Microbiome Dynamics of a Polychlorobiphenyl (PCB) Historically Contaminated Marine Sediment under Conditions Promoting Reductive Dechlorination.

Bruna Matturro,Simona Rossetti,Carla Ubaldi

doi:10.3389/fmicb.2016.01502

Abstract

The toxicity of polychlorinated biphenyls (PCB) can be efficiently reduced in contaminated marine sediments through the reductive dechlorination (RD) process lead by anaerobic organohalide bacteria. Although the process has been extensively investigated on PCB-spiked sediments, the knowledge on the identity and metabolic potential of PCB-dechlorinating microorganisms in real contaminated matrix is still limited. Aim of this study was to explore the composition and the dynamics of the microbial communities of the marine sediment collected from one of the largest Sites of National Interest (SIN) in Italy (Mar Piccolo, Taranto) under conditions promoting the PCBs RD. A long-term microcosm study revealed that autochthonous bacteria were able to sustain the PCB dechlorination at a high extent and the successive addition of an external fermentable organic substrate (lactate) caused the further depletion of the high-chlorinated PCBs (up to 70%). Next Generation Sequencing was used to describe the core microbiome of the marine sediment and to follow the changes caused by the treatments. OTUs affiliated to sulfur-oxidizing ε-proteobacteria, Sulfurovum, and Sulfurimonas, were predominant in the original sediment and increased up to 60% of total OTUs after lactate addition. Other OTUs detected in the sediment were affiliated to sulfate reducing (δ-proteobacteria) and to organohalide respiring bacteria within Chloroflexi phylum mainly belonging to Dehalococcoidia class. Among others, Dehalococcoides mccartyi was enriched during the treatments even though the screening of the specific reductive dehalogenase genes revealed the occurrence of undescribed strains, which deserve further investigations. Overall, this study highlighted the potential of members of Dehalococcoidia class in reducing the contamination level of the marine sediment from Mar Piccolo with relevant implications on the selection of sustainable bioremediation strategies to clean-up the site.

Highlights

Polychlorinated biphenyls (PCBs) form a family of 209 congeners characterized by physical and chemical properties desirable for various industrial and commercial purposes such as dielectric, heat transfer, hydraulic fluids, plasticizers, and fire retardants
As demonstrated in a previous treatability study (Matturro et al, 2016b), PCB reductive dechlorination (RD) was sustained by the sediment organic carbon under controlled anaerobic conditions with an overall decrement of the most high-chlorinated PCBs up to 50% and the simultaneous increment of low-chlorinated congeners usually found as byproducts of the PCB anaerobic RD (Bedard, 2008)
Despite the depletion of these compounds is possible through RD process, only little is known about the identity and role of microbial niches harboring microorganisms adapted at high level of PCB contamination and able to metabolize these compounds, in contaminated marine sediments where complex biogeochemical conditions may strongly affect the RD process

Summary

Introduction

Polychlorinated biphenyls (PCBs) form a family of 209 congeners characterized by physical and chemical properties desirable for various industrial and commercial purposes such as dielectric, heat transfer, hydraulic fluids, plasticizers, and fire retardants. Nowadays more than 1.5 million tons of PCBs are disseminated into the environment and accumulated in groundwater, soil, and sediments representing a serious risk for ecosystems and human health (Nogales et al, 2011). Despite their persistence into the environment, some microorganisms are able to reduce chlorinated compounds into less toxic or harmless products through the anaerobic reductive dechlorination (RD), a biological redox-based process, which occurs in the presence of an electron donor such as direct H2 or fermentable organic substrates (Passatore et al, 2015). Dehalococcoides mccartyi (Chloroflexi phylum) is considered the most important biomarker of chlorinated ethenes RD (Maymo-Gatell et al, 1997; Löffler et al, 2013; Hug and Edwards, 2013; Bedard, 2014)

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