating pesticide pollution of groundwater and surface water bodies. Specialized pesticide-degrading microbial populations are used, which can be endogenous to the ecosystem of interest or introduced by means of bioaugmentation. It involves (semi-)natural ecosystems like agricultural fields, vegetated filter strips, and riparian wetlands and man-made ecosystems like on-farm biopurification systems, groundwater treatment systems, and dedicated modules in drinking water treatment. Those ecosystems and applications impose challenges which are often different from those associated with bulk soil remediation. These include high or extreme low pesticide concentrations, mixed contamination, the presence of alternative carbon sources, specific hydraulic conditions, and spatial and temporal variation. Moreover, for various indicated ecosystems, limited knowledge exists about the microbiota present and their physiology and about the in situ degradation kinetics. This review reports on the current knowledge on applications of biodegradation in mitigating and remediating freshwater pesticide contamination. Attention is paid to the challenges involved and current knowledge gaps for improving those applications.

Impact of dry-wet and freeze-thaw events on pesticide mineralizing populations and their activity in wetland ecosystems: A microcosm study.

Riparian wetlands are proposed to mitigate diffuse pollution of surface water by pesticides in agricultural landscapes. Wetland ecosystems though are highly dynamic environments and seasonal disturbances such as freezing and drying can affect microbial population sizes in the sediment and their functionality including pesticide biodegradation, which has hardly been studied. This study examined the effect of artificially induced dry-wet or freeze-thaw events on the mineralization of the pesticides isoproturon (IPU) and 2-methoxy-4-chlorophenoxy acetic acid (MCPA) in wetland microcosms, either without or with prior enrichment of IPU/MCPA degrading populations. Without prior enrichment, mineralization of IPU and MCPA was significantly reduced after exposure to especially freeze-thaw events, as evidenced by lower mineralization rates and longer lag times compared to non-exposed microcosms. However, herbicide mineralization kinetics correlated poorly with cell numbers of herbicide mineralizers as estimated by a most probable number (MPN) approach and the number of IPU and MCPA mineralizers was unexpectedly higher in freeze-thaw and dry-wet cycle exposed setups compared to the control setups. This suggested that the observed effects of season-bound disturbances were due to other mechanisms than decay of pesticide mineralizers. In addition, in systems in which the growth of pesticide mineralizing bacteria was stimulated by amendment of IPU and MCPA, exposure to a freeze-thaw or dry-wet event only marginally affected the herbicide mineralization kinetics. Our results show that season bound environmental disturbances can affect pesticide mineralization kinetics in wetlands but that this effect can depend on the history of pesticide applications.

Diversity of dechlorination pathways and organohalide respiring bacteria in chlorobenzene dechlorinating enrichment cultures originating from river sludge.

Anaerobic reductive dechlorination of hexachlorobenzene (HCB) and three isomers of tetrachlorobenzene (TeCB) (1,2,3,4-, 1,2,3,5- and 1,2,4,5-TeCB) was investigated in microcosms containing chloroaromatic contaminated river sediment. All chlorobenzenes were dechlorinated to dichlorobenzene (DCB) or monochlorobenzene. From the sediment, a methanogenic sediment-free culture was obtained which dechlorinated HCB, pentachlorobenzene, three TeCB isomers, three trichlorobenzene (TCB) isomers (1,2,3-, 1,2,4- and 1,3,5-TCB) and 1,2-DCB. Dechlorination involved multiple pathways including the removal of doubly flanked, singly flanked and isolated chlorine substituents. 454-pyrosequencing of partial bacterial 16S rRNA genes amplified from selected chlorobenzene dechlorinating sediment-free enrichment cultures revealed the presence of a variety of bacterial species, including Dehalobacter and Dehalococcoides mccartyi, that were previously documented as organohalide respiring bacteria. A genus with apparent close relationship to Desulfitobacterium that also has been associated with organohalide respiration, composed the major fraction of the operational taxonomic units (OTUs). Another major OTU was linked with Sedimentibacter sp., a genus that was previously identified in strict co-cultures of consortia reductively dehalogenating chlorinated compounds. Our data point towards the existence of multiple interactions within highly chlorinated benzene dechlorinating communities.