Biolectrochemical systems to investigate the exoelectrogenic activity of hidrocarbon-degrading bacteria
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2017-06
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Universidad Técnica Federico Santa María
Universidad de Valparaíso
Università Degli Studi Di Milano-Bicocca
Universidad de Valparaíso
Università Degli Studi Di Milano-Bicocca
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Facultad
Facultad de Ciencias
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Programa Conjunto Doctorado en Ciencias Mencion Quimica
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Doctor en Ciencias, Mención Química
Resumen
Bioelectrochemistry and, more specifically, microbial electrochemistry, are
technologies based on the connection between microbes (named as
exoelectrogens or, focusing only on bacteria, electrochemically active bacteria)
and electrodes. The exchange of electrons to and from the electrode has been
studied primarily in mixed cultures but also with pure strains, mostly using model
species such as Geobacter and Shewanella; however, more efforts are needed
to elucidate the interaction between microbes and electrode and to find new
interesting niches of application for these microorganisms. A field of application is
bioelectrochemical remediation, an effective strategy in environments where the
absence of suitable electron acceptors limits classic bioremediation, and in which
bioelectrochemical systems are used for the removal of pollutants from
environmental matrices. Bioelectrochemical remediation of hydrocarbons with
pure strains and microbial communities has been reported; however, only few
exoelectrogenic hydrocarbonoclastic bacteria have been characterized, so far.
The degradative potential of several hydrocarbon-degrading strains has been
extensively studied, in terms of pollutants removal and mechanism of contaminant
mineralization, but not much is known about their exoelectrogenic capacity and
possible application for bioelectrochemical remediation. Bioelectrochemistry and
its application for bioremediation purposes, has primarily focused on testing the
hydrocarbonoclastic capacities of already known exoelectrogenic strains. In this
study we took a different approach, and we aimed at studying the exoelectrogenic
activity of three strains that showed great potential for bioremediation
applications: Cupriavidus metallidurans CH34, and Pseudomonas sp. strains
DN34 and DN36. C. metallidurans CH34 is a model metal-resistant strain, whose
hydrocarbonoclastic capacities have recently been individuated, and
Pseudomonas sp. strains DN34 and DN36 that are two hydrocarbon-degrading
strains isolated from an oil-polluted site in central Chile. By analyzing current
production, bacterial growth and substrate consumption in bioelectrochemical
systems (BES), we determined that the three strains possess exoelectrogenic
activity. Moreover, C. metallidurans CH34 showed the most promising results with
a non-recalcitrant substrate and was selected to assess bioremediation
experiments with toluene as model hydrocarbon. We demonstrated for the first
time that strain CH34 is able to degrade toluene under denitrifying conditions.
Further experiments in Microbial Fuel Cells (MFC) linked toluene degradation to
current production by strain CH34, showing current peaks after toluene respike
(maximum current density 0.24 mA/m2). Moreover, a Microbial Electrolysis Cell
(MEC) was operated by applying an external voltage (800 mV) between anode
and cathode to stimulate microbial metabolism of strain CH34 and to observe the
behavior of the strain in terms of toluene removal and current generation. Current
outputs increased by two orders of magnitude in comparison with MFC (up to 47
mA/m2), and coulombic efficiency raised up to 77%, demonstrating that the
bacterial cells adjusted progressively to the system conditions and that
electrochemical losses were, at least partially, overcome. In order to evaluate the
effect of an electron carrier on current production, Neutral Red (NR) was selected
as external transporter and amended in a MEC containing toluene and inoculated
with strain CH34, but no relevant effect was observed on current production nor
coulombic efficiency. Hence, we concluded that NR had no influence on current
generation in our system and that a mediated mechanism with this electron carrier
is not probable. The mechanism of extracellular electron transport (EET) is a key
feature in BESs and the efficiency of the microorganism to exchange electrons
with an electrode and to connect the EET to the cellular carbon metabolism,
significantly influences the overall process performance. We demonstrated that
the first step of the denitrification pathway is activated by nitrate reductases when
NO3
- was the only electron acceptor, but we also aimed at studying whether the
pathway of denitrification is still active in absence of nitrate, if a solid the anode is
potentiostetically-polarized at the same redox potential of nitrate reductase. Our
results indicate that nitrate reductase is not involved in the transport of electrons
in BES and that strain CH34 follows a different pathway of electron transport to
the anode. However, current production and cells viability demonstrated that
strain CH34 was actively performing oxidative phosphorylation, thus that, in a
mechanism that has not been elucidated yet, an extracellular electron transfer
takes place, either in a direct or indirect way
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BIODEGRADACION, BIOELECTROQUIMICA, HIDROCARBUROS