Antonin Prévoteau

Engineering electrodes for microbial electrocatalysis

Microbial electrocatalysis refers to the use of microorganisms to catalyze electrode reactions. Many processes have been developed on this principle, ranging from power generation to CO2 conversion using bioelectrochemical systems. The nature of the interface between the microorganisms and the electrodes determines the functioning and efficiency of these systems. This interface can be manipulated in terms of chemical and topographical features to better understand the interaction at nanometer and micrometer scales. Here we discuss how the electrode surface topography and chemistry impact the microorganism-electrode interaction both for direct and indirect electron transfer mechanisms. It appears that composite materials that combine high conductivity with excellent biocompatibility are most attractive towards application. In most cases this implies a combination of a metallic backbone with a carbon coating with a defined topography and chemistry.

Flame oxidation of stainless steel felt enhances anodic biofilm formation and current output in bioelectrochemical systems

Stainless steel (SS) can be an attractive material to create large electrodes for microbial bioelectrochemical systems (BESs), due to its low cost and high conductivity. However, poor biocompatibility limits its successful application today. Here we report a simple and effective method to make SS electrodes biocompatible by means of flame oxidation. Physicochemical characterization of electrode surface indicated that iron oxide nanoparticles (IONPs) were generated in situ on an SS felt surface by flame oxidation. IONPs-coating dramatically enhanced the biocompatibility of SS felt and consequently resulted in a robust electroactive biofilm formation at its surface in BESs. The maximum current densities reached at IONPs-coated SS felt electrodes were 16.5 times and 4.8 times higher than the untreated SS felts and carbon felts, respectively. Furthermore, the maximum current density achieved with the IONPs-coated SS felt (1.92 mA/cm(2), 27.42 mA/cm(3)) is one of the highest current densities reported thus far. These results demonstrate for the first time that flame oxidized SS felts could be a good alternative to carbon-based electrodes for achieving high current densities in BESs. Most importantly, high conductivity, excellent mechanical strength, strong chemical stability, large specific surface area, and comparatively low cost of flame oxidized SS felts offer exciting opportunities for scaling-up of the anodes for BESs.

Applications of biochar in redox-mediated reactions

Biochar is chemically more reduced and reactive than the original feedstock biomass. Graphite regions, functional groups, and redox-active metals in biochar contribute to its redox characteristics. While the functional groups such as phenolic species in biochar are the main electron donating moieties (i.e., reducers), the quinones and polycondensed aromatic functional groups are the components accepting electrons (oxidants). The redox capacity of biochar depends on feedstock properties and pyrolysis conditions. This paper aims to review and summarize the various synthesis techniques for biochars and the methods for probing their redox characteristics. We review the abiotic and microbial applications of biochars as electron donors, electron acceptors, or electron shuttles for pollutant degradation, metal(loid)s (im)mobilization, nutrient transformation, and discuss the underlying mechanisms. Furthermore, knowledge gaps that exist in the exploration and differentiation of the electron transfer mechanisms involving biochars are also identified.

The electron donating capacity of biochar is dramatically underestimated

Biochars have gathered considerable interest for agronomic and engineering applications. In addition to their high sorption ability, biochars have been shown to accept or donate considerable amounts of electrons to/from their environment via abiotic or microbial processes. Here, we measured the electron accepting (EAC) and electron donating (EDC) capacities of wood-based biochars pyrolyzed at three different highest treatment temperatures (HTTs: 400, 500, 600 °C) via hydrodynamic electrochemical techniques using a rotating disc electrode. EACs and EDCs varied with HTT in accordance with a previous report with a maximal EAC at 500 °C (0.4 mmol(e−).gchar−1) and a large decrease of EDC with HTT. However, while we monitored similar EAC values than in the preceding study, we show that the EDCs have been underestimated by at least 1 order of magnitude, up to 7 mmol(e−).gchar−1 for a HTT of 400 °C. We attribute this existing underestimation to unnoticed slow kinetics of electron transfer from biochars to the dissolved redox mediators used in the monitoring. The EDC of other soil organic constituents such as humic substances may also have been underestimated. These results imply that the redox properties of biochars may have a much bigger impact on soil biogeochemical processes than previously conjectured.

Hydrodynamic chronoamperometry for probing kinetics of anaerobic microbial metabolism – case study of Faecalibacterium prausnitzii

Monitoring in vitro the metabolic activity of microorganisms aids bioprocesses and enables better understanding of microbial metabolism. Redox mediators can be used for this purpose via different electrochemical techniques that are either complex or only provide non-continuous data. Hydrodynamic chronoamperometry using a rotating disc electrode (RDE) can alleviate these issues but was seldom used and is poorly characterized. The kinetics of Faecalibacterium prausnitzii A2-165, a beneficial gut microbe, were determined using a RDE with riboflavin as redox probe. This butyrate producer anaerobically ferments glucose and reduces riboflavin whose continuous monitoring on a RDE provided highly accurate kinetic measurements of its metabolism, even at low cell densities. The metabolic reaction rate increased linearly over a broad range of cell concentrations (9 × 104 to 5 × 107 cells.mL−1). Apparent Michaelis-Menten kinetics was observed with respect to riboflavin (KM = 6 μM; kcat = 5.3×105 s−1, at 37 °C) and glucose (KM = 6 μM; kcat = 2.4 × 105 s−1). The short temporal resolution allows continuous monitoring of fast cellular events such as kinetics inhibition with butyrate. Furthermore, we detected for the first time riboflavin reduction by another potential probiotic, Butyricicoccus pullicaecorum. The ability of the RDE for fast, accurate, simple and continuous measurements makes it an ad hoc tool for assessing bioprocesses at high resolution.

Effect of onion-type multilamellar liposomes on Trametes versicolor laccase activity and stability.

Trametes versicolor laccase was encapsulated into onion-type, lipid-based multilamellar vesicles (MLVs). When encapsulated, laccase was isolated from the assay medium but was still active once freed from its capsule. The encapsulation efficiency was larger than 65% at 25 °C and 37 °C and decreased to 55% by introducing 140 mM NaCl into the buffered medium (pH = 4.5). MLVs were shown to drastically improve both laccase stability and activity. At 25 °C, laccase activity was doubled in the presence of MLVs. At 37 °C in the salt-free medium, the half-life time of laccase was increased from 2 hr 30-65 h without and with MLVs, respectively. This effect was even more pronounced in the salted medium where laccase activity was unchanged for 6 days in the presence of MLVs. These beneficial effects were attributed to the immobilization of laccase onto MLV surface. Laccase activity as well as stability was notably shown to be directly correlated to MLV stability.

Electroactive Biofilms for Sensing: Reflections and Perspectives

Microbial electrochemistry has from the onset been recognized for its sensing potential due to the microbial ability to enhance signals through metabolic cascades, its relative selectivity toward substrates, and the higher stability conferred by the microbial ability to self-replicate. The greatest challenge has been to achieve stable and efficient transduction between a microorganism and an electrode surface. Over the past decades, a new kind of microbial architecture has been observed to spontaneously develop on polarized electrodes: the electroactive biofilm (EAB). The EAB conducts electrons over long distances and performs quasi-reversible electron transfer on conventional electrode surfaces. It also possesses self-regenerative properties. In only a few years, EABs have inspired considerable research interest for use as biosensors for environmental or bioprocess monitoring. Multiple challenges still need to be overcome before implementation at larger scale of this new kind of biosensors can be realized. This perspective first introduces the specific characteristics of the EAB with respect to other electrochemical biosensors. It summarizes the sensing applications currently proposed for EABs, stresses their limitations, and suggests strategies toward potential solutions. Conceptual prospects to engineer EABs for sensing purposes are also discussed.

Periodic polarization of electroactive biofilms increases current density and charge carriers concentration while modifying biofilm structure

Anodic electroactive biofilms (EABs) need to overcome low current densities for applications such as microbial fuel cells or biosensors. EABs can store charge in self-produced redox proteins when temporarily left in open circuit, and discharge them once the electrode is appropriately repolarized, thus behaving as pseudocapacitors. Here we investigated the effect of such periodic polarization on the intrinsic nature of the EABs during their entire growth (i.e. starting from inoculation and for 10 days) on glassy carbon electrodes. An optimal periodic polarization (half-period of 10 s) greatly increased the maximum steady-state current density delivered by the Geobacter-dominated EABs (up to 1.10 ± 0.02 mA cm-2, n = 3 electrodes) when compared to continuously polarized EABs (0.41 ± 0.04 mA cm-2); and increased the amount of electric charges produced per hour by 69 ± 17% even taking into account the half-periods of open circuit. This enhancement was highly correlated with a substantial increase in charge carriers concentration (10.6 ± 0.5 mMe- vs. 2.9 ± 0.6 mMe-), allowing higher charge storage capacity and higher electron mobility across the EABs. Our results suggest that appropriate periodic polarizations may upregulate the expression of heme-containing redox proteins associated with the matrix, such as c-type cytochromes. The EABs grown under periodic polarization presented mushroom-like structures on their top layers, while EABs grown under continuous polarization were flat.

Growth and current production of mixed culture anodic biofilms remain unaffected by sub-microscale surface roughness

Bioelectrochemical systems couple electricity demand/supply to the metabolic redox reactions of microorganisms. Generally, electrodes act not only as electron acceptors/donors, but also as physical support for an electroactive biofilm. The microorganism-electrode interface can be modified by changing the chemical and/or topographical features of the electrode surface. Thus far, studies have reported conflicting results on the impact of the electrode surface roughness on the growth and current production of biofilms. Here, the surface roughness of the glassy carbon electrodes was successfully modified at the sub-microscale using micro electrodischarge machining, while preserving the surface chemistry of the parent glassy carbon. All microbial electrodes showed similar startup time, maximum current density, charge transport ability across the biofilm and biomass production. Interestingly, an increase in the average surface cavity depth was observed for the biofilm top layer as a function of the electrode surface roughness (from 7 μm to 16 μm for a surface roughness of 5 nm to 682 nm, respectively). These results indicated that the surface roughness at a sub-microscale does not significantly impact the attachment or current production of mixed culture anodic biofilms on glassy carbon. Likely earlier observations were associated with changes in surface chemistry, rather than surface topography.

A new Shewanella isolate enhances corrosion by using metallic iron as electron donor with fumarate as electron acceptor

Shewanella spp. are frequently found on corroded metal structures. Their role in microbial influenced corrosion has been attributed mainly to their Fe(III)-reducing properties and, therefore, has been studied with the addition of an electron donor (lactate). Shewanella spp., however, can also use solid electron donors, such as cathodes and potentially Fe(0). In this work, we show that the electron acceptor fumarate supported the use of Fe(0) as the electron donor by Shewanella strain 4t3-1-2LB, which caused a (7.0 ± 0.6)-fold increase of the corrosion rate. The corrosion-enhancing mechanism likely involved cell surface-associated components in direct contact with the Fe(0) surface or maintenance of low hydrogen levels by attached cells, thereby favoring chemical hydrogen formation by Fe(0). This work sheds new light on the role of Shewanella spp. in biocorrosion, while the insights into the corrosion-enhancing mechanism contribute to the understanding of extracellular electron uptake processes. ABSTRACT The involvement of Shewanella spp. in biocorrosion is often attributed to their Fe(III)-reducing properties, but they could also affect corrosion by using metallic iron as an electron donor. Previously, we isolated Shewanella strain 4t3-1-2LB from an acetogenic community enriched with Fe(0) as the sole electron donor. Here, we investigated its use of Fe(0) as an electron donor with fumarate as an electron acceptor and explored its corrosion-enhancing mechanism. Without Fe(0), strain 4t3-1-2LB fermented fumarate to succinate and CO2, as was shown by the reaction stoichiometry and pH. With Fe(0), strain 4t3-1-2LB completely reduced fumarate to succinate and increased the Fe(0) corrosion rate (7.0 ± 0.6)-fold in comparison to that of abiotic controls (based on the succinate-versus-abiotic hydrogen formation rate). Fumarate reduction by strain 4t3-1-2LB was, at least in part, supported by chemical hydrogen formation on Fe(0). Filter-sterilized spent medium increased the hydrogen generation rate only 1.5-fold, and thus extracellular hydrogenase enzymes appear to be insufficient to explain the enhanced corrosion rate. Electrochemical measurements suggested that strain 4t3-1-2LB did not excrete dissolved redox mediators. Exchanging the medium and scanning electron microscopy (SEM) imaging indicated that cells were attached to Fe(0). It is possible that strain 4t3-1-2LB used a direct mechanism to withdraw electrons from Fe(0) or favored chemical hydrogen formation on Fe(0) through maintaining low hydrogen concentrations. In coculture with an Acetobacterium strain, strain 4t3-1-2LB did not enhance acetogenesis from Fe(0). This work describes a strong corrosion enhancement by a Shewanella strain through its use of Fe(0) as an electron donor and provides insights into its corrosion-enhancing mechanism. IMPORTANCE Shewanella spp. are frequently found on corroded metal structures. Their role in microbial influenced corrosion has been attributed mainly to their Fe(III)-reducing properties and, therefore, has been studied with the addition of an electron donor (lactate). Shewanella spp., however, can also use solid electron donors, such as cathodes and potentially Fe(0). In this work, we show that the electron acceptor fumarate supported the use of Fe(0) as the electron donor by Shewanella strain 4t3-1-2LB, which caused a (7.0 ± 0.6)-fold increase of the corrosion rate. The corrosion-enhancing mechanism likely involved cell surface-associated components in direct contact with the Fe(0) surface or maintenance of low hydrogen levels by attached cells, thereby favoring chemical hydrogen formation by Fe(0). This work sheds new light on the role of Shewanella spp. in biocorrosion, while the insights into the corrosion-enhancing mechanism contribute to the understanding of extracellular electron uptake processes.