Electrochemical activity and bacterial diversity of natural marine biofilm in laboratory closed-systems

Microbial survival and growth on non-corrodible conductive materials

Biofilms growing aerobically on conductive substrates are often correlated with a positive, sustained shift in their redox potential. This phenomenon has a beneficial impact on microbial fuel cells by increasing their overall power output but can be detrimental when occurring on stainless steel by enhancing corrosion. The biological mechanism behind this potential shift is unresolved and a metabolic benefit to cells has not been demonstrated. Here, biofilms containing the electroautotroph 'Candidatus Tenderia electrophaga' catalysed a shift in the open circuit potential of graphite and indium tin oxide electrodes by >100 mV. Biofilms on open circuit electrodes had increased biomass and a significantly higher proportion of 'Ca. Tenderia electrophaga' compared to those on plain glass. Addition of metabolic inhibitors showed that living cells were required to maintain the more positive potential. We propose a model to describe these observations, in which 'Ca. Tenderia electrophaga' drives the shift in open circuit potential through electron uptake for oxygen reduction and CO₂ fixation. We further propose that the electrode is continuously recharged by oxidation of trace redox-active molecules in the medium at the more positive potential. A similar phenomenon is possible on natural conductive substrates in the environment.

Enhanced oxygen reducing biocathode electroactivity by using sediment extract as inoculum

Autotrophic bacteria are able to catalyze cathodic oxygen reduction as a renewable and sustainable inexpensive catalyst. However, the performance of biocathode varied over reactors, and we still not know how inoculums affect this system. Using three different inoculum of wastewater (WW), sediment extract (SE) and soil extract (SO) in parallel reactors, we found that SE achieved the shortest setup time (17-25% shorter) as well as the highest power density compared to those of SO and WW. Cyclic voltammetry (CV) further revealed that the current densities of SE biocathodes (100±1A/m³) was 150% and 67% higher than those of WW biocathodes (40±1A/m³) and SO biocathodes (65±1A/m³). Community analysis showed the selective pressure on biocathode facilitated the growth of Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria families. Different from WW and SO biocathodes, Nitrospirae was selectively enriched in SE biocathodes, corresponding to an obvious increase in Unidentified Nitrospiraceae population at genus level, which may play an important role on the cathodic electroactivity. These results confirmed that sediment extract is a better bacteria source than soil and wastewater for the acclimation of autotrophic electroactive bacteria, and the community comparison provided broader knowledge on biocathode microbiology.

The biocathode of microbial electrochemical systems and microbially-influenced corrosion

The cathode reaction is one of the most important limiting factors in bioelectrochemical systems even with precious metal catalysts. Since aerobic bacteria have a much higher affinity for oxygen than any known abiotic cathode catalysts, the performance of a microbial fuel cell can be improved through the use of electrochemically-active oxygen-reducing bacteria acting as the cathode catalyst. These consume electrons available from the electrode to reduce the electron acceptors present, probably conserving energy for growth. Anaerobic bacteria reduce protons to hydrogen in microbial electrolysis cells (MECs). These aerobic and anaerobic bacterial activities resemble those catalyzing microbially-influenced corrosion (MIC). Sulfate-reducing bacteria and homoacetogens have been identified in MEC biocathodes. For sustainable operation, microbes in a biocathode should conserve energy during such electron-consuming reactions probably by similar mechanisms as those occurring in MIC. A novel hypothesis is proposed here which explains how energy can be conserved by microbes in MEC biocathodes.

Marine sediments microbes capable of electrode oxidation as a surrogate for lithotrophic insoluble substrate metabolism

Little is known about the importance and/or mechanisms of biological mineral oxidation in sediments, partially due to the difficulties associated with culturing mineral-oxidizing microbes. We demonstrate that electrochemical enrichment is a feasible approach for isolation of microbes capable of gaining electrons from insoluble minerals. To this end we constructed sediment microcosms and incubated electrodes at various controlled redox potentials. Negative current production was observed in incubations and increased as redox potential decreased (tested −50 to −400 mV vs. Ag/AgCl). Electrode-associated biomass responded to the addition of nitrate and ferric iron as terminal electron acceptors in secondary sediment-free enrichments. Elemental sulfur, elemental iron and amorphous iron sulfide enrichments derived from electrode biomass demonstrated products indicative of sulfur or iron oxidation. The microbes isolated from these enrichments belong to the genera Halomonas, Idiomarina, Marinobacter, and Pseudomonas of the Gammaproteobacteria, and Thalassospira and Thioclava from the Alphaproteobacteria. Chronoamperometry data demonstrates sustained electrode oxidation from these isolates in the absence of alternate electron sources. Cyclic voltammetry demonstrated the variability in dominant electron transfer modes or interactions with electrodes (i.e., biofilm, planktonic or mediator facilitated) and the wide range of midpoint potentials observed for each microbe (from 8 to −295 mV vs. Ag/AgCl). The diversity of extracellular electron transfer mechanisms observed in one sediment and one redox condition, illustrates the potential importance and abundance of these interactions. This approach has promise for increasing our understanding the extent and diversity of microbe mineral interactions, as well as increasing the repository of microbes available for electrochemical applications.

Corrosion of carbon steel by bacteria from North Sea offshore seawater injection systems: laboratory investigation

Influence of sulfidogenic bacteria, from a North Sea seawater injection system, on the corrosion of S235JR carbon steel was studied in a flow bioreactor; operating anaerobically for 100days with either inoculated or filtrated seawater. Deposits formed on steel placed in reactors contained magnesium and calcium minerals plus iron sulfide. The dominant biofilm-forming organism was an anaerobic bacterium, genus Caminicella, known to produce hydrogen sulfide and carbon dioxide. Open Circuit Potentials (OCP) of steel in the reactors was, for nearly the entire test duration, in the range -800<E(OCP)/mV (vs. SCE)<-700. Generally, the overall corrosion rate, expressed as 1/(Rp/Ω), was lower in the inoculated seawater though they varied significantly on both reactors. Initial and final corrosion rates were virtually identical, namely initial 1/(Rp/Ω)=2×10(-6)±5×10(-7) and final 1/(Rp/Ω)=1.1×10(-5)±2.5×10(-6). Measured data, including electrochemical noise transients and statistical parameters (0.05<Localized Index<1; -5<Skewness<-5; Kurtosis>45), suggested pitting on steel samples within the inoculated environment. However, the actual degree of corrosion could neither be directly correlated with the electrochemical data and nor with the steel corrosion in the filtrated seawater environment. Further laboratory tests are thought to clarify the noticed apparent discrepancies.

Essential data and techniques for conducting microbial fuel cell and other types of bioelectrochemical system experiments

Microbial fuel cells (MFCs) and other bioelectrochemical systems are new technologies that require expertise in a variety of technical areas, ranging from electrochemistry to biological wastewater treatment. There are certain data and critical information that should be included in every MFC study, such as specific surface area of the electrodes, solution conductivity, and power densities normalized to electrode surface area and volumes. Electrochemical techniques such as linear sweep voltammetry can be used to understand the performance of the MFC, but extremely slow scans are required for these biological systems compared to more traditional fuel cells. In this Minireview, the critical information needed for MFC studies is provided with examples of how results can be better conveyed through a full description of materials, the use of proper controls, and inclusion of a more complete electrochemical analysis.

Microbial catalysis of the oxygen reduction reaction for microbial fuel cells: a review

The slow kinetics of the electrochemical oxygen reduction reaction (ORR) is a crucial bottleneck in the development of microbial fuel cells (MFCs). This article firstly gives an overview of the particular constraints imposed on ORR by MFC operating conditions: neutral pH, slow oxygen mass transfer, sensitivity to reactive oxygen species, fouling and biofouling. A review of the literature is then proposed to assess how microbial catalysis could afford suitable solutions. Actually, microbial catalysis of ORR occurs spontaneously on the surface of metallic materials and is an effective motor of microbial corrosion. In this framework, several mechanisms have been proposed, which are reviewed in the second part of the article. The last part describes the efforts made in the domain of MFCs to determine the microbial ecology of electroactive biofilms and define efficient protocols for the formation of microbial oxygen-reducing cathodes. Although no clear mechanism has been established yet, several promising solutions have been recently proposed.

Convergent development of anodic bacterial communities in microbial fuel cells

Microbial fuel cells (MFCs) are often inoculated from a single wastewater source. The extent that the inoculum affects community development or power production is unknown. The stable anodic microbial communities in MFCs were examined using three inocula: a wastewater treatment plant sample known to produce consistent power densities, a second wastewater treatment plant sample, and an anaerobic bog sediment. The bog-inoculated MFCs initially produced higher power densities than the wastewater-inoculated MFCs, but after 20 cycles all MFCs on average converged to similar voltages (470±20 mV) and maximum power densities (590±170 mW m⁻²). The power output from replicate bog-inoculated MFCs was not significantly different, but one wastewater-inoculated MFC (UAJA3 (UAJA, University Area Joint Authority Wastewater Treatment Plant)) produced substantially less power. Denaturing gradient gel electrophoresis profiling showed a stable exoelectrogenic biofilm community in all samples after 11 cycles. After 16 cycles the predominance of Geobacter spp. in anode communities was identified using 16S rRNA gene clone libraries (58±10%), fluorescent in-situ hybridization (FISH) (63±6%) and pyrosequencing (81±4%). While the clone library analysis for the underperforming UAJA3 had a significantly lower percentage of Geobacter spp. sequences (36%), suggesting that a predominance of this microbe was needed for convergent power densities, the lower percentage of this species was not verified by FISH or pyrosequencing analyses. These results show that the predominance of Geobacter spp. in acetate-fed systems was consistent with good MFC performance and independent of the inoculum source.

Electroactivity of phototrophic river biofilms and constitutive cultivable bacteria

Electroactivity is a property of microorganisms assembled in biofilms that has been highlighted in a variety of environments. This characteristic was assessed for phototrophic river biofilms at the community scale and at the bacterial population scale. At the community scale, electroactivity was evaluated on stainless steel and copper alloy coupons used both as biofilm colonization supports and as working electrodes. At the population scale, the ability of environmental bacterial strains to catalyze oxygen reduction was assessed by cyclic voltammetry. Our data demonstrate that phototrophic river biofilm development on the electrodes, measured by dry mass and chlorophyll a content, resulted in significant increases of the recorded potentials, with potentials of up to +120 mV/saturated calomel electrode (SCE) on stainless steel electrodes and +60 mV/SCE on copper electrodes. Thirty-two bacterial strains isolated from natural phototrophic river biofilms were tested by cyclic voltammetry. Twenty-five were able to catalyze oxygen reduction, with shifts of potential ranging from 0.06 to 0.23 V, cathodic peak potentials ranging from -0.36 to -0.76 V/SCE, and peak amplitudes ranging from -9.5 to -19.4 μA. These isolates were diversified phylogenetically (Actinobacteria, Firmicutes, Bacteroidetes, and Alpha-, Beta-, and Gammaproteobacteria) and exhibited various phenotypic properties (Gram stain, oxidase, and catalase characteristics). These data suggest that phototrophic river biofilm communities and/or most of their constitutive bacterial populations present the ability to promote electronic exchange with a metallic electrode, supporting the following possibilities: (i) development of electrochemistry-based sensors allowing in situ phototrophic river biofilm detection and (ii) production of microbial fuel cell inocula under oligotrophic conditions.

Evidence of enzymatic catalysis of oxygen reduction on stainless steels under marine biofilm

Cathodic current trends on stainless steel samples with different surface percentages covered by biofilm and potentiostatically polarized in natural seawater were studied under oxygen concentration changes, temperature increases, and additions of enzymic inhibitors to the solution. The results showed that on each surface fraction covered by biofilm the oxygen reduction kinetics resembled a reaction catalyzed by an immobilised enzyme with high oxygen affinity (apparent Michaelis-Menten dissociation constant close to K(O(2))(M)  ≈ 10 μM) and low activation energy (W ≈ 20 KJ mole(-1)). The proposed enzyme rapidly degraded when the temperature was increased above the ambient (half-life time of ∼1 day at 25°C, and of a few minutes at 50°C). Furthermore, when reversible enzymic inhibitors (eg sodium azide and cyanide) were added, the cathodic current induced by biofilm growth was inhibited.

Exploiting a new electrochemical sensor for biofilm monitoring and water treatment optimization

Bacterial biofilm development is a serious problem in many fields, and the existing biofilm monitoring sensors often turn out to be inadequate. In this perspective, a new sensor (ALVIM) has been developed, exploiting the natural marine and freshwater biofilms electrochemical activity, proportional to surface covering. The results presented in this work, obtained testing the ALVIM system both in laboratory and in an industrial environment, show that the sensor gives a fast and accurate response to biofilm growth, and that this response can be used to optimize cleaning treatments inside pipelines. Compared to the existing biofilm sensors, the proposed system show significant technological innovations, higher sensitivity and precision.

Catalysis of the electrochemical reduction of oxygen by bacteria isolated from electro-active biofilms formed in seawater

Biofilms formed in aerobic seawater on stainless steel are known to be efficient catalysts of the electrochemical reduction of oxygen. Based on their genomic analysis, seven bacterial isolates were selected and a cyclic voltammetry (CV) procedure was implemented to check their electrocatalytic activity towards oxygen reduction. All isolates exhibited close catalytic characteristics. Comparison between CVs recorded with glassy carbon and pyrolytic graphite electrodes showed that the catalytic effect was not correlated with the surface area covered by the cells. The low catalytic effect obtained with filtered isolates indicated the involvement of released redox compounds, which was confirmed by CVs performed with adsorbed iron-porphyrin. None of the isolates were able to form electro-active biofilms under constant polarization. The capacity to catalyze oxygen reduction is shown to be a widespread property among bacteria, but the property detected by CV does not necessarily confer the ability to achieve stable oxygen reduction under constant polarization.