Title : Microbial diversity in bioelectrochemical systems (BES) used to address bioremediation of soils contaminated by halogenated aromatic compounds

Abstract:

A major effect of uncontrolled anthropogenic activity is the release and accumulations of organic pollutants in the environment. Among these, aromatic organo-halogenated pollutants [including polychlorinated biphenyls (PCBs)] are a major cause of concerns due to their high toxicity and recalcitrance. At the level of sediments that typically present conditions of anaerobiosis, the biodegradation of these toxic compounds occurs through a process known as reductive dehalogenation, in which the halogenated molecules are used as electron acceptors by anaerobic microorganisms. Generally, from the reductive dehalogenation of organo-halogenated compounds low halogen congeners are generated, which, in turn, can be further degraded under oxidative conditions through mono- and di-oxygenase activities. Therefore, the complete mineralization of organohalogenated contaminants requires the concerted action of both anaerobic and aerobic microorganisms. However, at the level of sediments the efficiency of both degradation processes can be hampered by the lack of electron donors available for the reductive dehalogenation and the limitation of oxygen levels used as final electron acceptor by aerobic microorganisms.

Bioelectrochemical systems (BES) are emerging tools based on the supply of electrical current to provide electron donors and acceptors necessary for efficient reductive and oxidative processes involved in organohalogenated compounds mineralization. In the present work, a BES was implemented to improve the biodegradation efficiency of polychlorinated biphenyls (PCBs) using the contaminated sediments collected from Mar Piccolo (Taranto, Italy) as initial inoculum. The supply of specific electrical currents had a positive effect on the PCB degradation and induced a shift in the composition of the microbial communities associated to cathode and anode. 16S rRNA targeting Illumina sequencing indicated that the microbial communities present in the initial Mar Piccolo sediments were composed by Proteobacteria (mainly Gamma- and Deltaproteobacteria), followed by Epsilonbacteraeota and Chloroflexi. Among these, members of Dehalococcoidia and Deltaproteobacteria classes were associated to dehalogenation activities, including dehalorespiration. After the 6 months-long treatment of these sediments in BESs, specific microbial taxa resulted to be differentially enriched in the cathode and anode in each bioreactor, unlike the open-circuit BES system, which served as control bioreactor. Further, the cathode- and anode-associated microbial communities were different among BESs depending on the value of electrical voltage applied. Specifically, the biocathode and bioanode of the best-performing BESs were enriched in members previously associated with chlorinated aromatic hydrocarbon (CAH) degradation belonging to Clostridiales and Flavobacteriales orders, respectively. Despite not being predominant in any BES, the presence of different halogenated aromatics-degrader belonging to Dehalococcoides order was confirmed using twenty-seven sets of degenerate primers targeting taxa-specific reductive dehalogenase genes (rdhA).

This project has received funding from the European Union’s Horizon 2020 project ELECTRA (Electricity driven Low Energy and Chemical input Technology foR Accelerated bioremediation); Grant agreement ID: 826244

Presentation Learning Outcome

• The potential application of bioelectrochemical systems (BES) in bioremediation approaches of sediments contaminated with halogenated aromatic hydrocarbons
• Our present knowledge on the microbial groups enriched in bioelectrochemical systems (BES) with contaminated soils from Mar Piccolo (Taranto, Italy) subject to different electrical currents