Abstract
BackgroundMicrobial reductive dechlorination of polychlorinated biphenyls (PCBs) plays a major role in detoxifying anoxic contaminated freshwater and marine sediments from PCBs. Known members of the phylum Chloroflexi are typically responsible for this activity in freshwater sediments, whereas less is known about the microorganisms responsible for this activity in marine sediments. PCB-respiring activities were detected in PCB-impacted marine sediments of the Venice Lagoon. The aim of this work was to identify the indigenous organohalide-respiring microorganisms in such environments and assess their dechlorination specificity against spiked Aroclor™ 1254 PCBs under laboratory conditions resembling the in situ biogeochemistry.ResultsHigh PCB dechlorination activities (from 150 ± 7 to 380 ± 44 μmol of chlorine removed kg−1 week−1) were detected in three out of six sediments sampled from different locations of the lagoon. An uncultured non-Dehalococcoides phylotype of the class Dehalococcoidia closely related to Dehalobium chlorocoercia DF-1, namely phylotype VLD-1, was detected and enriched up to 109 16S rRNA gene copies per gram of sediment where dechlorination activities were higher and 25-4/24-4 and 25-2/24-2/4-4 chlorobiphenyls (CB) accumulated as the main tri-/dichlorinated products. Conversely, a different phylotype closely related to the SF1/m-1 clade, namely VLD-2, also enriched highly where lower dechlorination activity and the accumulation of 25-3 CB as main tri-chlorinated product occurred, albeit in the simultaneous presence of VLD-1. Both phylotypes showed growth yields higher or comparable to known organohalide respirers and neither phylotypes enriched in sediment cultures not exhibiting dechlorination.ConclusionsThese findings confirm the presence of different PCB-respiring microorganisms in the indigenous microbial communities of Venice Lagoon sediments and relate two non-Dehalococcoides phylotypes of the class Dehalococcoidia to different PCB dechlorination rates and specificities.
Highlights
Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) plays a major role in detoxifying anoxic contaminated freshwater and marine sediments from PCBs
Dehalococcoides-like Dehalococcoidia belonging to the o-17/DF-1 clade have been more frequently linked to PCB dechlorination activity in estuarine environments [21, 23,24,25], where salinity and sulfate concentrations may shift over time and space between freshwater and marine conditions [26]
PCB dechlorination Six different sediments from the Venice Lagoon, historically contaminated by PCBs at low concentrations (Additional file 1: Table S1) were cultivated in biogeochemical conditions resembling those occurring in situ and spiked with Aroclor 1254
Summary
Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) plays a major role in detoxifying anoxic contaminated freshwater and marine sediments from PCBs. A different phylotype closely related to the SF1/m-1 clade, namely VLD-2, enriched highly where lower dechlorination activity and the accumulation of 25-3 CB as main tri-chlorinated product occurred, albeit in the simultaneous presence of VLD-1 Both phylotypes showed growth yields higher or comparable to known organohalide respirers and neither phylotypes enriched in sediment cultures not exhibiting dechlorination. Nuzzo et al Microb Cell Fact (2017) 16:127 activities, which sequentially remove chlorine atoms from the pollutants, making them be more susceptible to aerobic oxidative biodegradation and often less toxic and less prone to bioaccumulate than parent compounds [6, 7] Such activity is called microbial reductive dechlorination and represents a promising process for the sustainable remediation of contaminated sediments, currently managed through expensive and highly-impacting dredging operations or capping [8, 9]. A non-Dehalococcoides strain, Dehalobium chlorocoercia DF-1, is the sole Chloroflexi PCB respiring isolate obtained so far from estuarine sediments [24, 31]
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