Marine floating microbial fuel cell involving aerobic biofilm on stainless steel cathodes.

Here is presented a new design of a floating marine MFC in which the inter-electrode space is constant. This design allows the generation of stable current for applications in environments where the water column is large or subject to fluctuations such as tidal effects. The operation of the first prototype was validated by running a continuous test campaign for 6months. Performance in terms of electricity generation was already equivalent to what is conventionally reported in the literature with basic benthic MFCs despite the identification of a large internal resistance in the proposed design of the floating system. This high internal resistance is mainly explained by poor positioning of the membrane separating the anode compartment from the open seawater. The future objectives are to achieve more consistent performance and a second-generation prototype is now being developed, mainly incorporating a modification of the separator position and a stainless steel biocathode with a large bioavailable surface.

Modelling the growth and ethanol production of Brettanomyces bruxellensis at different glucose concentrations.

AIM: To study the effect of glucose concentrations on the growth by Brettanomyces bruxellensis yeast strain in batch experiments and develop a mathematical model for kinetic behaviour analysis of yeast growing in batch culture. METHODS AND RESULTS: A Matlab algorithm was developed for the estimation of model parameters. Glucose fermentation by B. bruxellensis was studied by varying its concentration (5, 9.3, 13.8, 16.5, 17.6 and 21.4%). The increase in substrate concentration up to a certain limit was accompanied by an increase in ethanol and biomass production; at a substrate concentration of 50-138 g l(-1), the ethanol and biomass production were 24, 59 and 6.3, 11.4 g l(-1), respectively. However, an increase in glucose concentration to 165 g l(-1) led to a drastic decrease in product formation and substrate utilization. CONCLUSIONS: The model successfully simulated the batch kinetic observed in all cases. The confidence intervals were also estimated at each phase at a 0.95 probability level in a t-Student distribution for f degrees of freedom. The maximum ethanol and biomass yields were obtained with an initial glucose concentration of 138 g l(-1). SIGNIFICANCE AND IMPACT OF THE STUDY: These experiments illustrate the importance of using a mathematical model applied to kinetic behaviour on glucose concentration by B. bruxellensis.

Effect of surface roughness, biofilm coverage and biofilm structure on the electrochemical efficiency of microbial cathodes.

Biofilms of Geobacter sulfurreducens were formed under chronoamperometry at -0.5 V and -0.6 V vs. Ag/AgCl on stainless steel cathodes and tested for fumarate reduction. Increasing the surface roughness Ra from 2.0 µm to 4.0 µm increased currents by a factor of 1.6. The overall current density increased with biofilm coverage. When the current density was calculated with respect to the biofilm-coated area only, values up to 280 A/m(2) were derived. These values decreased with biofilm coverage and indicated that isolated cells or small colonies locally provide higher current density than dense colonies. Steel composition affected the current values because of differences in biofilm structure and electron transfer rates. Biofilms formed under polarisation revealed better electrochemical characteristics than biofilm developed at open circuit. This work opens up new guidelines for the design of microbial cathodes: a uniform carpet of isolated bacteria or small colonies should be targeted, avoiding the formation of large colonies.

Electrochemical checking of aerobic isolates from electrochemically active biofilms formed in compost.

AIMS: To design a cyclic voltammetry (CV) procedure to check the electrochemical activity of bacterial isolates that may explain the electrochemical properties of biofilms formed in compost. METHODS AND RESULTS: Bacteria catalysing acetate oxidation in garden compost were able to form electrochemically active biofilms by transferring electrons to an electrode under chronoamperometry. They were recovered from the electrode surface and identification of the isolates using 16S rRNA sequencing showed that most of them were Gammaproteobacteria, mainly related to Enterobacter and Pseudomonas spp. A CV procedure was designed to check the electrochemical activity of both groups of isolates. Preliminary CVs suggested that the bacteria were not responsible for the catalysis of acetate oxidation. In contrast, both groups of isolates were found to catalyse the electrochemical reduction of oxygen under experimental conditions that favoured adsorption of the microbial cells on the electrode surface. CONCLUSIONS: Members of the genera Enterobacter and Pseudomonas were found to be able to catalyse the electrochemical reduction of oxygen. SIGNIFICANCE AND IMPACT OF THE STUDY: This study has shown the unexpected efficiency of Enterobacter and Pseudomonas spp. in catalysing the reduction of oxygen, suggesting a possible involvement of these species in biocorrosion, or possible application of these strains in designing bio-cathode for microbial fuel cells.

Modelling the growth dynamics of interacting mixed cultures: a case of amensalism.

In this work, a specific membrane bioreactor was used to perform co-cultures of two Saccharomyces cerevisiae yeast strains: a killer strain and a sensitive strain. Biomass could be segregated into four groups: viable killer yeasts, dead killer yeasts, viable sensitive yeasts and dead sensitive yeasts. An existing mathematical model describing the population dynamics in the mixed killer/sensitive cultures was confronted with the new experimental data. As it gave poor accuracy, some improvements were proposed and tested. In particular, a lag phase before the beginning of the lethal interaction between the two strains was introduced, in correspondence to the experimental observations.

Brettanomyces bruxellensis: effect of oxygen on growth and acetic acid production.

The influence of the oxygen supply on the growth, acetic acid and ethanol production by Brettanomyces bruxellensis in a glucose medium was investigated with different air flow rates in the range 0-300 l h(-1 ) x (0-0.5 vvm). This study shows that growth of this yeast is stimulated by moderate aeration. The optimal oxygen supply for cellular synthesis was an oxygen transfer rate (OTR) of 43 mg O(2) l(-1) x h(-1). In this case, there was an air flow rate of 60 l h(-1) (0.1 vvm). Above this value, the maximum biomass concentration decreased. Ethanol and acetic acid production was also dependent on the level of aeration: the higher the oxygen supply, the greater the acetic acid production and the lower the ethanol production. At the highest aeration rates, we observed a strong inhibition of the ethanol yield. Over 180 l h(-1) x (0.3 vvm, OTR =105 mg O(2) l(-1) x h(-1)), glucose consumption was inhibited and a high concentration of acetic acid (6.0 g x l(-1)) was produced. The ratio of "ethanol + acetic acid" produced per mole of consumed glucose using carbon balance calculations was analyzed. It was shown that this ratio remained constant in all cases. This makes it possible to establish a stoichiometric equation between oxygen supply and metabolite production.

Nutritional requirements of Brettanomyces bruxellensis: growth and physiology in batch and chemostat cultures.

The nutritional requirements of Brettanomyces bruxellensis have been investigated. Batch culture and chemostat pulse techniques were used to identify growth-limiting nutrients. The study included determination of the essential components of the culture medium and quantification of the effects of the components. Among the components tested, ammonium sulfate and yeast extract had a significant effect on glucose consumption, growth, and ethanol production. However, if the ammonium sulfate concentration is above 2 g/L, an inhibitory effect on B. bruxellensis growth is observed. The yeast extract appears to be the most important and significant component for growth. The maximum amount of synthesized biomass is proportional to the concentration of yeast extract added to the culture broth (in the tested range). Magnesium and phosphate ions are probably not essential for B. bruxellensis. These ions appear to be supplied in sufficient amounts by the yeast extract in the culture medium. Brettanomyces bruxellensis appears to have very low nutritional requirements for growth.