and to Nitrospira were detected (Yan et al. 2012) while a complex community was
described during the TPH degradation with a BES in contaminated soil (Lu et al.
2014a). The community was highly enriched in Proteobacteria (up to 94% on the
anodes) and the most abundant classes within this phylum were Betaproteobacteria
(main genera Achromobacter, Alcaligenes, Bordetella, Comamonas, and
Pusillimonas) and Gammaproteobacteria (main genera Enterobacter, Fulvimonas,
Pseudomonas, Pseudoxanthomonas, and Stenotrophomonas) and included microorganisms already described for their electrochemical activity (e.g., Comamonas and
Pseudomonas), but also bacteria that were not reported as exoelectrogens (e.g.,
Bordetella) (Lu et al. 2014a). Iron reducers represented only a minor fraction of
the microorganisms enriched in the above-reported studies. However, microorganisms of the genus Geobacter were enriched on the anodes of the bioelectrochemical
reactors during the phenol removal (Palma et al. 2018; Zhang et al. 2017). The
presence of Geobacter was also observed in microbial communities enriched in
bioelectrochemical reactors during the degradation of recalcitrant hydrocarbons such
as BTEX compounds (Daghio et al. 2018a) and PAHs (Yu et al. 2017). While in the
first case, the members of the family Geobacteraceae accounted for a minor fraction
of the microorganisms in anodic communities dominated by the family
Desulfobulbaceae (Daghio et al. 2018a), in the other report the genus Geobacter
represented nearly 39% of the microorganisms enriched on the electrodes of sediment MFCs in which removal of anthracene, phenanthrene, and pyrene was
observed (Yu et al. 2017).
It is intriguing that iron reducers, which are often enriched in the anodic
communities with easily degradable electron donors (e.g., acetate) (Kiely et al.
2011), seem to have a marginal role in several studies during the degradation of
recalcitrant substrates (e.g., hydrocarbons). Some authors have hypothesized that
when readily biodegradable carbon sources are provided, the selection of the
microbial community is mainly driven by the ability to use the electrodes as
electron acceptors (Daghio et al. 2017). Conversely, with complex and recalcitrant
substrates, the ability to use the electron donor represents a selective pressure that
can lead to the selection of microorganisms with less efficient EET mechanisms
(Daghio et al. 2017).
The occurrence of complex interactions within the microbial communities has
been described during bioelectrochemical hydrocarbon degradation in sulfur-rich
environments. Benzene removal in a BES reactor filled with sulfide-rich groundwater led to the selection of putative aerobic degraders probably because of the
diffusion of a small amount of oxygen into the reactor. The involvement of aerobic
hydrocarbon degraders in benzene removal was suggested by carbon and hydrogen
isotope fractionation patterns, which indicated that the benzene activation occurred
by a reaction catalyzed by a monooxygenase (Rakoczy et al. 2013). However, the
occurrence of the sulfide removal and the sulfate formation, together with the
presence of several groups of microorganisms linked to the sulfur cycle, led to the
hypothesis that benzene might have been degraded in the bulk and that the electron
transfer to the anode was performed via sulfide-sulfate (Rakoczy et al. 2013). A
confirmation of this hypothesis came from the characterization of the microbial
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described during the TPH degradation with a BES in contaminated soil (Lu et al.
2014a). The community was highly enriched in Proteobacteria (up to 94% on the
anodes) and the most abundant classes within this phylum were Betaproteobacteria
(main genera Achromobacter, Alcaligenes, Bordetella, Comamonas, and
Pusillimonas) and Gammaproteobacteria (main genera Enterobacter, Fulvimonas,
Pseudomonas, Pseudoxanthomonas, and Stenotrophomonas) and included microorganisms already described for their electrochemical activity (e.g., Comamonas and
Pseudomonas), but also bacteria that were not reported as exoelectrogens (e.g.,
Bordetella) (Lu et al. 2014a). Iron reducers represented only a minor fraction of
the microorganisms enriched in the above-reported studies. However, microorganisms of the genus Geobacter were enriched on the anodes of the bioelectrochemical
reactors during the phenol removal (Palma et al. 2018; Zhang et al. 2017). The
presence of Geobacter was also observed in microbial communities enriched in
bioelectrochemical reactors during the degradation of recalcitrant hydrocarbons such
as BTEX compounds (Daghio et al. 2018a) and PAHs (Yu et al. 2017). While in the
first case, the members of the family Geobacteraceae accounted for a minor fraction
of the microorganisms in anodic communities dominated by the family
Desulfobulbaceae (Daghio et al. 2018a), in the other report the genus Geobacter
represented nearly 39% of the microorganisms enriched on the electrodes of sediment MFCs in which removal of anthracene, phenanthrene, and pyrene was
observed (Yu et al. 2017).
It is intriguing that iron reducers, which are often enriched in the anodic
communities with easily degradable electron donors (e.g., acetate) (Kiely et al.
2011), seem to have a marginal role in several studies during the degradation of
recalcitrant substrates (e.g., hydrocarbons). Some authors have hypothesized that
when readily biodegradable carbon sources are provided, the selection of the
microbial community is mainly driven by the ability to use the electrodes as
electron acceptors (Daghio et al. 2017). Conversely, with complex and recalcitrant
substrates, the ability to use the electron donor represents a selective pressure that
can lead to the selection of microorganisms with less efficient EET mechanisms
(Daghio et al. 2017).
The occurrence of complex interactions within the microbial communities has
been described during bioelectrochemical hydrocarbon degradation in sulfur-rich
environments. Benzene removal in a BES reactor filled with sulfide-rich groundwater led to the selection of putative aerobic degraders probably because of the
diffusion of a small amount of oxygen into the reactor. The involvement of aerobic
hydrocarbon degraders in benzene removal was suggested by carbon and hydrogen
isotope fractionation patterns, which indicated that the benzene activation occurred
by a reaction catalyzed by a monooxygenase (Rakoczy et al. 2013). However, the
occurrence of the sulfide removal and the sulfate formation, together with the
presence of several groups of microorganisms linked to the sulfur cycle, led to the
hypothesis that benzene might have been degraded in the bulk and that the electron
transfer to the anode was performed via sulfide-sulfate (Rakoczy et al. 2013). A
confirmation of this hypothesis came from the characterization of the microbial
382
M. Daghio and A. Franzetti